mediadecode_ffmpeg/decoder/

mod.rs

use std::{collections::VecDeque, mem::ManuallyDrop, ptr};

use ffmpeg_next::{
  Codec, Packet, Rational,
  codec::{
    self,
    Context,
    // Bring the `Mut` / `Ref` traits into scope so `Packet::as_ptr` /
    // `Packet::as_mut_ptr` resolve. They are aliased to avoid shadowing
    // any future `Mut`/`Ref` types we might add — `cargo clippy` would
    // otherwise flag them as "unused" without the alias and the import
    // can mistakenly look unused. Confirmed in use by all `packet.as_ptr()`
    // / `packet.as_mut_ptr()` call sites in this module.
    packet::{Mut as PacketMut, Ref as PacketRef},
  },
  ffi::{
    AVBufferRef, AVCodec, AVFrame, AVHWFramesContext, AVMediaType, av_buffer_ref, av_buffer_unref,
    av_frame_move_ref, av_frame_unref, av_hwdevice_ctx_create, av_hwframe_transfer_data,
    av_packet_make_writable, av_packet_ref, avcodec_alloc_context3, avcodec_free_context,
    avcodec_parameters_to_context,
  },
  frame,
};

/// Local FFI shims: FFmpeg entry points re-declared with `c_int` where
/// the generated bindings use a closed Rust enum.
///
/// Constructing `AVCodecID` / `AVPixelFormat` / `AVSampleFormat` from a
/// runtime integer that is not in this build's discriminant set is UB —
/// and these are open C enums that FFmpeg extends in ABI-compatible
/// releases. Declaring the same C symbol with `c_int` sidesteps the
/// boundary entirely: both Rust declarations resolve to the same symbol
/// at link time, and the integer never becomes an enum on the Rust
/// side.
///
/// **This is the enum class inside this crate's own code.** The
/// dependency-API sweep closed every place *ffmpeg-next* formed an enum
/// out of FFmpeg memory; these three are places the crate's own new
/// code did the same thing. The census that walks the pixel-format
/// table to price the worst format is the sharpest instance: it exists
/// precisely to be correct about formats this build does not name, and
/// the binding it called returned those very ids as a closed
/// `AVPixelFormat`. Every id would have become an invalid enum value on
/// the way *into* the pricing that was supposed to handle it.
pub(crate) mod c_shims {
  use libc::c_int;

  use super::AVCodec;

  unsafe extern "C" {
    /// `AVCodecID` as `c_int`.
    pub fn avcodec_find_decoder(id: c_int) -> *const AVCodec;

    /// `AVCodecID` as `c_int`, answering the descriptor libavcodec keeps
    /// for that id — or null where this build names no codec for it.
    ///
    /// The pointer is into `codec_descriptors[]`, a `static const` table
    /// compiled into libavcodec, so the strings it names are live and
    /// unwritten for the process — the contract `crate::ffi`'s bounded
    /// reader is given. It is left **raw** at every call site: the
    /// struct embeds an `AVCodecID` and an `AVMediaType`, and forming a
    /// reference to it would assert two bindgen enums valid on a table
    /// that belongs to the linked library rather than to the bindings.
    /// See `crate::CodecId::descriptor`.
    ///
    /// Preferred over `avcodec_get_name`, which never answers null: for
    /// an id it cannot place it returns the string `"unknown_codec"`,
    /// a sentinel wearing a name's clothes. A descriptor that is absent
    /// is `None` here, and a consumer can tell the two apart.
    pub fn avcodec_descriptor_get(id: c_int) -> *const ffmpeg_next::ffi::AVCodecDescriptor;

    /// Returns `AVPixelFormat` as `c_int` — the id of a descriptor that
    /// may well name a format this build's bindings do not.
    pub fn av_pix_fmt_desc_get_id(desc: *const ffmpeg_next::ffi::AVPixFmtDescriptor) -> c_int;

    /// Takes `AVPixelFormat` as `c_int`, so an id straight out of
    /// [`av_pix_fmt_desc_get_id`] can be priced without ever being an
    /// enum.
    pub fn av_image_get_buffer_size(
      pix_fmt: c_int,
      width: c_int,
      height: c_int,
      align: c_int,
    ) -> c_int;

    /// Takes `AVSampleFormat` as `c_int`. Kept for the footprint
    /// sweep, which walks the format table to decide which cells
    /// exist; production pricing goes through
    /// `av_samples_get_buffer_size`, the allocator's own ruler.
    #[cfg(test)]
    pub fn av_get_bytes_per_sample(sample_fmt: c_int) -> c_int;

    /// The allocator's own audio ruler, with `AVSampleFormat` as
    /// `c_int`. `align = 0` asks for the alignment
    /// `av_frame_get_buffer` itself uses.
    pub fn av_samples_get_buffer_size(
      linesize: *mut c_int,
      nb_channels: c_int,
      nb_samples: c_int,
      sample_fmt: c_int,
      align: c_int,
    ) -> c_int;

    /// Writes an `AV_PIX_FMT_NONE`-terminated list of destination
    /// formats a transfer may produce. Declared `*mut *mut c_int` so
    /// the list is walked as integers — a driver may well offer a
    /// format this build's bindings do not name.
    pub fn av_hwframe_transfer_get_formats(
      hwframe_ctx: *mut ffmpeg_next::ffi::AVBufferRef,
      dir: c_int,
      formats: *mut *mut c_int,
      flags: c_int,
    ) -> c_int;
  }
}

use mediadecode::{Received, Sent, decoder::ScaledOutputCapability};

use crate::{
  backend::{self, Backend},
  error::{AllBackendsFailed, Error, HwDeviceInitFailed, Result},
  ffi::{CallbackState, codec_supports_hwaccel, get_hw_format},
  frame::Frame,
};

/// Hardware-accelerated video decoder.
///
/// Hardware-only — there is no software fallback inside this crate. If
/// every hardware backend in the platform's probe order fails to open,
/// `open` returns [`Error::AllBackendsFailed`] and the caller is
/// responsible for falling back to a software decoder of their choice
/// (e.g. `ffmpeg::decoder::Video`).
///
/// Mirrors `ffmpeg::decoder::Video`'s `send_packet`/`receive_frame` interface.
/// Decoded frames are returned through [`crate::Frame`], a CPU-side wrapper
/// whose accessors avoid the `AVPixelFormat`-enum UB that an unvalidated read
/// of FFmpeg's raw integer pixel formats can trigger.
///
/// `open` does a true probe: each backend opens with a strict `get_format`
/// callback. On the first non-transient error from a backend the decoder is
/// torn down and the next backend in probe order is tried, with all packets
/// seen so far replayed through it. The advance is *transactional* — the
/// candidate backend must successfully build and accept the replayed packets
/// before any probe state is consumed, so a failing backend in the middle of
/// the order does not strand the caller without history. Once the first frame
/// is delivered the probe collapses and subsequent calls go straight to the
/// active (committed) backend.
///
/// # After commit
///
/// A committed backend reports per picture, and nothing is classified or
/// remembered. Every failure is that picture's own error, reported as it
/// was minted — a refusal this crate made by its name, anything else as
/// libavcodec's errno — and the next call reaches libavcodec. That is all
/// this decoder promises about the next call: whether the hardware
/// session behind it recovers is FFmpeg's.
///
/// FFmpeg has no reliable signal that a hardware session is gone, so this
/// decoder never decides that one is. In FFmpeg 9.0.1 `AVERROR_EXTERNAL`
/// also answers a single picture: VideoToolbox's frame post-processing
/// and buffer creation (`libavcodec/videotoolbox.c` 126–129 and 556–559),
/// and every failed CUDA call under NVDEC (`libavutil/cuda_check.h` 52),
/// the decode of one picture included (`nvdec.c` 662). `ENOSYS` is
/// NVDEC's answer to one HEVC picture its tables cannot describe
/// (`nvdec_hevc.c` 200–227). A hardware format `ff_get_format` withdraws
/// says only that the hwaccel's setup failed, not why (`decode.c`
/// 1341–1343 and 1348–1357).
///
/// Nor does FFmpeg always recover a session. A malfunction or an
/// invalidated session marks a VideoToolbox session for a restart
/// (`videotoolbox.c` 1076–1077), as does an H.264 SPS whose profile/level
/// bytes differ (446–450), and the next picture stops the session and
/// starts a new one (1062–1068). A restart that fails answers
/// `AVERROR_EXTERNAL` (1066–1067) and leaves no session behind, because
/// the stop released it (518–522): VideoToolbox answers every picture
/// after it with `AVERROR_INVALIDDATA` (1071–1072) until a new parameter
/// set re-arms the restart (446–450). [`Self::flush`] does not rebuild
/// one: `avcodec_flush_buffers` reaches only the codec's own flush
/// (`libavcodec/avcodec.c` 417–418).
///
/// So a caller that sees failures persist rebuilds: it opens a software
/// decoder from the same parameters and feeds it forward. When to do so
/// is the caller's policy — the caller sees the errors, the packets' key
/// flags and what it has delivered — and [`DecodePath`](crate::DecodePath)
/// describes one.
pub struct VideoDecoder {
  /// Live FFmpeg state for the currently active backend.
  state: DecoderState,
  /// Reusable frame buffer used for hw-side decoding before transfer / move.
  /// Internal use only — never handed to callers.
  hw_frame: frame::Video,
  /// Probe state: present until the first frame is received from the active
  /// backend, then `None`. While `Some`, packets are buffered for replay and
  /// non-transient errors / decoder failures advance to the next backend.
  probe: Option<ProbeState>,
  /// CPU-side frames produced by a candidate decoder during probe replay
  /// (when its internal queue filled and we had to drain output before the
  /// next `send_packet`). Already transferred from the candidate's
  /// `AVHWFramesContext` to a CPU frame, so they remain valid after the
  /// candidate state is committed. [`Self::receive_frame`] dequeues these
  /// FIFO before reading from `state.inner`.
  pending_frames: VecDeque<frame::Video>,
  /// Per-decoder byte budget for [`Self::pending_frames`] during probe
  /// replay. Defaults to [`DEFAULT_MAX_PROBE_PENDING_BYTES`]; override via
  /// [`Self::with_max_probe_pending_bytes`].
  max_probe_pending_bytes: usize,
  /// Resource ceilings for the frames this decoder produces. Fixed at
  /// open, because [`FrameLimits::max_pixels`] is written into every
  /// `AVCodecContext` this decoder builds — including the ones a probe
  /// advance builds later — and a context's ceiling cannot be moved
  /// after `avcodec_open2`.
  frame_limits: crate::limits::DecoderLimits,
  /// The stream's packet timebase, written into every
  /// `AVCodecContext` this decoder opens — including the ones a probe
  /// advance opens later, which is why it is held rather than passed.
  ///
  /// `None` for a standalone decoder: [`Self::open`] and its siblings
  /// are handed codec parameters and no stream, so there is no ruler
  /// to declare. The stream decoder one tier up has one.
  pkt_timebase: Option<mediadecode::Timebase>,
  /// `true` once [`Self::send_eof`] has been accepted, until
  /// [`Self::flush`].
  ///
  /// **It lives here rather than in [`ProbeState`], and that move is
  /// the point.** It used to be a probe field, so it vanished the
  /// moment the probe collapsed — a committed decoder could not tell
  /// whether the caller had signalled the end, and therefore could not
  /// answer the one question [`SessionPhase`] exists to answer without
  /// guessing. The probe machinery still reads it for replay; it simply
  /// no longer owns it.
  eof_sent: bool,
  /// The GPU-side scaled-output stage — see [`crate::vtscale`]. Carries
  /// the caller's standing [`Self::request_scaled_output`] and, on the
  /// VideoToolbox road, the cached pixel-transfer session and fitted
  /// frames context the CPU download reads from.
  ///
  /// **It rides the decoder rather than the wrapper** because the seam
  /// it inserts itself into is here: between the decoded hardware frame
  /// and `av_hwframe_transfer_data`, the one point at which a picture is
  /// still a GPU surface and can still be resized without a
  /// full-resolution round-trip through main memory.
  ///
  /// Declared last, so it drops **after** the [`DecoderState`] holding
  /// the VideoToolbox device its fitted frames context was built over —
  /// and that order is safe by the FFmpeg contract rather than by luck:
  /// `av_hwframe_ctx_alloc` "will make a new reference for internal
  /// use", so the device outlives the decoder that opened it for exactly
  /// as long as this stage still holds a context over it.
  scaled_output: crate::vtscale::ScaledOutput,
}

/// Owned FFmpeg state for one open codec context. Has its own `Drop` so we
/// can swap it out cleanly during a probe advance via `mem::replace`.
struct DecoderState {
  /// Wrapped FFmpeg decoder. `ManuallyDrop` so we can sequence its drop
  /// before freeing the callback state.
  inner: ManuallyDrop<ffmpeg_next::decoder::Video>,
  /// Backend driving this state.
  backend: Backend,
  /// Owned reference produced by `av_hwdevice_ctx_create`.
  hw_device_ref: *mut AVBufferRef,
  /// Owned `Box<CallbackState>` raw pointer; `AVCodecContext::opaque`
  /// aliases it.
  callback_state: *mut CallbackState,
}

/// Maximum number of packets we are willing to buffer for probe replay
/// before abandoning the fallback safety net. Set high enough to absorb
/// long B-frame GOPs and codec setup latency, low enough to bound memory
/// against malicious / pathological streams that never produce a first
/// frame.
const MAX_PROBE_PACKETS: usize = 256;

/// Maximum total compressed-byte size of buffered probe packets. Each
/// buffered packet holds its own copy of the payload (see
/// [`try_clone_packet`]), so this bounds bytes the probe itself holds.
/// 64 MiB is generous for normal video and gives untrusted media a hard
/// ceiling.
const MAX_PROBE_PACKET_BYTES: usize = 64 * 1024 * 1024;

/// Hard cap on the number of side-data entries we tolerate per buffered
/// packet. `av_packet_ref` allocates an `AVPacketSideData` descriptor and
/// an `AVBufferRef` per entry, so a packet stuffed with many tiny or
/// zero-sized entries can consume significant memory in descriptor /
/// allocator overhead even after [`packet_side_data_bytes`] charges
/// [`SIDE_DATA_ENTRY_OVERHEAD`] bytes per entry. Refusing to clone such
/// packets short-circuits the descriptor explosion path.
///
/// Sized for legitimate streams (typical video packets carry 0-5 side-
/// data entries; SEI-heavy HEVC/AV1 maybe a dozen) while comfortably
/// rejecting weaponised input.
///
/// Shared with the [`crate::FfmpegVideoStreamDecoder`] rolling GOP buffer,
/// which charges the same side-data budget so its byte cap is a true upper
/// bound on retained memory rather than counting bare payloads.
pub(crate) const MAX_PROBE_PACKET_SIDE_DATA_ENTRIES: usize = 64;

/// Conservative per-side-data-entry overhead estimate used by both
/// [`packet_side_data_bytes`] and the budget accounting in
/// [`VideoDecoder::send_packet`]. Counts the `AVPacketSideData`
/// descriptor (24 bytes per the FFmpeg 9.x bindings), the `AVBufferRef`
/// FFmpeg allocates per entry, and a margin for malloc bookkeeping
/// (header bytes, alignment slack). Setting it on the high side keeps
/// the byte cap a true upper bound on retained memory; under-charging
/// would let many tiny entries slip past the cap.
const SIDE_DATA_ENTRY_OVERHEAD: usize = 80;

/// Conservative upper-bound bytes-per-pixel multiplier used to estimate
/// the size of a CPU frame **before** `av_hwframe_transfer_data`
/// allocates its pixel buffers. Covers every HW download format this
/// crate produces (worst case is `P416LE` / `P412LE` at 6 bytes/pixel
/// for 16-bit 4:4:4 semi-planar) plus a margin for FFmpeg's per-row
/// stride alignment (typically 32-byte aligned, ~5% extra at HD widths
/// and below).
///
/// Used by [`drain_into_pending`] as a pre-transfer guard: if the
/// product `width * height * WORST_CASE_BYTES_PER_PIXEL` would already
/// push `pending_bytes` past `max_probe_pending_bytes`, the candidate
/// replay refuses the frame *before* allocating. Without this, FFmpeg
/// would perform the full HW→CPU download (potentially ~100 MiB for
/// 8K HDR) and we would only reject the frame after RSS had already
/// spiked. The post-transfer accounting via [`cpu_frame_bytes`] stays in
/// place as a backstop using the frame's actual stride/format.
///
/// Slightly over-charges true 4:2:0 NV12 / P010 frames (which dominate
/// real workloads) — that's the right side to err on. Callers feeding
/// 8K+ workloads through the probe path can tune
/// [`VideoDecoder::with_max_probe_pending_bytes`] upward to compensate.
const WORST_CASE_BYTES_PER_PIXEL: usize = 8;

/// Maximum number of CPU frames we are willing to queue from a candidate
/// during probe replay. Each frame is a fully-allocated CPU buffer
/// (~3 MiB for 1080p NV12, ~24 MiB for 4K P010, ~96 MiB for 8K P010), so
/// an unbounded queue would OOM on a candidate with a shallow internal
/// queue against a deep replay history. This cap, together with
/// [`DEFAULT_MAX_PROBE_PENDING_BYTES`], is enforced as a hard limit during
/// replay: once either limit is reached, probe buffering fails for the
/// candidate (returns `ENOMEM` from `drain_into_pending`) instead of
/// queueing additional drained frames. The probe loop then advances to
/// the next backend or returns `Error::AllBackendsFailed` if exhausted.
const MAX_PROBE_PENDING_FRAMES: usize = 16;

/// Default byte budget for probe-replay drained frames. 256 MiB is enough
/// for 16 frames at 4K P010 (~24 MiB each = 384 MiB worst case under the
/// count cap), and is the cap that fires first for very high-resolution
/// content (8K P010: ~96 MiB per frame → only ~2 frames fit).
///
/// Override per-decoder with [`VideoDecoder::with_max_probe_pending_bytes`]
/// when targeting 8K+ workloads or memory-constrained environments.
///
/// TODO: when frames significantly exceed typical sizes, consider
/// memmap-backed pending buffers (write transferred frames to a temp file
/// or shared-memory segment) so the resident set stays bounded even when
/// the byte cap is raised. Out of scope for now.
pub const DEFAULT_MAX_PROBE_PENDING_BYTES: usize = 256 * 1024 * 1024;

/// Where a decoding session is in its life — **the one derived fact the
/// classifiers read, and the only place the latches are interpreted.**
///
/// Every road that must decide what an errno *means* needs the same two
/// questions answered, and answering them ad hoc at each road is what
/// let them disagree. `EAGAIN` means "send me more" only where more can
/// come; `AVERROR_EOF` means "the stream is over" only where a backend
/// has committed to producing it. Read those wrong and a caller is
/// handed a state with no satisfying operation, or a candidate that
/// will never produce a frame is mistaken for a finished stream.
///
/// The two questions are exactly the two dimensions the machinery
/// already keeps latches for — whether an end has been recorded, and
/// whether a backend is still on trial — so this enum is a census of
/// those latches rather than a new idea. Deriving it lives in
/// [`VideoDecoder::phase`] and its siblings, one per session type;
/// nothing else reads a latch to answer a classification question.
#[derive(Copy, Clone, Debug, PartialEq, Eq)]
pub(crate) enum SessionPhase {
  /// A committed backend, and no end-of-stream recorded. Both flow
  /// signals mean what they say.
  Streaming,
  /// A committed backend draining its tail after a recorded end. "Send
  /// me more" is no longer satisfiable here — the caller has nothing
  /// left to send and the send gates refuse — so it reads as the end.
  Draining,
  /// A candidate backend on trial, no end recorded. It may legitimately
  /// want more input; what it may not do is quietly end the stream on
  /// the caller's behalf, which is the committed backend's privilege.
  Auditioning,
  /// A candidate on trial that has already been handed the whole
  /// history, **end-of-stream included**.
  ///
  /// The arm that had no name. A candidate here has been given
  /// everything there is and answers about a stream that is already
  /// over: if it has produced no frame, it never will. That is a
  /// candidate failing — the probe's business — and neither "send me
  /// more" (nothing left to send) nor "the stream ended" (this backend
  /// never decoded a thing) is a true reading of it.
  AuditioningPastEnd,
}

impl SessionPhase {
  /// Whether more input can still reach this session.
  ///
  /// The satisfiability question: [`Received::NeedsInput`] and
  /// [`Sent::MustDrain`] are both instructions, and both are honest
  /// only where the caller can carry them out.
  pub(crate) const fn accepts_input(self) -> bool {
    matches!(self, Self::Streaming | Self::Auditioning)
  }

  /// Whether a backend has committed, and so may speak for the stream.
  ///
  /// Only a committed backend's `AVERROR_EOF` is the stream's end. A
  /// candidate's is its own: it drained to nothing without ever proving
  /// it could decode this content.
  pub(crate) const fn is_committed(self) -> bool {
    matches!(self, Self::Streaming | Self::Draining)
  }
}

/// How a funnel verdict routes. See [`VideoDecoder::verdict_routing`].
enum VerdictRouting {
  /// The name says this backend cannot take this stream: while it is on
  /// trial, the candidate fails. After commit the verdict is reported as
  /// it was minted, as every verdict is.
  CandidateFailed,
  /// The name says retrying a backend cannot help — report it as it is.
  Direct,
  /// Nothing was named; the road's own reading decides.
  Unnamed,
}

/// What an **unnamed** verdict means on the road that produced it — the
/// one thing the shared routing policy cannot know for itself.
enum BareVerdict {
  /// A flow signal's own fault: `avcodec_send_packet` answering
  /// `AVERROR_EOF` to a submission past the end. The probe must not
  /// advance on it — the candidate did nothing wrong, the caller did.
  Reported,
  /// A real failure. While a candidate is on trial, that is the
  /// candidate failing.
  CandidateFailure,
}

/// What the caller should do with a routed hardware failure.
enum HwRoute {
  /// Hand this to the caller.
  Report(Error),
  /// The active candidate failed: advance the probe and retry.
  Advance(Error),
}

/// How [`VideoDecoder::advance_probe_with`] builds a candidate: the
/// signature of [`VideoDecoder::build_state`], which every production
/// road passes.
type BuildCandidate = fn(
  codec::Parameters,
  Codec,
  Backend,
  crate::limits::DecoderLimits,
  Option<mediadecode::Timebase>,
) -> Result<DecoderState>;

/// State carried only during the probe window (before the first successful
/// frame). Holds enough information to tear down the current decoder and
/// retry with the next backend.
struct ProbeState {
  parameters: codec::Parameters,
  codec: Codec,
  /// Backends still to try, in order. Empty means "no more options after
  /// the active one fails" — `advance_probe` then surfaces
  /// [`Error::AllBackendsFailed`] so the contract is the same on
  /// single-backend platforms (e.g. macOS) as on multi-backend ones.
  remaining_backends: Vec<Backend>,
  /// Packets sent so far, kept for replay through any candidate backend.
  /// Preserved across failed candidates — only cleared when the probe
  /// collapses on a successful first frame, or when the probe is
  /// abandoned due to the size caps.
  buffered_packets: Vec<Packet>,
  /// Cumulative size (in compressed bytes) of `buffered_packets`. Tracked
  /// incrementally so we don't have to re-sum on every send.
  buffered_bytes: usize,
  /// Whether `send_eof` has been called; replayed alongside packets.
  /// Per-backend errors captured since the probe window opened. Pushed
  /// whenever a backend's failure triggers `advance_probe` (the active
  /// backend that just failed) or a candidate's build / replay rejects
  /// it. Drained into [`Error::AllBackendsFailed`] when the probe
  /// exhausts every option.
  attempts: Vec<(Backend, Box<Error>)>,
}

// SAFETY: All raw pointers are exclusively owned by `DecoderState` and never
// shared. `ffmpeg::decoder::Video` is itself `Send` (its `Context` carries an
// `unsafe impl Send`). The decoder is not safe for concurrent use, hence not
// `Sync`.
//
// `VideoDecoder`'s claim additionally covers what `crate::vtscale` holds on
// Apple targets: a `VTPixelTransferSessionRef` and an `AVBufferRef` to a
// fitted `AVHWFramesContext`. Both are exclusively owned by the stage — no
// `Clone`, no handing out — and every use of the session goes through
// `&mut ScaledOutput`, so the one-transfer-at-a-time protocol
// `VTPixelTransferSessionTransferImage` expects is serialized by the borrow
// rather than by a lock. VideoToolbox sessions are Core Foundation objects
// with no thread affinity (unlike the UI-framework types that have one), so
// moving one between threads is exactly as sound as moving the codec context
// beside it. Still not `Sync`, for the same reason nothing else here is.
unsafe impl Send for DecoderState {}
unsafe impl Send for VideoDecoder {}

impl Drop for DecoderState {
  fn drop(&mut self) {
    // Order matters:
    //  1. Drop the codec context first. While it lives, FFmpeg may invoke
    //     `get_format`, which dereferences `callback_state` via `opaque`.
    //  2. Free the callback state heap allocation.
    //  3. Release our hw device reference (FFmpeg released its own when
    //     the codec context was freed in step 1).
    unsafe {
      ManuallyDrop::drop(&mut self.inner);
      if !self.callback_state.is_null() {
        drop(Box::from_raw(self.callback_state));
        self.callback_state = ptr::null_mut();
      }
      if !self.hw_device_ref.is_null() {
        av_buffer_unref(&mut self.hw_device_ref);
      }
    }
  }
}

impl VideoDecoder {
  /// Auto-probe hardware backends in the platform's default order.
  ///
  /// Each backend opens with a strict `get_format` callback. The first
  /// backend whose `avcodec_open2` succeeds becomes active; if its first
  /// frame is unusable (decode error, transfer failure, or a CPU-format
  /// frame from a HW context) the decoder is torn down and the next backend
  /// is tried — packets sent so far are replayed through the new decoder
  /// transparently. The probe advance is transactional: the next backend
  /// must build *and* accept the replayed history before any probe state is
  /// consumed, so a misbehaving middle backend cannot strand the caller.
  ///
  /// [`Self::backend`] reflects whichever backend ultimately produced the
  /// first frame.
  ///
  /// [`Error::AllBackendsFailed`] surfaces in two places, with the same
  /// meaning ("no hardware backend can decode this stream — fall back to
  /// software yourself"):
  /// - From `open` itself, when no backend even opens.
  /// - From [`Self::send_packet`] / [`Self::send_eof`] /
  ///   [`Self::receive_frame`], when the initially-opened backend fails
  ///   at decode time and every remaining backend in the probe order
  ///   either also fails or doesn't exist. On single-backend platforms
  ///   (e.g. macOS, where the order is `[VideoToolbox]`), this is the
  ///   only place a HW-only failure surfaces.
  ///
  /// In both cases, `attempts` carries the per-backend error log. When
  /// the runtime path fires, `unconsumed_packets` also contains the
  /// packets the decoder consumed from the caller before the probe
  /// exhausted (the probe's own copies of them); for non-seekable inputs
  /// (live streams, pipes) the caller can replay these directly into
  /// a software decoder of their choice without re-demuxing. From the
  /// open-time path the vec is empty since no packets have been sent.
  ///
  /// On `Ok`, the returned decoder **always** has an active probe
  /// rescue safety net. If a parameters clone fails under memory
  /// pressure before the probe state can be set up, `open` returns
  /// `Err(Error::Ffmpeg(Other { errno: ENOMEM }))` rather than handing
  /// back a live decoder with no fallback contract. No packets have
  /// been sent yet, so the caller can retry or fall back to software
  /// with the original `parameters` directly.
  pub fn open(parameters: codec::Parameters) -> Result<Self> {
    Self::open_with_frame_limits(parameters, crate::limits::DecoderLimits::default())
  }

  /// [`Self::open`], with the stream's packet timebase declared.
  ///
  /// **Prefer this one whenever the caller knows the timebase**, which
  /// is nearly always: the packets fed to a decoder carry timestamps in
  /// their stream's units, and `AVCodecContext.pkt_timebase` is how
  /// libavcodec is told what those units are. Not owning the
  /// `AVStream` does not mean not knowing its ruler.
  ///
  /// The value reaches every `AVCodecContext` this decoder opens,
  /// including the ones a later hardware probe advance opens.
  pub fn open_timed(
    parameters: codec::Parameters,
    timebase: mediadecode::Timebase,
  ) -> Result<Self> {
    Self::open_with_frame_limits_timed(
      parameters,
      crate::limits::DecoderLimits::default(),
      timebase,
    )
  }

  /// [`Self::open`], with the frame ceilings named.
  ///
  /// Taken at open for the reason [`Self::open_with_limits`] gives:
  /// [`FrameLimits::max_pixels`] is written into every `AVCodecContext`
  /// this decoder opens — including the ones a later probe advance
  /// opens — and a context's ceiling cannot be moved after
  /// `avcodec_open2`.
  pub fn open_with_frame_limits(
    parameters: codec::Parameters,
    limits: crate::limits::DecoderLimits,
  ) -> Result<Self> {
    // **Explicitly untimed.** No packet timebase is declared, so
    // `AVCodecContext.pkt_timebase` is left at libavcodec's own
    // default and anything it derives from that field — a subtitle
    // cue's PTS, a packet-duration fallback — is unavailable. Reach
    // for [`Self::open_with_frame_limits_timed`] when the stream's
    // ruler is known, which is nearly always.
    Self::open_with_frame_limits_timed_in(parameters, limits, None)
  }

  /// [`Self::open_with_frame_limits`], with the stream's packet
  /// timebase declared — see [`Self::open_timed`].
  pub fn open_with_frame_limits_timed(
    parameters: codec::Parameters,
    limits: crate::limits::DecoderLimits,
    timebase: mediadecode::Timebase,
  ) -> Result<Self> {
    Self::open_with_frame_limits_timed_in(parameters, limits, Some(timebase))
  }

  fn open_with_frame_limits_timed_in(
    parameters: codec::Parameters,
    limits: crate::limits::DecoderLimits,
    pkt_timebase: Option<mediadecode::Timebase>,
  ) -> Result<Self> {
    Self::open_probing(parameters, limits, pkt_timebase, backend::probe_order())
  }

  /// The probe itself, across `order` — [`backend::probe_order`] for every
  /// public constructor.
  ///
  /// Taken as an argument so a lane can stand on a platform with no
  /// hardware backend at all, which no machine that runs the suite is: an
  /// empty `order` is exactly that platform's probe, and it answers
  /// [`Error::AllBackendsFailed`] with no attempt and no packet.
  pub(crate) fn open_probing(
    parameters: codec::Parameters,
    limits: crate::limits::DecoderLimits,
    pkt_timebase: Option<mediadecode::Timebase>,
    order: &[Backend],
  ) -> Result<Self> {
    let codec = find_decoder(&parameters)?;

    let mut attempts: Vec<(Backend, Box<Error>)> = Vec::new();
    for (i, &backend) in order.iter().enumerate() {
      // Use the checked clone — ffmpeg-next's `Parameters::clone` does
      // `avcodec_parameters_alloc` without a null check and ignores the
      // return of `avcodec_parameters_copy`. Under OOM that path silently
      // produces a Parameters with a null inner pointer.
      let cloned_for_build =
        match try_clone_parameters(&parameters, limits.max_codec_parameter_bytes()) {
          Ok(p) => p,
          Err(e) => {
            tracing::warn!(?backend, error = %e, "hwdecode: parameters clone failed");
            attempts.push((backend, Box::new(e)));
            continue;
          }
        };
      match Self::build_state(cloned_for_build, codec, backend, limits, pkt_timebase) {
        Ok(state) => {
          tracing::info!(?backend, "hwdecode: opened video decoder (probing)");
          let remaining = order[(i + 1)..].to_vec();
          // Deep-copy the caller's `parameters` before storing in ProbeState.
          // `codec::Parameters` from `stream.parameters()` carries an Rc
          // owner pointing at the demuxer; moving that Rc to a worker
          // thread (when VideoDecoder is sent) would race with the demuxer's
          // Rc on the original thread. The checked clone copies the bytes
          // into a fresh allocation with `owner: None`, severing the link.
          //
          // We always create ProbeState — even when `remaining` is empty
          // (single-backend platforms like macOS) — so that a first-frame
          // failure on the only backend surfaces as
          // `Error::AllBackendsFailed` from `receive_frame` /
          // `send_packet` rather than as a raw FFmpeg error. That keeps
          // the API contract the same regardless of how many HW backends
          // the platform exposes.
          //
          // If the clone fails (ENOMEM), fail the **whole open call**
          // rather than returning a live decoder with `probe: None`.
          // Returning Ok here would let the caller send packets that the
          // active backend consumes, and a subsequent backend failure
          // would then surface as a raw FFmpeg error with no
          // `unconsumed_packets` — silently breaking the rescue contract
          // for non-seekable inputs (live streams, pipes). Dropping the
          // already-built `state` here runs its FFmpeg cleanup, and the
          // caller can retry / fall back to software with the original
          // parameters in their hand (no packets were consumed yet).
          // Seed the probe's attempt log with any backends that failed
          // to open earlier in this loop (including
          // `BackendUnsupportedByCodec` and parameters-clone errors).
          // Without this, a runtime exhaustion on the active backend
          // would surface an `AllBackendsFailed` containing only the
          // active backend's runtime failure — losing the original
          // open-time causes that, on multi-backend platforms (Linux,
          // Windows), are usually the more diagnostic signal. E.g. a
          // VAAPI-then-CUDA host where VAAPI fails to open and CUDA
          // later fails at first-frame must report both failures in
          // probe order, not just CUDA.
          let probe = match try_clone_parameters(&parameters, limits.max_codec_parameter_bytes()) {
            Ok(probe_params) => ProbeState {
              parameters: probe_params,
              codec,
              remaining_backends: remaining,
              buffered_packets: Vec::new(),
              buffered_bytes: 0,
              attempts: std::mem::take(&mut attempts),
            },
            Err(e) => {
              tracing::warn!(
                error = %e,
                "hwdecode: parameters clone failed for probe state at open; \
                 failing closed instead of returning a decoder without rescue"
              );
              return Err(e);
            }
          };
          return Ok(Self {
            state,
            hw_frame: alloc_av_frame().map_err(Error::Ffmpeg)?,
            probe: Some(probe),
            pending_frames: VecDeque::new(),
            max_probe_pending_bytes: DEFAULT_MAX_PROBE_PENDING_BYTES,
            frame_limits: limits,
            pkt_timebase,
            eof_sent: false,
            scaled_output: crate::vtscale::ScaledOutput::new(),
          });
        }
        Err(e) => {
          tracing::warn!(?backend, error = %e, "hwdecode: backend open failed");
          attempts.push((backend, Box::new(e)));
        }
      }
    }
    // No packets have been consumed at open time.
    Err(Error::AllBackendsFailed(AllBackendsFailed::new(
      attempts,
      Vec::new(),
    )))
  }

  /// Open the decoder with a specific backend. No probe, no fallback.
  ///
  /// If `backend` cannot actually decode this stream, the failure surfaces
  /// from [`Self::receive_frame`] (the strict `get_format` callback returns
  /// `AV_PIX_FMT_NONE`, the decoder errors out). The caller is responsible
  /// for retrying with another hardware backend or falling back to a
  /// software decoder of their choice (e.g. `ffmpeg::decoder::Video`).
  pub fn open_with(parameters: codec::Parameters, backend: Backend) -> Result<Self> {
    Self::open_with_limits(parameters, backend, crate::limits::DecoderLimits::default())
  }

  /// [`Self::open_with`], with the stream's packet timebase declared —
  /// see [`Self::open_timed`].
  pub fn open_with_timed(
    parameters: codec::Parameters,
    backend: Backend,
    timebase: mediadecode::Timebase,
  ) -> Result<Self> {
    Self::open_with_limits_timed(
      parameters,
      backend,
      crate::limits::DecoderLimits::default(),
      timebase,
    )
  }

  /// [`Self::open_with`], with the frame ceilings named.
  ///
  /// The limits are taken **at open**, not through a `with_*` builder,
  /// because [`FrameLimits::max_pixels`] is written straight into the
  /// `AVCodecContext` this call opens — that is the layer that makes
  /// libavcodec refuse an oversized picture before allocating it, and a
  /// context's ceiling cannot be moved after `avcodec_open2`. A builder
  /// would have silently applied to only half the enforcement.
  pub fn open_with_limits(
    parameters: codec::Parameters,
    backend: Backend,
    limits: crate::limits::DecoderLimits,
  ) -> Result<Self> {
    // Explicitly untimed — see [`Self::open_with_frame_limits`].
    Self::open_with_limits_timed_in(parameters, backend, limits, None)
  }

  /// [`Self::open_with_limits`], with the stream's packet timebase
  /// declared — see [`Self::open_timed`].
  pub fn open_with_limits_timed(
    parameters: codec::Parameters,
    backend: Backend,
    limits: crate::limits::DecoderLimits,
    timebase: mediadecode::Timebase,
  ) -> Result<Self> {
    Self::open_with_limits_timed_in(parameters, backend, limits, Some(timebase))
  }

  fn open_with_limits_timed_in(
    parameters: codec::Parameters,
    backend: Backend,
    limits: crate::limits::DecoderLimits,
    pkt_timebase: Option<mediadecode::Timebase>,
  ) -> Result<Self> {
    let codec = find_decoder(&parameters)?;
    let state = Self::build_state(parameters, codec, backend, limits, pkt_timebase)?;
    Ok(Self {
      state,
      hw_frame: alloc_av_frame().map_err(Error::Ffmpeg)?,
      probe: None,
      pending_frames: VecDeque::new(),
      max_probe_pending_bytes: DEFAULT_MAX_PROBE_PENDING_BYTES,
      frame_limits: limits,
      pkt_timebase,
      eof_sent: false,
      scaled_output: crate::vtscale::ScaledOutput::new(),
    })
  }

  /// Builds a decoder around a **software** `ffmpeg::decoder::Video`,
  /// for tests that need [`VideoDecoder`]'s own send/receive arms driven
  /// against real libavcodec.
  ///
  /// **Why this exists.** Those arms classify libavcodec's flow control
  /// themselves, and the only other way to reach them is
  /// [`VideoDecoder::open`], which needs a working hardware backend and
  /// a sample file — so every existing lane through them is
  /// `#[ignore]`-gated and runs nowhere. A regression that never runs is
  /// a claim, not a check. This keeps the arms, the probe state and the
  /// funnels exactly as production builds them and swaps only the
  /// backend behind `state.inner`, which is the one thing a test cannot
  /// otherwise supply.
  ///
  /// `auditioning` opens the probe window with **no backends left to
  /// try**, which is what makes the candidate-failure road observable:
  /// `advance_probe` has nowhere to advance to, so it surfaces
  /// [`Error::AllBackendsFailed`] and a lane can tell "the probe road
  /// was taken" from "a status was answered". Passing `false` leaves the
  /// probe collapsed, for lanes that mean to exercise a committed
  /// backend.
  ///
  /// The `backend` label is cosmetic here — it is read only for the
  /// attempt log and [`Self::backend`], neither of which a software
  /// decoder reaches — and `hw_device_ref` is null, which
  /// [`DecoderState`]'s `Drop` already handles.
  #[cfg(test)]
  pub(crate) fn from_software_for_test(
    parameters: codec::Parameters,
    limits: crate::limits::DecoderLimits,
    auditioning: bool,
  ) -> Result<Self> {
    let codec = find_decoder(&parameters)?;
    let probe = auditioning.then(|| ProbeState {
      parameters: try_clone_parameters(&parameters, limits.max_codec_parameter_bytes())
        .expect("a clonable parameter set"),
      codec,
      remaining_backends: Vec::new(),
      buffered_packets: Vec::new(),
      buffered_bytes: 0,
      attempts: Vec::new(),
    });
    let backend = backend::probe_order()
      .first()
      .copied()
      .unwrap_or(Backend::VideoToolbox);
    let state = Self::software_state_for_test(parameters, codec, backend, limits, None)?;
    Ok(Self {
      state,
      hw_frame: alloc_av_frame().map_err(Error::Ffmpeg)?,
      probe,
      pending_frames: VecDeque::new(),
      max_probe_pending_bytes: DEFAULT_MAX_PROBE_PENDING_BYTES,
      frame_limits: limits,
      pkt_timebase: None,
      eof_sent: false,
      scaled_output: crate::vtscale::ScaledOutput::new(),
    })
  }

  /// A [`BuildCandidate`] that opens the decoder in software: the state
  /// [`Self::from_software_for_test`] stands on, labelled `backend`.
  ///
  /// Passed to [`Self::advance_probe_with`], it is a candidate the probe
  /// can build, replay into and commit on any machine.
  #[cfg(test)]
  fn software_state_for_test(
    parameters: codec::Parameters,
    codec: Codec,
    backend: Backend,
    limits: crate::limits::DecoderLimits,
    pkt_timebase: Option<mediadecode::Timebase>,
  ) -> Result<DecoderState> {
    let (ctx, callback_state) = build_codec_context(&parameters, limits, pkt_timebase)?;
    let opened = ctx.decoder().open_as(codec).map_err(Error::Ffmpeg)?;
    ensure_video_codec_type(&opened)?;
    Ok(DecoderState {
      inner: ManuallyDrop::new(ffmpeg_next::decoder::Video(opened)),
      backend,
      hw_device_ref: ptr::null_mut(),
      callback_state: Box::into_raw(callback_state),
    })
  }

  /// Override the byte budget for probe-replay queued frames. Defaults to
  /// [`DEFAULT_MAX_PROBE_PENDING_BYTES`]. Use a higher value when targeting
  /// 8K+ workloads where 16 frames at full size could exceed the default;
  /// use a lower value in memory-constrained services to bound peak
  /// allocation more tightly.
  ///
  /// Setting after the first frame has been delivered is harmless but has
  /// no observable effect — the probe has already collapsed and the cap
  /// only applies during replay drain.
  ///
  /// Returns `self` for builder-style chaining:
  /// ```ignore
  /// let decoder = VideoDecoder::open(params)?
  ///     .with_max_probe_pending_bytes(1024 * 1024 * 1024); // 1 GiB
  /// ```
  #[must_use]
  pub fn with_max_probe_pending_bytes(mut self, bytes: usize) -> Self {
    self.max_probe_pending_bytes = bytes;
    self
  }

  /// The backend currently producing frames. While the probe is still in
  /// progress (no frame received yet) this returns the optimistically
  /// selected backend; after the first frame, it is the backend that
  /// actually produced it. Once stable, never changes again.
  pub fn backend(&self) -> Backend {
    self.state.backend
  }

  /// Whether this decoder can emit pictures at a caller-requested size
  /// instead of full coded size.
  ///
  /// [`ScaledOutputCapability::Supported`] on exactly one road: the
  /// VideoToolbox backend on an Apple target, where
  /// [`crate::vtscale`]'s `VTPixelTransferSession` sits between the
  /// decoded hardware frame and the CPU download. Every other backend
  /// this crate wires — [`Backend::Vaapi`], [`Backend::Cuda`],
  /// [`Backend::D3d11va`] — answers `Unsupported`, each with its own
  /// filed native scaling seam.
  ///
  /// # And it stops saying `Supported` the moment the promise breaks
  ///
  /// The trait's contract is that a `Supported` answer lets a caller
  /// skip its own resampler, so a session that quietly went back to
  /// full-size pictures under that answer would hand the caller mixed
  /// extents with nothing to notice them by. The stage can stand down
  /// per frame — a padded pixel buffer, a crop rectangle, side data a
  /// resize would strand, a resolution change that turns the standing
  /// request into an upscale, a transfer that fails — and the **first**
  /// frame that goes out at anything other than the requested extent
  /// flips this answer to [`ScaledOutputCapability::Unsupported`]. That
  /// is the explicit transition a caller is told to look for: query
  /// again and learn that resampling is theirs once more. A fresh
  /// [`Self::request_scaled_output`] buys a fresh promise.
  ///
  /// A request the stream already satisfies is not a broken promise:
  /// the picture arrives at exactly the size asked for.
  ///
  /// A pure query: it neither requests anything nor changes what
  /// [`Self::receive_frame`] delivers.
  pub fn scaled_output_capability(&self) -> ScaledOutputCapability {
    if crate::vtscale::ScaledOutput::supported()
      && self.state.backend.is_video_toolbox()
      && self.scaled_output.promise_stands()
    {
      ScaledOutputCapability::Supported
    } else {
      ScaledOutputCapability::Unsupported
    }
  }

  /// Asks this decoder to emit pictures at `size` from the next frame
  /// on, and reports whether the request was recorded.
  ///
  /// # What "from the next frame on" means, exactly
  ///
  /// The stage is consulted per frame, on the way out of the decoder
  /// and before the GPU→CPU download. So a request placed mid-stream
  /// takes effect on the next picture `receive_frame` produces — never
  /// retroactively on one already decoded and queued, and never later
  /// than that.
  ///
  /// # Refusal is not an error
  ///
  /// [`ScaledOutputCapability::Unsupported`] comes back when this road
  /// has no stage at all (see [`Self::scaled_output_capability`]), when
  /// either requested extent is zero, or when the request is an
  /// **upscale** of the stream's coded size — the stage exists to move
  /// fewer bytes across the GPU→CPU bus, and enlarging a picture moves
  /// more. Nothing about [`Self::send_packet`] or
  /// [`Self::receive_frame`] can fail because of it.
  ///
  /// **A refusal returns the session to full coded size**, dropping any
  /// request already standing. That is what the trait says this answer
  /// means, and the only reading a caller can act on: told
  /// `Unsupported` it resamples for itself, and a session that went on
  /// quietly fitting to an older request would have it resample an
  /// already-fitted picture. Like an acceptance, it takes effect from
  /// the next picture; one already decoded keeps the extent it was
  /// decoded at.
  ///
  /// See [`ScaledOutputCapability`] for the determinism trade a caller
  /// takes on by acting on a `Supported` answer.
  pub fn request_scaled_output(&mut self, size: (u32, u32)) -> ScaledOutputCapability {
    // **The road, not the standing promise.** A session whose promise
    // broke on an earlier request answers `Unsupported` from
    // [`Self::scaled_output_capability`] until somebody asks again —
    // and asking again is exactly this, so gating it on that answer
    // would make the break permanent and unrecoverable. What is checked
    // here is what cannot change: whether this build and this backend
    // have a stage at all.
    if !crate::vtscale::ScaledOutput::supported() || !self.state.backend.is_video_toolbox() {
      return ScaledOutputCapability::Unsupported;
    }
    let coded = (self.width(), self.height());
    self.scaled_output.request(size, coded)
  }

  /// Withdraws any standing scaled-output request, returning this
  /// decoder to full coded size from the next frame.
  ///
  /// The refusal road that does not go through
  /// [`Self::request_scaled_output`]: the wrapper refuses a request
  /// placed while a decoded picture is parked, and a refusal has to
  /// mean the same thing there as everywhere else — the session is
  /// returning to full coded size. Without this the old request would
  /// stay armed behind an `Unsupported` answer, and a caller acting on
  /// that answer would resample pictures that were already fitted.
  pub fn cancel_scaled_output(&mut self) {
    self.scaled_output.cancel();
  }

  /// Decoder width in pixels.
  pub fn width(&self) -> u32 {
    self.state.inner.width()
  }

  /// Decoder height in pixels.
  pub fn height(&self) -> u32 {
    self.state.inner.height()
  }

  /// Codec context time base.
  pub fn time_base(&self) -> Rational {
    self.state.inner.time_base()
  }

  /// Frame rate from the codec context, if known.
  pub fn frame_rate(&self) -> Option<Rational> {
    self.state.inner.frame_rate()
  }

  /// Mints a verdict for a road that holds none yet, then routes it.
  ///
  /// The funnel runs **exactly once** here, which is the law the doors
  /// carry: see the invariant on [`software_receive`]. Roads that have
  /// already minted (the receive arm) call [`Self::hw_route`] straight,
  /// and the route never funnels again — a funnel **consumes** the latch
  /// it reads, so a second one would find nothing and report
  /// libavcodec's errno over a refusal already collected.
  fn hw_failure(&self, e: ffmpeg_next::Error, bare: BareVerdict) -> HwRoute {
    self.hw_route(self.hw_exit(Error::Ffmpeg(e)), bare)
  }

  /// How a funnel verdict routes, before the road's own reading of an
  /// unnamed one applies.
  ///
  /// **Exhaustive on purpose, and with no wildcard.** A `_ => false`
  /// stood here and was a hazard rather than a convenience: a future
  /// named verdict that *did* say a backend cannot take the stream would
  /// inherit the silence and be reported plain, which is a bug that
  /// compiles. The match is total over this crate's own error
  /// vocabulary, so adding an arm forces whoever adds it to say how it
  /// routes.
  fn verdict_routing(reason: &Error) -> VerdictRouting {
    match reason {
      // The hardware pool declined the coded surface. Software is not
      // subject to that ceiling, and neither is the next backend.
      Error::HwSurfaceTooLarge(_) => VerdictRouting::CandidateFailed,
      // Software would decode the same oversized frame and be refused
      // by the same ceiling; so would the next backend. A fallback here
      // invites an action that cannot succeed.
      Error::FrameBudgetExceeded(_) => VerdictRouting::Direct,
      // The funnel handed its fallback straight back: nothing was named,
      // so the errno is all there is and the road decides.
      Error::Ffmpeg(_) => VerdictRouting::Unnamed,
      // None of these can leave a funnel — `hw_exit` mints only the two
      // refusals above or returns its argument — and each is already a
      // decided fact that did not ask for a backend to be retried. They
      // are listed rather than swept up so a new arm cannot join them
      // silently.
      Error::PacketBuild(_)
      | Error::ParametersTooLarge(_)
      // A malformed channel layout is a fact about the *stream*, not
      // about a backend: every backend would be handed the same
      // parameters and refuse them the same way, and the refusal
      // happens before any backend is chosen at all.
      | Error::MalformedChannelLayout(_)
      | Error::ChannelMapMissing(_)
      | Error::NoCodec(_)
      | Error::HwTransferTooLarge(_)
      | Error::BackendUnsupportedByCodec(_)
      | Error::HwDeviceInitFailed(_)
      | Error::AllBackendsFailed(_)
      | Error::FallbackFailed(_) => VerdictRouting::Direct,
    }
  }

  /// **One policy for what a funnelled hardware failure means, shared by
  /// every road that can produce one.**
  ///
  /// The send roads used to return their funnel's result the moment they
  /// had it. That was right for a flow signal and wrong for anything
  /// else: `hw_send` can mint [`Error::HwSurfaceTooLarge`], and returning
  /// it plain meant a probe still auditioning never advanced past the
  /// candidate that had just declined the surface.
  ///
  /// So minting and routing are one move now, and the receive road's
  /// policy is the policy. What differs between roads is only what an
  /// *unnamed* verdict means, which is why [`BareVerdict`] is a
  /// parameter rather than an assumption.
  ///
  /// Only a running probe acts on what a verdict says. While a candidate
  /// is on trial, a verdict that fails it advances the probe. Once a
  /// backend has committed, every verdict is that picture's own error,
  /// reported as it was minted: nothing is classified, nothing is
  /// remembered, and the next call reaches libavcodec. FFmpeg gives no
  /// reliable signal that a hardware session is gone — see the type's
  /// documentation.
  fn hw_route(&self, reason: Error, bare: BareVerdict) -> HwRoute {
    let candidate_failed = match Self::verdict_routing(&reason) {
      VerdictRouting::CandidateFailed => true,
      VerdictRouting::Direct => false,
      VerdictRouting::Unnamed => matches!(bare, BareVerdict::CandidateFailure),
    };
    if candidate_failed && self.probe.is_some() {
      return HwRoute::Advance(reason);
    }
    HwRoute::Report(reason)
  }

  /// **Where this session is, derived here and nowhere else.**
  ///
  /// The two latches this reads — whether a backend is still on trial,
  /// and whether an end has been recorded — are the only inputs any
  /// classification question has ever needed. Reading them at the point
  /// of a decision is what let the roads disagree; reading them once,
  /// here, is what stops it.
  pub(crate) const fn phase(&self) -> SessionPhase {
    match (self.probe.is_some(), self.eof_sent) {
      (false, false) => SessionPhase::Streaming,
      (false, true) => SessionPhase::Draining,
      (true, false) => SessionPhase::Auditioning,
      (true, true) => SessionPhase::AuditioningPastEnd,
    }
  }

  /// Submit a packet to the decoder.
  ///
  /// On success — and only on success — the packet is buffered for potential
  /// replay through a fallback backend while the probe is active. `EAGAIN`
  /// (the decoder needs `receive_frame` to drain output first) is
  /// [`Sent::MustDrain`]: nothing was consumed, so the caller drains and
  /// offers the same packet again. `AVERROR_EOF` is **not** back pressure
  /// on this face — it means this decoder was already told the stream
  /// ended — so it stays a fault. See [`send_status`].
  ///
  /// While the probe is active, a non-transient error (e.g. the active HW
  /// backend rejecting this stream's geometry on first packet) advances the
  /// probe to the next candidate and retries the packet there. The caller
  /// observes only the eventual success or, if the probe is exhausted, the
  /// final error.
  ///
  /// **Atomic probe rescue.** While the probe is active, the rescue
  /// invariant is that everything FFmpeg has consumed since open is
  /// reflected in `buffered_packets` (so a future
  /// [`Error::AllBackendsFailed`] can hand a complete replay history
  /// back to the caller for software fallback on a non-seekable input).
  /// If we cannot prove this packet is buffer-able — its side-data
  /// entry count exceeds [`MAX_PROBE_PACKET_SIDE_DATA_ENTRIES`], its
  /// bytes would push the probe past [`MAX_PROBE_PACKETS`] or
  /// [`MAX_PROBE_PACKET_BYTES`], or the probe's copy of it cannot be
  /// made ([`av_packet_ref`] or `av_packet_make_writable` failing) —
  /// `send_packet` returns [`Error::AllBackendsFailed`] **without
  /// invoking** `state.inner.send_packet` on this packet. The caller's
  /// packet stays in their hand and `unconsumed_packets` carries the
  /// pre-existing buffered history, so they can replay
  /// `unconsumed_packets` plus the current packet through their
  /// software decoder of choice. The post-probe path (after the first
  /// frame, when `self.probe` is `None`) skips this pre-flight
  /// entirely.
  ///
  /// After commit a failure is this packet's own error, reported as it
  /// was minted, and the next packet reaches libavcodec — nothing is
  /// remembered from one call to the next. See the type's documentation.
  pub fn send_packet(&mut self, packet: &Packet) -> Result<Sent> {
    loop {
      // Re-read each iteration: a probe advance moves this session from
      // one phase to another underneath the loop.
      let phase = self.phase();
      // Pre-flight while probe is active: prove we can record this
      // packet for replay BEFORE the active decoder consumes it.
      // `staged_clone` carries the probe's copy and the new
      // `buffered_bytes` value through the send below; we only commit
      // them to the probe state if FFmpeg accepts the packet.
      let staged_clone: Option<(Packet, usize)> = if let Some(probe) = self.probe.as_ref() {
        // Step 1: side-data entry count cap. Read just `side_data_elems`
        // (no array walk yet) so a corrupt or weaponised value cannot
        // drive an unbounded loop from the safe entry point.
        let side_count = packet_side_data_count(packet);
        if side_count > MAX_PROBE_PACKET_SIDE_DATA_ENTRIES {
          let probe = self.probe.take().expect("probe present");
          tracing::warn!(
            side_data_entries = side_count,
            max_side_data_entries = MAX_PROBE_PACKET_SIDE_DATA_ENTRIES,
            trigger = "side_data_entry_cap",
            "hwdecode: probe rescue exhausted before consuming packet; \
             returning AllBackendsFailed without invoking decoder"
          );
          return Err(Error::AllBackendsFailed(AllBackendsFailed::new(
            probe.attempts,
            probe.buffered_packets,
          )));
        }
        // Step 2: byte / packet count cap. `packet_side_data_bytes`
        // clamps its walk to MAX_PROBE_PACKET_SIDE_DATA_ENTRIES as
        // defense-in-depth even though the count check above already
        // bounded the array length.
        let pkt_size = packet.size().saturating_add(packet_side_data_bytes(
          packet,
          MAX_PROBE_PACKET_SIDE_DATA_ENTRIES,
        ));
        let new_count = probe.buffered_packets.len() + 1;
        let new_bytes = probe.buffered_bytes.saturating_add(pkt_size);
        if new_count > MAX_PROBE_PACKETS || new_bytes > MAX_PROBE_PACKET_BYTES {
          let probe = self.probe.take().expect("probe present");
          tracing::warn!(
            packets = new_count,
            bytes = new_bytes,
            side_data_entries = side_count,
            max_packets = MAX_PROBE_PACKETS,
            max_bytes = MAX_PROBE_PACKET_BYTES,
            trigger = "byte_or_packet_cap",
            "hwdecode: probe rescue exhausted before consuming packet; \
             returning AllBackendsFailed without invoking decoder"
          );
          return Err(Error::AllBackendsFailed(AllBackendsFailed::new(
            probe.attempts,
            probe.buffered_packets,
          )));
        }
        // Step 3: copy before consuming. The history goes to the caller
        // as owned, mutable `Packet`s, so it keeps a payload of its own:
        // a reference to this one would carry the caller's later writes
        // into the history, and `data_mut` on the two would alias. The
        // copy is the size step 2 charged — payload and side data — so
        // `new_bytes` stands. If it cannot be made we bail rather than
        // consuming a packet we can't track; the packet stays the
        // caller's, unsent.
        match try_clone_packet(packet) {
          Ok(c) => Some((c, new_bytes)),
          Err(e) => {
            let probe = self.probe.take().expect("probe present");
            tracing::warn!(
              error = %e,
              "hwdecode: packet copy failed before consuming; \
               returning AllBackendsFailed without invoking decoder"
            );
            return Err(Error::AllBackendsFailed(AllBackendsFailed::new(
              probe.attempts,
              probe.buffered_packets,
            )));
          }
        }
      } else {
        None
      };

      match self.state.inner.send_packet(packet) {
        Ok(()) => {
          if let Some((cloned, new_bytes)) = staged_clone {
            // Probe is still Some here: the only paths that take it are
            // the bailouts above (which return) and `advance_probe`'s
            // exhaustion (which would have propagated via `?`). Commit
            // the clone now that FFmpeg has accepted the packet.
            if let Some(probe) = self.probe.as_mut() {
              probe.buffered_packets.push(cloned);
              probe.buffered_bytes = new_bytes;
            }
          }
          return Ok(Sent::Accepted);
        }
        // **libavcodec's send-side flow control, guarded here and read
        // through the funnel — the same door the software road uses.**
        //
        // The guard and the classification are two different questions
        // and they get two different answers. `is_transient` decides
        // *whether the probe may advance*: neither `EAGAIN` nor
        // `AVERROR_EOF` is a candidate failing, so both are taken here,
        // ahead of the failure road, exactly as this one guard always
        // took them. `send_status` then decides *which* of the two this
        // is — back pressure or the double-EOF fault — and that
        // decision is not written here at all, so this arm cannot drift
        // away from the road the software decoders take. The staged
        // clone drops; the caller drains and re-offers, and we re-clone
        // at the top of the loop.
        //
        // It reads the errno through [`Self::hw_send`] rather than
        // classifying it raw: a refusal this crate latched during the
        // submission — `get_format` declining a coded surface as the
        // decoder configures on its first packet — must be what the
        // caller is told, not the `EAGAIN` libavcodec reported over the
        // top of it.
        // **Mint, then route — not mint and return.** A flow signal
        // leaves immediately; anything else is a verdict, and a verdict
        // that names a declined surface has to reach a running probe,
        // which advances on it. `BareVerdict::Reported`
        // is the road's own reading of an *unnamed* verdict here: the
        // double-EOF is the caller's fault, not the candidate's, so the
        // probe must not advance on it.
        Err(e) if is_transient(&e) => match self.hw_send(e, phase) {
          Ok(status) => return Ok(status),
          Err(reason) => match self.hw_route(reason, BareVerdict::Reported) {
            HwRoute::Report(err) => return Err(err),
            HwRoute::Advance(err) => {
              self.advance_probe(err)?;
              continue;
            }
          },
        },
        // A real failure. Minted once and routed by the shared policy:
        // while a candidate is on trial this is that candidate failing,
        // so `advance_probe` consumes the reason into `attempts` and
        // either installs the next candidate or surfaces
        // `AllBackendsFailed`. Any staged clone drops without entering
        // history; the next iteration clones afresh.
        Err(e) => match self.hw_failure(e, BareVerdict::CandidateFailure) {
          HwRoute::Report(err) => return Err(err),
          HwRoute::Advance(err) => {
            self.advance_probe(err)?;
            continue;
          }
        },
      }
    }
  }

  /// Signal end-of-stream to the decoder.
  ///
  /// Recorded for replay only if the underlying `send_eof` succeeds. While
  /// the probe is active, non-transient errors trigger probe advance and
  /// retry, matching `send_packet`'s behaviour.
  ///
  /// Answers [`Sent::MustDrain`] on `EAGAIN` — the end-of-stream was
  /// **not** recorded, so drain and signal again. A second EOF is a
  /// caller fault and stays one; see [`send_status`].
  ///
  /// After commit a failure here is reported as it was minted, as on
  /// [`Self::send_packet`], and nothing is remembered.
  pub fn send_eof(&mut self) -> Result<Sent> {
    loop {
      // Re-read each iteration: a probe advance moves this session from
      // one phase to another underneath the loop.
      let phase = self.phase();
      match self.state.inner.send_eof() {
        Ok(()) => {
          self.eof_sent = true;
          return Ok(Sent::Accepted);
        }
        // The same guard, the same door and the same routing as
        // `send_packet`; see the note there.
        Err(e) if is_transient(&e) => match self.hw_send(e, phase) {
          Ok(status) => return Ok(status),
          Err(reason) => match self.hw_route(reason, BareVerdict::Reported) {
            HwRoute::Report(err) => return Err(err),
            HwRoute::Advance(err) => {
              self.advance_probe(err)?;
              continue;
            }
          },
        },
        // The same shared policy; see `send_packet`.
        Err(e) => match self.hw_failure(e, BareVerdict::CandidateFailure) {
          HwRoute::Report(err) => return Err(err),
          HwRoute::Advance(err) => {
            self.advance_probe(err)?;
            continue;
          }
        },
      }
    }
  }

  /// Receive a CPU-side decoded frame.
  ///
  /// The frame is downloaded with `av_hwframe_transfer_data` and metadata
  /// is copied via `av_frame_copy_props`. The caller's frame is always
  /// unref'd first, so reuse across resolution changes or different
  /// decoders is safe.
  ///
  /// While the probe window is open, *any* non-transient failure (decode
  /// error, transfer error, copy_props error, or a CPU-format frame from a
  /// HW-opened context) tears down the current decoder and advances to the
  /// next hardware backend in probe order, replaying buffered packets
  /// through it. Frames the candidate produced during replay (drained when
  /// `send_packet` returned EAGAIN) are queued and delivered FIFO via this
  /// method, so the caller never loses initial frames after a fallback.
  ///
  /// This crate is hardware-only: there is no software fallback inside the
  /// decoder. When every backend in the probe order has been exhausted —
  /// including the case of a single-backend platform whose only backend
  /// failed — this returns [`Error::AllBackendsFailed`] with the per-
  /// backend attempt log so the caller can branch into a software
  /// decoder of their choice.
  ///
  /// After commit a failure is that picture's own error, reported as it
  /// was minted, and nothing is remembered: the next call reaches
  /// libavcodec. See the type's documentation.
  ///
  /// Answers the same three states `ffmpeg::decoder::Video` does, in
  /// the shape the trait tier publishes: [`Received::NeedsInput`] where
  /// libavcodec says `EAGAIN`, [`Received::Ended`] where it says `EOF`,
  /// and [`Received::Frame`] when `frame` was written. **The errno
  /// stops here** — the two flow signals never leave this crate as
  /// `Error::Ffmpeg`, so a caller has nothing to decode.
  pub fn receive_frame(&mut self, frame: &mut Frame) -> Result<Received> {
    // Pre-drain frames queued during probe replay. They are already CPU-side
    // (transferred at drain time, when the candidate's HW context was alive)
    // so we just move them into the caller's slot.
    if self.try_pop_pending(frame) {
      return Ok(Received::Frame);
    }

    loop {
      // Re-read each iteration: a probe advance moves this session from
      // one phase to another underneath the loop.
      let phase = self.phase();
      let res = self.state.inner.receive_frame(&mut self.hw_frame);
      match res {
        Err(e) => {
          // **The phase decides whether this errno is a protocol state
          // at all, and this arm holds no opinion of its own.**
          //
          // `EAGAIN` used to short-circuit here unconditionally, which
          // was right for three of the four phases and quietly wrong for
          // the fourth: a candidate that has been replayed the whole
          // history *including the end* and still answers "nothing yet"
          // has produced zero frames and never will. Answering the
          // caller `NeedsInput` there asked for input nothing could
          // supply; answering `Ended` would have credited a backend that
          // never decoded a thing. It is a candidate failing, and the
          // classifier says so by handing it back — straight into the
          // probe road below, which is where candidate failures have
          // always gone.
          //
          // **And it reads the errno through the funnel, which is the
          // law this road lost and got back.** A `get_format`
          // declination or an allocator-judge refusal sits in the
          // callback state waiting to be collected; classifying the raw
          // errno first answers `Ended` or `NeedsInput` for a frame this
          // crate itself declined, and the reason dies unread. The
          // funnel is no longer a step to remember — [`Self::hw_receive`]
          // is the only way in, and the classifiers are private to this
          // module so no road can take a shortcut past it.
          let reason = match self.hw_receive(e, phase) {
            Ok(status) => return Ok(status),
            // The funnel's verdict: the latched refusal when there was
            // one, the original error when there was not. It travels
            // onward as it is — rebuilding `Error::Ffmpeg(e)` here would
            // throw away the collection that just happened.
            Err(reason) => reason,
          };
          // **The same shared policy every hardware road uses.** This
          // road mints its own verdict (above), so it routes rather than
          // minting again. `CandidateFailure` is its reading of an
          // unnamed verdict: a candidate that drains to `EOF` without
          // ever producing a frame is a candidate failing, not a stream
          // ending — which is why this road hands `AVERROR_EOF` to the
          // probe while the send roads report it.
          match self.hw_route(reason, BareVerdict::CandidateFailure) {
            HwRoute::Report(err) => return Err(err),
            HwRoute::Advance(err) => {
              self.advance_probe(err)?;
              // Probe advance may have populated `pending_frames`;
              // deliver one of those before reading more from the new
              // candidate.
              if self.try_pop_pending(frame) {
                return Ok(Received::Frame);
              }
              continue;
            }
          }
        }
        Ok(()) => {
          // Always attempt the HW→CPU transfer. With strict `get_format`,
          // libavcodec can only deliver frames in the wired-up HW format
          // (or fail). If a misbehaving codec ever hands us a CPU-side
          // frame anyway, `av_hwframe_transfer_data` returns AVERROR(EINVAL)
          // (neither src nor dst has an AVHWFramesContext attached) and we
          // route through the same error path below.
          // **The transfer is priced before it is paid, and a refusal
          // here is final.** See [`judge_hw_transfer`]: neither ceiling
          // hook reaches this allocation — `hwaccel->alloc_frame`
          // bypasses `get_buffer2` entirely, and the CPU destination is
          // allocated by `av_hwframe_transfer_data` outside both — so
          // this is the seat that bounds what the hardware road hands
          // back.
          //
          // Judged out here rather than inside `transfer_hw_frame`
          // deliberately. Errors from that function are FFmpeg's, and
          // the arms below route them as hardware failures — a candidate
          // failing while the probe runs, and after commit the picture's
          // own error. A byte ceiling is not a hardware failure: software
          // would decode the same oversized frame and be refused again,
          // so routing it as one is exactly the wrong answer. The named
          // refusal returns straight to the caller.
          if let Err(e) =
            unsafe { judge_hw_transfer(self.hw_frame.as_ptr(), self.frame_limits.frame()) }
          {
            return Err(Error::HwTransferTooLarge(e));
          }
          // **The scaled-output stage, and the one place it sits.** A
          // standing request turns this into a GPU resize followed by a
          // download of the *fitted* surface; with no request, or on any
          // condition the stage cannot honor, `stage` answers `None` and
          // the full-size hardware frame is downloaded exactly as
          // before. It cannot fail — see [`crate::vtscale`].
          //
          // The byte ceiling above is deliberately **not** re-priced
          // against the fitted size. `judge_hw_transfer` bounds what
          // this road may allocate, and pricing the full-size frame is
          // the conservative reading of that bound: a stream refused
          // without scaled output is refused identically with it, so
          // turning the stage on can never widen what the ceiling lets
          // through. What it saves is the bus traffic and the CPU
          // allocation actually paid, which is the trade #55 is about.
          //
          // **And a fitted surface that will not download is the
          // stage's failure, not the backend's.** The scale can succeed
          // and the crossing still fail — an unsupported destination
          // pixel format, a metadata copy that runs out of memory — and
          // routing that into the arms below would let an optional
          // bandwidth optimisation reject a VideoToolbox decode that
          // was working, or cost the caller a picture the hardware
          // decoded. So the fitted attempt is made first and
          // separately: if it fails, the stage latches the key off, the
          // destination is reset, and the original full-size frame —
          // still live in `hw_frame`, still the path this crate took
          // before any of this existed — is downloaded instead. Only
          // *that* failing is a hardware failure.
          let scaled = self
            .scaled_output
            .stage(&self.hw_frame)
            .map(|fitted| unsafe { transfer_hw_frame(frame, fitted) });
          match scaled {
            Some(Ok(())) => {
              self.probe = None;
              return Ok(Received::Frame);
            }
            Some(Err(e)) => {
              tracing::warn!(
                error = %e,
                "hwdecode: the fitted surface would not download; retiring scaled output for \
                 this size and delivering the full-size frame instead"
              );
              self.scaled_output.latch_failure();
              // SAFETY: `frame` is the caller's slot; unreferencing it
              // discards whatever the failed transfer left behind
              // before the retry writes it again.
              unsafe { av_frame_unref(frame.as_inner_mut().as_mut_ptr()) };
            }
            None => {}
          }
          match unsafe { transfer_hw_frame(frame, &self.hw_frame) } {
            Ok(()) => {
              self.probe = None;
              return Ok(Received::Frame);
            }
            Err(e) => {
              // The same shared policy. A transfer failure is an
              // HW-output problem — an unsupported CPU pix_fmt surfaces
              // as `AVERROR(EINVAL)`, a context loss as Bug/Bug2/Unknown
              // — never input corruption, so while a candidate is on
              // trial it is that candidate failing.
              match self.hw_failure(e, BareVerdict::CandidateFailure) {
                HwRoute::Report(err) => return Err(err),
                HwRoute::Advance(err) => {
                  self.advance_probe(err)?;
                  unsafe { av_frame_unref(frame.as_inner_mut().as_mut_ptr()) };
                  if self.try_pop_pending(frame) {
                    return Ok(Received::Frame);
                  }
                  continue;
                }
              }
            }
          }
        }
      }
    }
  }

  /// Pop one queued frame (produced by a candidate decoder during probe
  /// replay) into the caller's slot. Returns `true` when a frame was
  /// delivered, `false` when the queue was empty.
  fn try_pop_pending(&mut self, frame: &mut Frame) -> bool {
    let Some(mut buffered) = self.pending_frames.pop_front() else {
      return false;
    };
    // SAFETY: `buffered` is a CPU-side AVFrame we previously transferred
    // and pushed into the queue; both pointers are valid.
    unsafe {
      av_frame_unref(frame.as_inner_mut().as_mut_ptr());
      av_frame_move_ref(frame.as_inner_mut().as_mut_ptr(), buffered.as_mut_ptr());
    }
    // Probe semantics: delivering a frame collapses the probe.
    self.probe = None;
    true
  }

  /// Flush internal buffers (e.g. after a seek).
  ///
  /// Discards every frame buffered by the decoder, every frame queued during
  /// probe replay (`pending_frames`), and the residual `hw_frame` scratch
  /// buffer. Probe-time replay state (buffered packets, EOF marker) is also
  /// cleared since post-seek packets do not align with the previously
  /// captured history. After a flush, the next `receive_frame` waits for new
  /// post-seek input.
  ///
  /// It rebuilds no hardware session. `avcodec_flush_buffers` reaches only
  /// the codec's own flush (`libavcodec/avcodec.c` 417–418 in FFmpeg
  /// 9.0.1), which drops the pictures and references the codec holds. See
  /// the type's documentation for a session FFmpeg leaves without one.
  pub fn flush(&mut self) {
    self.state.inner.flush();
    // SAFETY: hw_frame is a valid AVFrame we own; av_frame_unref is a no-op
    // for an already-empty frame.
    unsafe { av_frame_unref(self.hw_frame.as_mut_ptr()) };
    self.pending_frames.clear();
    // The end belongs to the position being abandoned.
    self.eof_sent = false;
    if let Some(probe) = self.probe.as_mut() {
      probe.buffered_packets.clear();
      probe.buffered_bytes = 0;
    }
  }

  /// Takes the coded-surface refusal the `get_format` callback left
  /// behind, if it left one, clearing it for the next candidate.
  fn take_ceiling_declination(&self) -> Option<Error> {
    ceiling_declination_of(self.state.callback_state)
  }

  /// **The single hardware-exit funnel.** Every road that turns a
  /// hardware failure — or an end-of-stream that is really a refusal —
  /// into an `Error` goes through here, and it reads the callback's
  /// declination *before* anything wraps or tears down state.
  ///
  /// The reason there is a funnel at all: a `get_format` callback
  /// cannot return a reason, so it leaves one behind, and every exit
  /// that forgets to collect it hands the caller libavcodec's
  /// `Invalid data found when processing input` for a refusal this
  /// crate made — or, on the explicit-backend road, a stream that
  /// simply drains to EOF with nothing said at all.
  ///
  /// The lesson this encodes: R14 claimed four consumers of the
  /// declination and production had exactly one. Consumers added
  /// helper-by-helper are lost the next time the surrounding code is
  /// restructured; a single funnel that every exit *must* call is the
  /// only version of this that stays true. The per-road table in
  /// `decoder/tests.rs` is what checks that it did.
  /// The hardware road's funnel-and-classify entry — [`software_receive`]'s
  /// twin, and the same law: what a caller reads is what the funnel
  /// found, never the errno that reached it.
  ///
  /// Answers `Err` with the funnel's verdict, which is the latched
  /// refusal when there was one. Callers route *that* value onward
  /// rather than rebuilding the raw error, or the collection is undone
  /// the moment it is used.
  fn hw_receive(&self, e: ffmpeg_next::Error, phase: SessionPhase) -> Result<Received> {
    receive_status(self.hw_exit(Error::Ffmpeg(e)), phase)
  }

  /// The send road's half of [`Self::hw_receive`].
  fn hw_send(&self, e: ffmpeg_next::Error, phase: SessionPhase) -> Result<Sent> {
    send_status(self.hw_exit(Error::Ffmpeg(e)), phase)
  }

  fn hw_exit(&self, fallback: Error) -> Error {
    self
      .take_ceiling_declination()
      .or_else(|| frame_budget_declination_of(self.state.callback_state))
      .unwrap_or(fallback)
  }

  /// Try the next backend in `remaining_backends`. Transactional: a
  /// candidate must successfully build and accept the replayed history
  /// before any probe state is consumed. Backends that fail to build or
  /// reject the replay are recorded into `probe.attempts` and the loop
  /// continues to the next one.
  ///
  /// `last_error` is the error that triggered this advance — i.e. the
  /// failure of the currently active backend on `send_packet` /
  /// `send_eof` / `receive_frame`. It is recorded against the active
  /// backend before any candidate is tried so that a final
  /// `AllBackendsFailed` carries the full attempt log including the
  /// initially-opened backend's runtime failure.
  ///
  /// Returns:
  /// - `Ok(())` when a candidate is installed and replay completed —
  ///   caller should retry the operation.
  /// - `Err(Error::AllBackendsFailed(p))` when every remaining
  ///   backend has been exhausted (including the just-failed active one).
  ///   `p.attempts()` carries the per-backend failure log.
  ///   This is what the documented `open` contract promises, surfaced at
  ///   runtime so the caller can branch into a software fallback. On a
  ///   single-backend platform (e.g. macOS), this fires after the only
  ///   backend's first-frame failure; on multi-backend platforms it
  ///   fires after the last candidate's failure.
  /// - `Err(_)` for other fatal conditions surfaced by probe machinery
  ///   itself (e.g. `alloc_av_frame` ENOMEM during replay drain).
  fn advance_probe(&mut self, last_error: Error) -> Result<()> {
    self.advance_probe_with(last_error, Self::build_state)
  }

  /// [`Self::advance_probe`], with the candidate builder named.
  ///
  /// [`Self::build_state`] on every production road. A lane passes a
  /// builder that opens the candidate in software, which is how the
  /// replay and the commit below run on a machine with one hardware
  /// backend or none.
  fn advance_probe_with(&mut self, last_error: Error, build: BuildCandidate) -> Result<()> {
    // Record the failure that triggered this advance against the active
    // backend. If the probe was somehow already gone (shouldn't happen —
    // call sites guard with `self.probe.is_some()`), just propagate the
    // error so behaviour matches the pre-fix code path.
    let active_backend = self.state.backend;
    // **The reason the callback could not return.** Declining a format
    // in `get_format` surfaces from libavcodec as
    // `Invalid data found when processing input` — true about what it
    // saw, false about what happened, because the data was fine and
    // this crate declined it over the coded surface's size. The
    // callback leaves the real reason in its own state; this is where
    // it becomes the error the caller reads.
    // **Mint or no-op, and never a re-derivation.** Three of this
    // method's callers hand it a raw `Error::Ffmpeg` — no funnel has run
    // on their roads — so this is where their verdict is minted. The
    // receive road hands it one already minted, and this call then finds
    // the latch empty and returns its argument unchanged: `hw_exit`
    // answers with the recorded refusal when there is one and with its
    // fallback when there is not, so a verdict passed in comes back out.
    // Either way the caller's reason is what gets recorded. See the
    // invariant on [`software_receive`].
    let last_error = self.hw_exit(last_error);
    match self.probe.as_mut() {
      Some(probe) => probe.attempts.push((active_backend, Box::new(last_error))),
      None => return Err(last_error),
    }

    // Drop frames previously queued from the backend we're now abandoning.
    // They came from a candidate that just failed for cause and cannot be
    // trusted alongside frames we may queue from the next candidate. (If
    // this method is called repeatedly via chained probe advances, this
    // also keeps `pending_frames` from accumulating frames from multiple
    // rejected backends.)
    self.pending_frames.clear();
    // Read before any `probe` borrow: the end is the *decoder's* fact
    // now, not the probe's, and a candidate must be handed it along
    // with the replayed history or it will sit at `EAGAIN` forever on a
    // stream that is already over.
    let eof_sent = self.eof_sent;

    loop {
      // Snapshot inputs without mutating probe state. Use the checked
      // clone helper rather than `Parameters::clone` (which masks ENOMEM).
      let (next_backend, parameters, codec) = match self.probe.as_ref() {
        Some(probe) if !probe.remaining_backends.is_empty() => {
          let parameters = match try_clone_parameters(
            &probe.parameters,
            self.frame_limits.max_codec_parameter_bytes(),
          ) {
            Ok(p) => p,
            Err(e) => {
              tracing::warn!(
                error = %e,
                "hwdecode: parameters clone failed during probe advance; popping backend and trying next"
              );
              let popped = self
                .probe
                .as_mut()
                .expect("probe state present")
                .remaining_backends
                .remove(0);
              self
                .probe
                .as_mut()
                .expect("probe state present")
                .attempts
                .push((popped, Box::new(e)));
              continue;
            }
          };
          (probe.remaining_backends[0], parameters, probe.codec)
        }
        // No more candidates — surface the accumulated attempt log as
        // AllBackendsFailed so single- and multi-backend platforms have
        // the same contract for "every HW backend failed."
        //
        // Hand the buffered packet history back to the caller along
        // with the attempt log: those packets were consumed from the
        // caller's demuxer (and copied into `buffered_packets`)
        // before the probe exhausted, and for non-seekable inputs the
        // caller cannot re-demux them. Returning them here lets a
        // caller-side software fallback replay the same byte history
        // through `ffmpeg::decoder::Video` without losing initial frames.
        // Dropping `ProbeState` after the take frees the codec/params
        // refs we no longer need; only `attempts` and `buffered_packets`
        // are retained.
        _ => {
          let (attempts, unconsumed_packets) = self
            .probe
            .take()
            .map(|p| (p.attempts, p.buffered_packets))
            .unwrap_or_default();
          return Err(Error::AllBackendsFailed(AllBackendsFailed::new(
            attempts,
            unconsumed_packets,
          )));
        }
      };

      let prev_backend = self.state.backend;
      tracing::warn!(from = ?prev_backend, to = ?next_backend, "hwdecode: advancing probe");

      // Build candidate. On failure, record into attempts and continue
      // without touching the packet buffer.
      let mut candidate_state = match build(
        parameters,
        codec,
        next_backend,
        self.frame_limits,
        self.pkt_timebase,
      ) {
        Ok(s) => s,
        Err(e) => {
          tracing::warn!(?next_backend, error = %e, "hwdecode: candidate build failed");
          self
            .probe
            .as_mut()
            .expect("probe state present")
            .remaining_backends
            .remove(0);
          self
            .probe
            .as_mut()
            .expect("probe state present")
            .attempts
            .push((next_backend, Box::new(e)));
          continue;
        }
      };

      // Replay buffered history through the candidate WITHOUT installing it.
      // We borrow the buffer immutably; if replay fails the candidate's Drop
      // releases the FFmpeg state and the buffer is preserved for the next
      // attempt.
      //
      // EAGAIN handling: `avcodec_send_packet` may return EAGAIN when its
      // internal queue is full and the user is expected to drain output
      // first (B-frame buffering, candidate-specific queue depth, etc.).
      // This is normal flow — we drain frames out of the candidate, transfer
      // each one to a CPU frame, and stash them in `local_pending`. After
      // commit they move to `self.pending_frames` and are delivered FIFO
      // by `receive_frame`, so the caller never loses initial frames.
      let mut local_pending: VecDeque<frame::Video> = VecDeque::new();
      let mut local_pending_bytes: usize = 0;
      let max_pending_bytes = self.max_probe_pending_bytes;
      let replay_result: std::result::Result<(), ffmpeg_next::Error> = {
        let probe = self.probe.as_ref().expect("probe state present");
        let mut hw_buf = match alloc_av_frame() {
          Ok(f) => f,
          Err(e) => return Err(Error::Ffmpeg(e)),
        };
        let mut r: std::result::Result<(), ffmpeg_next::Error> = Ok(());

        'replay: for kept in &probe.buffered_packets {
          // **The candidate is sent a copy, never the history's own
          // packet.** `avcodec_send_packet` takes a reference to what it
          // is sent (`libavcodec/decode.c` 748 in FFmpeg 9.0.1) and the
          // decoder may hold it past the call, while a candidate
          // committed here stays the session's when the history later
          // goes to the caller as owned, mutable `Packet`s. A copy that
          // cannot be made fails this candidate's replay.
          let pkt = match try_clone_packet(kept) {
            Ok(pkt) => pkt,
            Err(e) => {
              r = Err(e);
              break 'replay;
            }
          };
          loop {
            match candidate_state.inner.send_packet(&pkt) {
              Ok(()) => break,
              Err(e) if is_eagain(&e) => {
                // Drain candidate output (transferring + queueing each frame)
                // and retry the same packet.
                if let Err(de) = drain_into_pending(
                  &mut candidate_state.inner,
                  &mut hw_buf,
                  &mut local_pending,
                  &mut local_pending_bytes,
                  max_pending_bytes,
                  self.frame_limits.frame(),
                ) {
                  r = Err(de);
                  break 'replay;
                }
              }
              Err(e) => {
                r = Err(e);
                break 'replay;
              }
            }
          }
        }
        if r.is_ok() && eof_sent {
          // `avcodec_send_packet(NULL)` (which `send_eof` becomes) can
          // return EAGAIN with the same drain-output-first semantics as
          // a regular send_packet. Loop drain+retry instead of failing
          // the candidate on backpressure.
          loop {
            match candidate_state.inner.send_eof() {
              Ok(()) => break,
              Err(e) if is_eagain(&e) => {
                if let Err(de) = drain_into_pending(
                  &mut candidate_state.inner,
                  &mut hw_buf,
                  &mut local_pending,
                  &mut local_pending_bytes,
                  max_pending_bytes,
                  self.frame_limits.frame(),
                ) {
                  r = Err(de);
                  break;
                }
              }
              Err(e) => {
                r = Err(e);
                break;
              }
            }
          }
        }
        r
      };

      if let Err(e) = replay_result {
        tracing::warn!(?next_backend, error = %e, "hwdecode: candidate replay failed");
        // **The candidate's own refusal, read before the candidate
        // dies.** `hw_exit` consults `self.state` — the backend that is
        // still active — but the error being recorded here belongs to
        // `candidate_state`, whose `get_format` callback is the one
        // that may have declined. Classifying through the wrong state
        // and then dropping the right one lost the reason entirely: the
        // attempt log recorded FFmpeg's `Invalid data found when
        // processing input` for a coded surface this crate refused.
        //
        // Order matters and is the whole fix — read, then drop.
        let recorded =
          ceiling_declination_of(candidate_state.callback_state).unwrap_or(Error::Ffmpeg(e));
        // Drop candidate explicitly so its FFI cleanup runs now. Discard any
        // frames we drained from this candidate — they're tied to a decoder
        // we're throwing away.
        drop(candidate_state);
        drop(local_pending);
        self
          .probe
          .as_mut()
          .expect("probe state present")
          .remaining_backends
          .remove(0);
        self
          .probe
          .as_mut()
          .expect("probe state present")
          .attempts
          .push((next_backend, Box::new(recorded)));
        continue;
      }

      // Commit: install the candidate, clear residual hw_frame, queue the
      // drained frames for the caller, and pop the now-active backend.
      self.state = candidate_state;
      // **The scaled-output stage's session belongs to the device that
      // just went away.** Its fitted frames context was built over the
      // outgoing backend's hardware device; the caller's standing
      // request outlives the advance, but nothing built for the old
      // device may. Unreachable on the one platform where the stage has
      // a body — `probe_order` names a single backend on Apple targets,
      // so this method never reaches a commit there — and written all
      // the same, because "unreachable today" is not a property a
      // resource-owning cache should depend on.
      self.scaled_output.retire();
      unsafe { av_frame_unref(self.hw_frame.as_mut_ptr()) };
      self.pending_frames.append(&mut local_pending);
      self
        .probe
        .as_mut()
        .expect("probe state present")
        .remaining_backends
        .remove(0);
      return Ok(());
    }
  }

  /// Build raw FFmpeg state for one hardware backend. Strict `get_format`
  /// (NONE on missing HW format); cross-backend fallback is the caller's job.
  fn build_state(
    parameters: codec::Parameters,
    codec: Codec,
    backend: Backend,
    limits: crate::limits::DecoderLimits,
    pkt_timebase: Option<mediadecode::Timebase>,
  ) -> Result<DecoderState> {
    // Use our checked allocator instead of Context::from_parameters, which
    // does not null-check avcodec_alloc_context3 and would feed a null
    // AVCodecContext into FFmpeg under OOM.
    let (mut ctx, mut state) = build_codec_context(&parameters, limits, pkt_timebase)?;
    let av_type = backend.av_hwdevice_type();

    // Verify the codec advertises this hwaccel **with the exact HW pix_fmt
    // we're about to wire up in `get_format`**. FFmpeg's HW config table
    // is keyed per (device_type, pix_fmt); a codec can advertise the same
    // device with several HW pix_fmts, so matching only on device_type
    // would let probing succeed for a backend whose pix_fmt the codec
    // never offers — the failure would then surface deep inside the
    // probe/decode loop. Matching the exact pix_fmt keeps the strict
    // `get_format` honest and gives `open_with` a clean rejection.
    let hw_pix_fmt = backend.hw_pixel_format();
    if !codec_supports_hwaccel(unsafe { codec.as_ptr() }, av_type, hw_pix_fmt as i32) {
      return Err(Error::BackendUnsupportedByCodec(backend));
    }

    // Create the device context.
    let mut hw_device_ref: *mut AVBufferRef = ptr::null_mut();
    // SAFETY: `hw_device_ref` is a stack ptr we hand FFmpeg to fill.
    let ret = unsafe {
      av_hwdevice_ctx_create(&mut hw_device_ref, av_type, ptr::null(), ptr::null_mut(), 0)
    };
    if ret < 0 {
      return Err(Error::HwDeviceInitFailed(HwDeviceInitFailed::new(
        backend,
        ffmpeg_next::Error::from(ret),
      )));
    }

    // The state `build_codec_context` already installed in `opaque`,
    // told which format this backend wants. One allocation, one seat:
    // the budget the judge reads and the declination the funnel reads
    // are the same object, and `Box::into_raw` hands its ownership to
    // the guard below without moving it — so the pointer the context
    // holds stays the one that is freed.
    state.wanted = hw_pix_fmt;
    state.wanted_int = hw_pix_fmt as i32;
    let callback_state = Box::into_raw(state);
    // RAII guard: from now until the end-of-function `into_owned()`, every
    // early return — `av_buffer_ref` failure, `open_as` failure, codec_type
    // mismatch, or any future error path added between here and the
    // `DecoderState` construction — frees `hw_device_ref` and
    // `callback_state` via the guard's Drop. Without it, each error site
    // had to remember to clean up these two FFI-owned resources by hand;
    // the codec_type-mismatch branch was missed and silently leaked one
    // device ref + one heap allocation per bad input.
    let guard = PartialBuildState {
      hw_device_ref,
      callback_state,
    };

    // SAFETY: ctx is a freshly-constructed AVCodecContext we own;
    // av_buffer_ref bumps the refcount of the device buffer for FFmpeg's
    // use (we keep our own ref in `hw_device_ref` for cleanup).
    // av_buffer_ref returns NULL on allocation failure; we must check it
    // before assigning, otherwise the codec context would be opened with a
    // HW-flagged setup but no actual device reference.
    let device_ref_for_ctx = unsafe { av_buffer_ref(hw_device_ref) };
    if device_ref_for_ctx.is_null() {
      // guard's Drop frees hw_device_ref (the first ref) and callback_state.
      return Err(Error::Ffmpeg(ffmpeg_next::Error::Other {
        errno: libc::ENOMEM,
      }));
    }
    // SAFETY: device_ref_for_ctx is a valid AVBufferRef* from av_buffer_ref;
    // ctx is freshly built and owned by us. After this point ctx aliases
    // `callback_state` via `opaque` (FFmpeg never frees opaque, so
    // `callback_state` ownership stays with us / the guard) and aliases
    // `device_ref_for_ctx` (the second ref) via `hw_device_ctx` (FFmpeg
    // unrefs that on codec context drop, independent of the guard's first
    // ref).
    unsafe {
      let raw = ctx.as_mut_ptr();
      (*raw).hw_device_ctx = device_ref_for_ctx;
      (*raw).opaque = callback_state.cast();
      (*raw).get_format = Some(get_hw_format);
    }

    // Open the decoder. On failure `ctx`/`opened` Drop releases the codec
    // context (and via that the second device ref); the guard releases the
    // first device ref and the callback state.
    //
    // We deliberately bypass `Opened::video()` because it calls
    // `Context::medium()`, which reads `AVCodecContext.codec_type` as the
    // bindgen `AVMediaType` enum — the same UB hazard we've been
    // systematically removing. Instead: validate `codec_type` as a raw
    // `c_int` ourselves, then construct the `decoder::Video` wrapper
    // directly via its public tuple field.
    // Through the funnel's free-standing half — there is no decoder yet
    // to ask, and the guard frees the callback state on the way out, so
    // the reason has to be collected here or not at all.
    let opened = match ctx.decoder().open_as(codec) {
      Ok(opened) => opened,
      Err(e) => return Err(ceiling_declination_of(callback_state).unwrap_or(Error::Ffmpeg(e))),
    };

    // Validate codec_type as a raw integer — never construct AVMediaType
    // from an unvalidated runtime value. On failure `opened`'s Drop
    // releases the codec context; the guard releases the first
    // hw_device_ref and the callback state.
    if let Err(e) = ensure_video_codec_type(&opened) {
      // Same exit, same collection: a declined format can leave the
      // context looking like the wrong medium.
      return Err(ceiling_declination_of(callback_state).unwrap_or(e));
    }
    // SAFETY of construction: `decoder::Video` is `pub struct Video(pub Opened)`.
    // We construct via the public field; this is the same wrapping
    // `Opened::video()` does on success, just without the enum read.
    let opened = ffmpeg_next::decoder::Video(opened);

    // Disarm the guard and transfer ownership of both resources into the
    // returned DecoderState (whose own Drop handles their lifetime).
    let (hw_device_ref, callback_state) = guard.into_owned();
    Ok(DecoderState {
      inner: ManuallyDrop::new(opened),
      backend,
      hw_device_ref,
      callback_state,
    })
  }
}

/// RAII guard for the partially-owned FFmpeg state that
/// [`VideoDecoder::build_state`] holds between the
/// `av_hwdevice_ctx_create` and `Box::into_raw(CallbackState)`
/// allocations and the final `DecoderState` construction.
///
/// If `build_state` returns `Err` for any reason in that window
/// (`av_buffer_ref` ENOMEM, `open_as` failure, codec_type mismatch, or
/// any future error path), this guard's `Drop` releases
/// `hw_device_ref` — the first ref returned by `av_hwdevice_ctx_create`,
/// distinct from the second ref FFmpeg unrefs when the codec context
/// drops — and the boxed `CallbackState`, which FFmpeg never touches
/// because `AVCodecContext::opaque` is purely user-owned.
///
/// Successful construction calls [`Self::into_owned`] to disarm the
/// guard and hand both pointers to the new `DecoderState`.
struct PartialBuildState {
  hw_device_ref: *mut AVBufferRef,
  callback_state: *mut CallbackState,
}

impl PartialBuildState {
  /// Disarm the guard: return the owned pointers and replace the guard's
  /// fields with null so its Drop is a no-op.
  fn into_owned(mut self) -> (*mut AVBufferRef, *mut CallbackState) {
    let hw = std::mem::replace(&mut self.hw_device_ref, ptr::null_mut());
    let cb = std::mem::replace(&mut self.callback_state, ptr::null_mut());
    (hw, cb)
  }
}

impl Drop for PartialBuildState {
  fn drop(&mut self) {
    // SAFETY: pointers are either freshly allocated by `build_state` (via
    // `av_hwdevice_ctx_create` and `Box::into_raw`) or null after
    // `into_owned`. Both `av_buffer_unref` and `Box::from_raw` need the
    // null check we apply here; both are otherwise sound on resources we
    // own.
    unsafe {
      if !self.hw_device_ref.is_null() {
        let mut hw = self.hw_device_ref;
        av_buffer_unref(&mut hw);
      }
      if !self.callback_state.is_null() {
        drop(Box::from_raw(self.callback_state));
      }
    }
  }
}

/// Download a HW frame into a CPU [`Frame`]. Always unrefs the destination
/// first so reuse across resolution changes is safe.
///
/// `src` is whichever surface the scaled-output stage nominated: the
/// decoder's own hardware frame in the ordinary case, or the fitted
/// surface [`crate::vtscale`] scaled it into when a request is standing.
/// Both are VideoToolbox frames carrying their own `AVHWFramesContext`,
/// and the fitted one already carries this frame's metadata — so this
/// function's contract is unchanged either way, and the CPU frame's
/// extent is the nominated surface's.
///
/// Deliberately does **not** call `av_frame_copy_props`. That FFmpeg
/// helper deep-copies AVFrame side data (SEI, mastering display, ICC
/// profiles, dynamic HDR, etc.), the metadata dict, and bumps both
/// `opaque_ref` and `private_ref` on every receive — none of which
/// `Frame` exposes via its public accessors. On a crafted stream with
/// megabytes of per-frame metadata that would mean an unbounded
/// allocation per receive, with no caller-visible benefit. We instead
/// copy only the scalar fields the public API can read (today: `pts`);
/// pixel layout (`width`, `height`, `format`, `linesize`, `data`) is
/// already set by `av_hwframe_transfer_data`. If `Frame` ever grows
/// accessors for timing extras (`duration`, `time_base`, `pkt_dts`) or
/// color metadata, add those to `copy_frame_props_minimal` at the same
/// time.
unsafe fn transfer_hw_frame(
  dst: &mut Frame,
  src: &frame::Video,
) -> std::result::Result<(), ffmpeg_next::Error> {
  unsafe {
    av_frame_unref(dst.as_inner_mut().as_mut_ptr());
    let ret = av_hwframe_transfer_data(dst.as_inner_mut().as_mut_ptr(), src.as_ptr(), 0);
    if ret < 0 {
      return Err(ffmpeg_next::Error::from(ret));
    }
    // Validate the post-transfer CPU pix_fmt against the safe `Frame`
    // accessor's supported set. FFmpeg picks the destination format
    // when `dst.format == AV_PIX_FMT_NONE` on entry (which it always is
    // here — `av_frame_unref` clears it) by walking the result of
    // `av_hwframe_transfer_get_formats`. Driver/version ordering can
    // pick a layout outside our NV*/P0xx/P2xx/P4xx set; the call would
    // return success while the resulting frame is unreadable through
    // `Frame::row` / `Frame::as_ptr` (those return `None` for
    // unsupported formats). Surface the unsupported result as a
    // transfer failure so `receive_frame`'s probe-active path advances
    // to the next backend rather than collapsing on an unusable frame;
    // post-probe, the caller gets that picture's error.
    let dst_raw_fmt: i32 = (*dst.as_inner_mut().as_ptr()).format;
    let dst_pix_fmt = crate::boundary::from_av_pixel_format(dst_raw_fmt);
    if !crate::frame::is_supported_cpu_pix_fmt(&dst_pix_fmt) {
      tracing::warn!(
        pix_fmt = dst_raw_fmt,
        "hwdecode: hw->cpu transfer produced unsupported pix_fmt; \
         treating as backend failure"
      );
      av_frame_unref(dst.as_inner_mut().as_mut_ptr());
      return Err(ffmpeg_next::Error::Other {
        errno: libc::EINVAL,
      });
    }
    if let Err(e) = copy_frame_props_minimal(dst.as_inner_mut().as_mut_ptr(), src.as_ptr()) {
      // Failed to propagate metadata. Reset the destination so the
      // partial frame doesn't leak (its pixel buffers were attached
      // by `av_hwframe_transfer_data` above) and surface as a
      // backend failure — the probe path will advance to the next
      // candidate; post-probe, the caller gets that picture's error.
      av_frame_unref(dst.as_inner_mut().as_mut_ptr());
      return Err(e);
    }
  }
  Ok(())
}

/// Copies AVFrame metadata (timestamps, color metadata, crop rect,
/// flags, side data, etc.) from the source HW frame to the destination
/// CPU frame so the post-transfer frame surfaces the same metadata a
/// SW-decoded frame would.
///
/// Defers to FFmpeg's `av_frame_copy_props`, which handles the per-
/// `side_data[i]` allocation, dict copy, and refcounted buffer
/// replacements internally. The cost is bounded by what the source
/// frame attaches — typical HDR streams carry 1–3 side-data entries
/// (mastering display, content light level, dolby/HDR10+ dynamic
/// metadata) totalling a few hundred bytes, so per-frame allocation
/// overhead stays negligible relative to the pixel data already
/// transferred via `av_hwframe_transfer_data`.
///
/// # Safety
/// Both pointers must be valid `AVFrame` pointers we own. We do not
/// form `&AVFrame` — `av_frame_copy_props` operates on raw pointers
/// directly.
/// Sum the byte sizes of every entry in `(*frame).side_data[]`.
/// Used by the probe replay queue's byte-cap accounting so a
/// frame's deep-copied side-data is charged against
/// `max_probe_pending_bytes` along with its pixel buffers.
///
/// # Safety
/// `frame` must be a live `*const AVFrame`. Reads only `nb_side_data`,
/// the `side_data` pointer array, and each `AVFrameSideData.size` —
/// no `&AVFrame` reference is formed.
unsafe fn sum_side_data_bytes(frame: *const AVFrame) -> usize {
  // Clamp `nb_side_data` to the same entry cap the copy path
  // enforces. Without the clamp, a decoder-controlled or
  // version-skew `nb_side_data` value (the bindgen field is
  // `c_int`, signed) could drive this walk arbitrarily long
  // before the cap downstream kicks in. Negative values are
  // pinned to zero before casting.
  let raw = unsafe { (*frame).nb_side_data };
  let arr = unsafe { (*frame).side_data };
  if raw <= 0 || arr.is_null() {
    return 0;
  }
  let count = (raw as usize).min(HW_COPY_SIDE_DATA_MAX_ENTRIES);
  let mut total: usize = 0;
  for i in 0..count {
    // SAFETY: `arr` points to `nb_side_data` valid `*mut AVFrameSideData`
    // entries per FFmpeg's contract; `i < count` is in-bounds.
    let entry = unsafe { *arr.add(i) };
    if entry.is_null() {
      continue;
    }
    let sz = unsafe { (*entry).size };
    total = total.saturating_add(sz);
    if total >= HW_COPY_SIDE_DATA_MAX_TOTAL_BYTES {
      // Already at or above the byte cap — further entries can't
      // change the projected-vs-cap decision the caller makes.
      total = HW_COPY_SIDE_DATA_MAX_TOTAL_BYTES;
      break;
    }
  }
  total
}

/// Hard cap on the number of `AVFrameSideData` entries we copy from
/// HW source frame to CPU destination frame on the HW transfer
/// path. Mirrors `convert::SIDE_DATA_MAX_ENTRIES`; the public
/// converter re-enforces the same cap so this is defense in depth.
pub(crate) const HW_COPY_SIDE_DATA_MAX_ENTRIES: usize = 64;
/// Hard cap on the total side-data byte budget per HW transfer.
/// Mirrors `convert::SIDE_DATA_MAX_TOTAL_BYTES`.
const HW_COPY_SIDE_DATA_MAX_TOTAL_BYTES: usize = 256 * 1024;

/// Maps a raw `AV_FRAME_DATA_*` integer to the matching bindgen
/// `AVFrameSideDataType` enum value when (and only when) the integer
/// is a known discriminant in the linked FFmpeg's bindgen output.
/// Returns `None` for unknown / version-skew / corrupt values —
/// the caller drops those entries instead of `transmute`-ing an
/// arbitrary integer back into the enum (which would be immediate
/// UB if the discriminant isn't in the enum's set).
///
/// The whitelist covers the entries safe to preserve across HW
/// transfer:
/// - HDR10 / HDR10+ / Dolby Vision / Vivid / ambient HDR metadata
/// - SMPTE / GOP timecodes
/// - ICC color profile
/// - A53 closed captions
/// - Spherical / display matrix orientation
/// - Stereo3D layout
///
/// Other AV_FRAME_DATA_* constants exist (motion vectors, encoder
/// params, RPU buffers, …) but are either decoder-internal or
/// rarely useful through the public mediadecode API; dropping them
/// is the safe default.
pub(crate) fn whitelisted_side_data_kind(
  kind_raw: i32,
) -> Option<ffmpeg_next::ffi::AVFrameSideDataType> {
  use ffmpeg_next::ffi::AVFrameSideDataType;
  // Each match arm compares `kind_raw` against the i32 cast of a
  // known constant, then returns the constant itself — we never
  // construct the enum from arbitrary integer bytes.
  let kind = match kind_raw {
    x if x == AVFrameSideDataType::AV_FRAME_DATA_PANSCAN as i32 => {
      AVFrameSideDataType::AV_FRAME_DATA_PANSCAN
    }
    x if x == AVFrameSideDataType::AV_FRAME_DATA_A53_CC as i32 => {
      AVFrameSideDataType::AV_FRAME_DATA_A53_CC
    }
    x if x == AVFrameSideDataType::AV_FRAME_DATA_STEREO3D as i32 => {
      AVFrameSideDataType::AV_FRAME_DATA_STEREO3D
    }
    x if x == AVFrameSideDataType::AV_FRAME_DATA_DISPLAYMATRIX as i32 => {
      AVFrameSideDataType::AV_FRAME_DATA_DISPLAYMATRIX
    }
    x if x == AVFrameSideDataType::AV_FRAME_DATA_AFD as i32 => {
      AVFrameSideDataType::AV_FRAME_DATA_AFD
    }
    x if x == AVFrameSideDataType::AV_FRAME_DATA_MASTERING_DISPLAY_METADATA as i32 => {
      AVFrameSideDataType::AV_FRAME_DATA_MASTERING_DISPLAY_METADATA
    }
    x if x == AVFrameSideDataType::AV_FRAME_DATA_GOP_TIMECODE as i32 => {
      AVFrameSideDataType::AV_FRAME_DATA_GOP_TIMECODE
    }
    x if x == AVFrameSideDataType::AV_FRAME_DATA_SPHERICAL as i32 => {
      AVFrameSideDataType::AV_FRAME_DATA_SPHERICAL
    }
    x if x == AVFrameSideDataType::AV_FRAME_DATA_CONTENT_LIGHT_LEVEL as i32 => {
      AVFrameSideDataType::AV_FRAME_DATA_CONTENT_LIGHT_LEVEL
    }
    x if x == AVFrameSideDataType::AV_FRAME_DATA_ICC_PROFILE as i32 => {
      AVFrameSideDataType::AV_FRAME_DATA_ICC_PROFILE
    }
    x if x == AVFrameSideDataType::AV_FRAME_DATA_S12M_TIMECODE as i32 => {
      AVFrameSideDataType::AV_FRAME_DATA_S12M_TIMECODE
    }
    x if x == AVFrameSideDataType::AV_FRAME_DATA_DYNAMIC_HDR_PLUS as i32 => {
      AVFrameSideDataType::AV_FRAME_DATA_DYNAMIC_HDR_PLUS
    }
    x if x == AVFrameSideDataType::AV_FRAME_DATA_REGIONS_OF_INTEREST as i32 => {
      AVFrameSideDataType::AV_FRAME_DATA_REGIONS_OF_INTEREST
    }
    x if x == AVFrameSideDataType::AV_FRAME_DATA_SEI_UNREGISTERED as i32 => {
      AVFrameSideDataType::AV_FRAME_DATA_SEI_UNREGISTERED
    }
    x if x == AVFrameSideDataType::AV_FRAME_DATA_FILM_GRAIN_PARAMS as i32 => {
      AVFrameSideDataType::AV_FRAME_DATA_FILM_GRAIN_PARAMS
    }
    x if x == AVFrameSideDataType::AV_FRAME_DATA_DOVI_RPU_BUFFER as i32 => {
      AVFrameSideDataType::AV_FRAME_DATA_DOVI_RPU_BUFFER
    }
    x if x == AVFrameSideDataType::AV_FRAME_DATA_DOVI_METADATA as i32 => {
      AVFrameSideDataType::AV_FRAME_DATA_DOVI_METADATA
    }
    x if x == AVFrameSideDataType::AV_FRAME_DATA_DYNAMIC_HDR_VIVID as i32 => {
      AVFrameSideDataType::AV_FRAME_DATA_DYNAMIC_HDR_VIVID
    }
    x if x == AVFrameSideDataType::AV_FRAME_DATA_AMBIENT_VIEWING_ENVIRONMENT as i32 => {
      AVFrameSideDataType::AV_FRAME_DATA_AMBIENT_VIEWING_ENVIRONMENT
    }
    _ => return None,
  };
  Some(kind)
}

pub(crate) unsafe fn copy_frame_props_minimal(
  dst: *mut AVFrame,
  src: *const AVFrame,
) -> std::result::Result<(), ffmpeg_next::Error> {
  // We deliberately do NOT use `av_frame_copy_props` here, despite
  // its convenience. Upstream `av_frame_copy_props` deep-copies
  // *every* `AVFrameSideData` entry, the metadata `AVDictionary`,
  // and refcounted `opaque_ref` / `private_ref` buffers — all from
  // attacker-controlled decoder output. A crafted stream with many
  // multi-MiB side-data entries could drive the per-frame
  // allocation cost arbitrarily high (one alloc per entry, with the
  // entry's bytes copied via `memcpy`). The downstream
  // `convert::collect_side_data` cap helps the *Rust* side but the
  // FFmpeg-side allocations have already happened.
  //
  // Instead we copy scalar fields manually (timestamps, color
  // metadata, picture type, flags) and copy side-data with a hard
  // cap matching the converter's. Metadata dict and opaque_ref /
  // private_ref are intentionally NOT copied — they're rarely
  // populated on decoded frames and represent unbounded surfaces.
  use core::ptr::{addr_of, addr_of_mut, read_unaligned, write_unaligned};
  use ffmpeg_next::ffi::av_frame_new_side_data;
  unsafe {
    // Scalar timestamps / flags / color / SAR / crop. None of
    // these allocate.
    (*dst).pts = (*src).pts;
    (*dst).pkt_dts = (*src).pkt_dts;
    (*dst).duration = (*src).duration;
    (*dst).best_effort_timestamp = (*src).best_effort_timestamp;
    (*dst).quality = (*src).quality;
    (*dst).repeat_pict = (*src).repeat_pict;
    (*dst).flags = (*src).flags;
    (*dst).sample_aspect_ratio = (*src).sample_aspect_ratio;
    (*dst).crop_left = (*src).crop_left;
    (*dst).crop_top = (*src).crop_top;
    (*dst).crop_right = (*src).crop_right;
    (*dst).crop_bottom = (*src).crop_bottom;
    (*dst).time_base = (*src).time_base;

    // Enum-typed fields: bit-copy raw to avoid materializing an
    // invalid `AVColorPrimaries` etc. on either side. `read_unaligned`
    // / `write_unaligned` on `i32` projections sidestep the bindgen
    // enum's discriminant-validity invariant.
    let pict_type_raw = read_unaligned(addr_of!((*src).pict_type) as *const i32);
    write_unaligned(addr_of_mut!((*dst).pict_type) as *mut i32, pict_type_raw);
    let cp_raw = read_unaligned(addr_of!((*src).color_primaries) as *const i32);
    write_unaligned(addr_of_mut!((*dst).color_primaries) as *mut i32, cp_raw);
    let trc_raw = read_unaligned(addr_of!((*src).color_trc) as *const i32);
    write_unaligned(addr_of_mut!((*dst).color_trc) as *mut i32, trc_raw);
    let cs_raw = read_unaligned(addr_of!((*src).colorspace) as *const i32);
    write_unaligned(addr_of_mut!((*dst).colorspace) as *mut i32, cs_raw);
    let cr_raw = read_unaligned(addr_of!((*src).color_range) as *const i32);
    write_unaligned(addr_of_mut!((*dst).color_range) as *mut i32, cr_raw);
    let cl_raw = read_unaligned(addr_of!((*src).chroma_location) as *const i32);
    write_unaligned(addr_of_mut!((*dst).chroma_location) as *mut i32, cl_raw);

    // Side-data: bounded copy. `av_frame_new_side_data(dst, type,
    // size)` allocates the entry and returns a pointer to write
    // the payload bytes into; a null return is the OOM signal.
    // Callers (`transfer_hw_frame`, `drain_into_pending`) hand us
    // freshly-unref'd `dst` frames, so any prior side-data has
    // already been freed by `av_frame_unref` — we don't need to
    // strip dst's existing side-data here.
    // Read `nb_side_data` as the bindgen `c_int` and clamp non-
    // positive values BEFORE casting to `usize`. A negative value
    // (corrupt / version-skew decoder output) cast directly to
    // `usize` becomes a huge positive count and would walk OOB
    // memory below; pinning to zero up front collapses that to a
    // no-op. Same signed-count guard `sum_side_data_bytes` applies.
    let nb_side_data_raw = (*src).nb_side_data;
    let src_arr = (*src).side_data;
    if nb_side_data_raw > 0 && !src_arr.is_null() {
      let count_raw = nb_side_data_raw as usize;
      let count = count_raw.min(HW_COPY_SIDE_DATA_MAX_ENTRIES);
      if count_raw > HW_COPY_SIDE_DATA_MAX_ENTRIES {
        tracing::warn!(
          cap = HW_COPY_SIDE_DATA_MAX_ENTRIES,
          requested = count_raw,
          "mediadecode-ffmpeg: HW->CPU transfer side-data entry cap reached; truncating",
        );
      }
      let mut total_bytes: usize = 0;
      for i in 0..count {
        let entry = *src_arr.add(i);
        if entry.is_null() {
          continue;
        }
        let kind_raw = read_unaligned(addr_of!((*entry).type_) as *const i32);
        let size = (*entry).size;
        let data_ptr = (*entry).data;
        if size == 0 || data_ptr.is_null() {
          continue;
        }
        // Whitelist gate: only proceed when `kind_raw` matches a
        // known `AV_FRAME_DATA_*` constant the linked FFmpeg's
        // bindgen output knows about. Without this gate, a
        // version-skew or hostile decoder could write a side-data
        // type integer outside our bindgen's discriminant set, and
        // constructing the `AVFrameSideDataType` enum value (so
        // we could pass it to `av_frame_new_side_data`) would be
        // immediate UB before the call. Unknown types are dropped
        // with a debug-level log — the public converter's
        // `collect_side_data` walks the destination raw and would
        // also surface them as bare integers in `SideDataEntry.kind`.
        let Some(kind_enum) = whitelisted_side_data_kind(kind_raw) else {
          tracing::debug!(
            kind_raw,
            "mediadecode-ffmpeg: unknown AV_FRAME_DATA type during HW->CPU transfer; dropping",
          );
          continue;
        };
        let projected = total_bytes.saturating_add(size);
        if projected > HW_COPY_SIDE_DATA_MAX_TOTAL_BYTES {
          tracing::warn!(
            cap = HW_COPY_SIDE_DATA_MAX_TOTAL_BYTES,
            projected,
            "mediadecode-ffmpeg: HW->CPU transfer side-data byte cap reached; dropping rest",
          );
          break;
        }
        let new_entry = av_frame_new_side_data(dst, kind_enum, size);
        if new_entry.is_null() {
          // **OOM is reported, not absorbed.** This used to `break` and
          // return `Ok(())`, which published a frame carrying whatever
          // side data happened to fit before the allocator gave out —
          // silently dropping the entries behind it. Those entries are
          // the HDR mastering metadata, the ICC profile and the display
          // matrix: a picture that comes back with its colours or its
          // orientation quietly missing is worse than one that does not
          // come back, because nothing downstream can tell.
          //
          // The caller already knows what to do with an error here: it
          // unrefs the partial destination and either advances to the
          // next backend or surfaces the failure for a software retry.
          tracing::warn!("mediadecode-ffmpeg: av_frame_new_side_data OOM during HW->CPU transfer",);
          return Err(ffmpeg_next::Error::Other {
            errno: libc::ENOMEM,
          });
        }
        // SAFETY: `(*new_entry).data` is allocated for `size` bytes
        // per av_frame_new_side_data's contract; `data_ptr` is
        // valid for `size` reads per AVFrameSideData's contract.
        core::ptr::copy_nonoverlapping(data_ptr, (*new_entry).data, size);
        total_bytes = projected;
      }
    }
  }
  Ok(())
}

/// `EAGAIN` and `EOF` together: "this decoder has no more output for
/// now", either because it wants input or because it is finished.
///
/// **What is left of a predicate that used to guard both roads.** Both
/// public faces classify at their boundary now — [`receive_status`] and
/// [`send_status`] — and the send face had to stop treating the two
/// alike, since `AVERROR_EOF` there is a caller fault rather than a
/// state. The one caller that still wants them together is
/// [`drain_into_pending`], the probe-replay drain: it reads a raw
/// `ffmpeg_next::decoder::Video` that never crosses a public seam, and
/// for it "wants input" and "finished" really are one answer — stop
/// draining, the candidate produced everything it is going to.
fn is_transient(e: &ffmpeg_next::Error) -> bool {
  is_eagain(e) || matches!(e, ffmpeg_next::Error::Eof)
}

/// **The receive road's single errno gate, and it cannot be spent
/// without saying where the session is.**
///
/// Turns what a funnel (`software_exit` / `hw_exit`) handed back into
/// the trait's status vocabulary, keeping the two flow signals inside
/// this crate.
///
/// Takes the funnel's *output*, never libavcodec's raw error, and that
/// ordering is the point: the funnels collect a `get_format` or
/// allocator-judge refusal that the callback state is holding, and a
/// classifier placed in front of them would answer "needs input" for a
/// road that had a named refusal waiting. So every receive site funnels
/// first and gates second.
///
/// # Why the phase is a parameter and not a guess
///
/// The same errno means different things at different points in a
/// session's life, and every road that guessed guessed differently:
///
/// * `EAGAIN` is [`Received::NeedsInput`] only where more input can
///   arrive. Past a recorded end it is an instruction the caller cannot
///   carry out — the send gates refuse — so it is the end instead. And
///   on a candidate that has already been handed the whole history
///   including the end, it is neither: that candidate has produced no
///   frame and never will, which is a candidate failing, so it goes
///   back as an error for the probe machinery to act on.
/// * `AVERROR_EOF` is [`Received::Ended`] only from a backend that has
///   committed. A candidate's is its own exhaustion, not the stream's.
///
/// Making the phase an argument is what stops a road from having an
/// opinion about this. A classification without it does not compile.
fn receive_status(e: Error, phase: SessionPhase) -> Result<Received> {
  match &e {
    Error::Ffmpeg(f) if is_eagain(f) => {
      if phase.accepts_input() {
        Ok(Received::NeedsInput)
      } else if phase.is_committed() {
        // Draining. A committed backend with nothing more to give has
        // ended, whichever errno it chose — libavcodec is not supposed
        // to answer `EAGAIN` after a flush packet, but this crate has
        // met a codec that does (see `ImageDecodeError::NoImage`), and
        // the alternative is handing back a state nothing can satisfy.
        Ok(Received::Ended)
      } else {
        // `AuditioningPastEnd`: a candidate that has been given
        // everything and produced nothing. The probe road owns it.
        Err(e)
      }
    }
    Error::Ffmpeg(ffmpeg_next::Error::Eof) if phase.is_committed() => Ok(Received::Ended),
    _ => Err(e),
  }
}

/// **The send road's gate, and it is deliberately narrower than its
/// sibling.** Only `EAGAIN` is back pressure here.
///
/// `avcodec_send_packet` answers `AVERROR_EOF` for a different fact than
/// `avcodec_receive_frame` does: not "the stream is over" but *"this
/// decoder has already been told the stream is over, and you sent
/// something anyway"* — a caller usage fault rather than a session
/// state, so it stays in `Err`. Reading it as `Accepted` would silently
/// drop the submission; reading it as `MustDrain` would send the caller
/// into a drain loop that can never make the next offer succeed.
///
/// The same line puts [`crate::ResampleError::AfterEof`] and the
/// WebCodecs adapter's `AfterEof` on the error side.
///
/// # The phase, here too
///
/// [`Sent::MustDrain`] is a promise — *drain, and this same offer
/// becomes acceptable* — and past a recorded end it is one no session
/// can keep. The send gates refuse there first, so this is the second
/// lock rather than the first; what it buys is that the classifier
/// itself becomes incapable of making the promise, which is the whole
/// point of moving the phase into the signature.
fn send_status(e: Error, phase: SessionPhase) -> Result<Sent> {
  match &e {
    Error::Ffmpeg(f) if is_eagain(f) && phase.accepts_input() => Ok(Sent::MustDrain),
    _ => Err(e),
  }
}

/// Reject a `codec::Parameters` whose inner `*mut AVCodecParameters` is
/// null. This guards the public trust boundary: ffmpeg-next can produce
/// such a `Parameters` under OOM (`Parameters::new()` does not check
/// `avcodec_parameters_alloc`), and a safe caller can legally hand one
/// in. Without this check, the very next `(*p.as_ptr()).field` read
/// would be a null deref.
fn ensure_parameters_non_null(parameters: &codec::Parameters) -> Result<()> {
  // SAFETY: as_ptr() returns the inner *const AVCodecParameters; we just
  // inspect the pointer value (no deref).
  if unsafe { parameters.as_ptr() }.is_null() {
    return Err(Error::Ffmpeg(ffmpeg_next::Error::Other {
      errno: libc::ENOMEM,
    }));
  }
  Ok(())
}

/// Allocate a fresh `frame::Video`, checking that `av_frame_alloc` did not
/// return NULL. ffmpeg-next's `frame::Video::empty()` does not surface that
/// failure and the resulting null pointer would be UB on the next field
/// access; this wrapper catches it and surfaces it as `ENOMEM`.
fn alloc_av_frame() -> std::result::Result<frame::Video, ffmpeg_next::Error> {
  let inner = frame::Video::empty();
  // SAFETY: as_ptr() just exposes the inner pointer for inspection.
  if unsafe { inner.as_ptr() }.is_null() {
    return Err(ffmpeg_next::Error::Other {
      errno: libc::ENOMEM,
    });
  }
  Ok(inner)
}

/// Build a fresh `Context` from `parameters`, checking the underlying
/// `avcodec_alloc_context3` for NULL before passing it to
/// `avcodec_parameters_to_context`. ffmpeg-next's `Context::from_parameters`
/// skips that check and would feed a null pointer into FFmpeg under OOM —
/// undefined behavior. This helper surfaces the failure as `ENOMEM` and
/// frees the context if `parameters_to_context` itself errors.
pub(crate) fn build_codec_context(
  parameters: &codec::Parameters,
  limits: crate::limits::DecoderLimits,
  pkt_timebase: Option<mediadecode::Timebase>,
) -> Result<(Context, Box<CallbackState>)> {
  ensure_parameters_non_null(parameters)?;
  // **The choke point.** `avcodec_parameters_to_context` below is a
  // wholesale copy *into* FFmpeg — it duplicates `extradata`, every
  // `coded_side_data` entry and the channel map into the context, at
  // whatever size the caller's parameters declare. Every road that
  // opens a decoder in this crate arrives here, so measuring and
  // admitting once, right here, is what stops a caller handing
  // libavcodec parameters nobody budgeted: the four session `open`s,
  // the HW probe's `build_state`, its per-backend advances, and the
  // software fallback all pass through this function and none of them
  // can reach `avcodec_parameters_to_context` any other way.
  //
  // The outbound clone (`extras::bounded_clone_parameters`) closed the
  // Rust-side copy; this closes the FFmpeg-side one. They are the same
  // budget.
  //
  // **Structure before size, and before FFmpeg is handed anything.**
  //
  // `measure_parameters` below prices a custom channel map from
  // `nb_channels` alone and never looks at `u.map`, so a layout with a
  // positive count and a null map passed this budget and went on to
  // `avcodec_parameters_to_context`, which reaches
  // `av_channel_layout_copy` — a `memcpy` from that null. The
  // non-custom orders were no better: nothing bounded `nb_channels` for
  // a `NATIVE` or `AMBISONIC` layout, and FFmpeg's own arithmetic over
  // it assumes invariants it does not check.
  //
  // Every decoder this crate opens arrives here, and the refusal is
  // structural rather than an allocation failure, so it carries its own
  // error rather than the ceiling's or the allocator's.
  //
  // SAFETY: `ensure_parameters_non_null` just proved the pointer is
  // live; for a custom order its map is FFmpeg's own allocation of
  // `nb_channels` entries, and the preflight allocates nothing.
  unsafe {
    crate::channel_layout::layout_preflight(ptr::addr_of!((*parameters.as_ptr()).ch_layout))
  }
  .map_err(
    |fault| match crate::demuxer::layout_fault_to_demux(0, fault) {
      crate::demuxer::DemuxError::ParametersLayoutShape(shape) => {
        Error::MalformedChannelLayout(shape)
      }
      crate::demuxer::DemuxError::ParametersChannelMap(map) => Error::ChannelMapMissing(map),
      // The preflight allocates nothing, so no other arm is reachable;
      // reporting the allocator keeps the match total without an
      // `unreachable!`.
      _ => Error::Ffmpeg(ffmpeg_next::Error::Other {
        errno: libc::ENOMEM,
      }),
    },
  )?;

  // SAFETY: `ensure_parameters_non_null` just proved the pointer is
  // live; the measurement allocates nothing.
  let footprint = unsafe { crate::extras::measure_parameters(parameters.as_ptr()) };
  let declared = footprint.and_then(|f| f.total()).unwrap_or(usize::MAX);
  if declared > limits.max_codec_parameter_bytes() {
    return Err(Error::ParametersTooLarge(
      crate::demuxer::ParametersTooLarge::new(0, declared, limits.max_codec_parameter_bytes()),
    ));
  }
  // SAFETY: avcodec_alloc_context3(NULL) returns a fresh AVCodecContext
  // or NULL on allocation failure.
  let ctx_ptr = unsafe { avcodec_alloc_context3(ptr::null()) };
  if ctx_ptr.is_null() {
    return Err(Error::Ffmpeg(ffmpeg_next::Error::Other {
      errno: libc::ENOMEM,
    }));
  }
  // SAFETY: ctx_ptr is non-null and freshly allocated; parameters.as_ptr()
  // returns a valid AVCodecParameters pointer; the function copies bytes
  // out of parameters into the context.
  let ret = unsafe { avcodec_parameters_to_context(ctx_ptr, parameters.as_ptr()) };
  if ret < 0 {
    // SAFETY: ctx_ptr was allocated by us and never handed to anyone else.
    let mut p = ctx_ptr;
    unsafe { avcodec_free_context(&mut p) };
    return Err(Error::Ffmpeg(ffmpeg_next::Error::from(ret)));
  }
  // **The push-down.** The same pixel ceiling this crate checks against
  // a decoded frame is written into the decoder itself, so libavcodec
  // refuses an oversized picture *before allocating it*. Checking only
  // on our side would mean FFmpeg had already paid for the frame by the
  // time we declined to copy it — two layers, one number, and this is
  // the layer that matters.
  //
  // FFmpeg's own default here is `INT_MAX`, i.e. no ceiling worth the
  // name. `max_pixels` is a plain `int64_t` field on `AVCodecContext`
  // (and has been since FFmpeg 4.0), so it is set directly rather than
  // through `av_opt_set_int` and a stringly-typed option name.
  //
  // **And the byte ceiling, pushed down through the same field.**
  //
  // The pixel ceiling alone does not bound bytes, because a pixel is not
  // a fixed price: 10000x10000 is 100 Mpx — comfortably under the 256
  // Mpx default — and in `rgba64` it is 800 MB, well over the 512 MiB
  // byte ceiling. A highly compressible frame of that shape is a few KB
  // on disk, so nothing upstream sees it coming.
  //
  // **`max_pixels` carries the caller's number, verbatim.** It used to
  // carry `min(that, max_frame_bytes / worst-bytes-per-pixel)`, so the
  // byte ceiling could be enforced before libavcodec allocated — and
  // that translation charged every stream the widest format in
  // existence, 16 bytes a pixel. A 1920x1080 `yuv420p` frame costs
  // 3.14 MiB and was refused under a 4 MiB budget, at
  // `ff_set_dimensions`, before anything accurate had a chance to look
  // at it. Over-refusing ordinary video is not a conservative failure;
  // it is a broken decoder.
  //
  // The translation is gone because it is no longer needed: the byte
  // ceiling is enforced by [`judge_buffer`], which is *also* a
  // pre-allocation seat — `get_buffer2` is the allocator, so it runs
  // before the allocation and prices the frame's real format at its
  // real aligned dimensions. Nothing is lost on the software road by
  // stating the pixel limit as what it is.
  //
  // SAFETY: `ctx_ptr` is the non-null context just allocated and
  // populated above; `max_pixels` is a public field.
  unsafe {
    (*ctx_ptr).max_pixels = i64::try_from(limits.frame().max_pixels()).unwrap_or(i64::MAX);
  }

  // **The byte ceiling's own seat, in the allocator itself.**
  // `max_pixels` bounds an extent; what an extent costs depends on its
  // format and on how the allocator aligns it — a `gray8` frame of
  // 65536x1 is 64 KiB by `w * h` and 2 MiB once its single row is
  // rounded up. No scalar compared against a pixel product can bound
  // that, so the byte question is asked where the answer is knowable:
  // in `get_buffer2`, which *is* the allocation, against the caller's
  // own `max_frame_bytes`.
  //
  // See [`judge_buffer`] for why this hook rather than `get_format`
  // (measured: `get_format` never fires for a one-shot `png` decode).
  //
  // SAFETY: `ctx_ptr` is the non-null context; `get_buffer2` is a
  // public function-pointer field, and `judge_buffer` delegates every
  // frame it accepts to the allocator libavcodec would have used.
  unsafe {
    (*ctx_ptr).get_buffer2 = Some(judge_buffer);
  }

  // **The packet timebase, for every decoder opened against a stream.**
  //
  // `AVCodecContext.pkt_timebase` is documented as caller-supplied, and
  // libavcodec reads it: its generic subtitle path derives
  // `AVSubtitle.pts` and the packet-duration fallback *only* when the
  // field is set, so a subtitle decoder opened without it returns
  // timestamped cues carrying neither. It was never written here, on
  // any road.
  //
  // **A zero numerator is deliberately not written.** Left alone the
  // field is `0/1`, and `0/1` is also exactly what a container that
  // declared no timebase hands this crate — the two are the same
  // statement, so writing it would be ceremony, and treating it as a
  // real ruler would claim one the file never gave. See
  // [`crate::demuxer`]'s own note on why that value is passed through
  // rather than refused on the track road.
  if let Some(timebase) = pkt_timebase.filter(|timebase| timebase.num() > 0) {
    // SAFETY: `ctx_ptr` is the non-null context allocated above and
    // `pkt_timebase` is a public field; the value is built from a
    // `Timebase`, which cannot be negative or zero-denominatored.
    unsafe {
      (*ctx_ptr).pkt_timebase =
        ffmpeg_next::Rational::new(timebase.num(), timebase.den().get()).into();
    }
  }

  // **`max_samples` is deliberately left alone.**
  //
  // It bounds `nb_samples * channels`, so bounding *bytes* with it
  // means dividing by a per-channel-sample cost — and the only sound
  // divisor is the widest sample format the build can emit, 8 bytes.
  // That charged every stream `f64` rates: a 6-channel `s16` frame
  // fitting a 64 KiB budget was refused, because the translation
  // priced it at four times its real cost.
  //
  // The audio pre-allocation story is now the same as the video one,
  // and it is stronger than the translation was: [`judge_buffer`] runs
  // in `get_buffer2`, before the planes are allocated, and prices the
  // frame's real sample format at its real channel count through
  // [`crate::footprint`] — which asks `av_samples_get_buffer_size`, the
  // allocator's own ruler. An exact judge at the allocation beats an
  // approximate one before it.

  // **The judge's budget seat.** `judge_buffer` runs as a C callback
  // with nothing but the context to read, and the byte ceiling is not
  // recoverable from any field on it — see
  // [`CallbackState::max_frame_bytes`]. So the state that already
  // carries the `get_format` declination carries the budget too, and
  // every road gets one: this is the single point every decoder in the
  // crate is built through.
  //
  // Ownership stays with the caller, which keeps the box alive for as
  // long as the context. `Box` contents do not move when the box does,
  // so the pointer installed here stays valid across the return.
  let mut state = Box::new(CallbackState {
    wanted: ffmpeg_next::ffi::AVPixelFormat::AV_PIX_FMT_NONE,
    wanted_int: ffmpeg_next::ffi::AVPixelFormat::AV_PIX_FMT_NONE as i32,
    ceiling_declined: core::sync::atomic::AtomicBool::new(false),
    declined_pixels: core::sync::atomic::AtomicI64::new(0),
    declined_limit: core::sync::atomic::AtomicI64::new(0),
    max_frame_bytes: limits.frame().max_frame_bytes() as u64,
    frame_budget_declined: core::sync::atomic::AtomicBool::new(false),
    declined_frame_bytes: core::sync::atomic::AtomicU64::new(0),
    declined_frame_audio: core::sync::atomic::AtomicBool::new(false),
  });
  // SAFETY: `ctx_ptr` is the non-null context; `opaque` is a public
  // field FFmpeg never reads or frees.
  unsafe {
    (*ctx_ptr).opaque = (&raw mut *state).cast();
  }

  // SAFETY: ctx_ptr is valid; passing `owner: None` means our wrapper owns
  // the allocation and `Context::drop` will run `avcodec_free_context`.
  Ok((unsafe { Context::wrap(ctx_ptr, None) }, state))
}

/// Checked deep-clone of `codec::Parameters`. ffmpeg-next's
/// `Parameters::clone` allocates via `avcodec_parameters_alloc` without
/// checking for NULL and runs `avcodec_parameters_copy` without checking
/// the return code. On `ENOMEM` the result is a `Parameters` with a null
/// inner pointer, which becomes UB when later passed to FFmpeg.
///
/// This helper performs both calls explicitly, frees a partial allocation
/// on failure, and surfaces the AVERROR. The returned `Parameters` has
/// `owner: None`, severing any Rc link to the caller's demuxer (the
/// reason we deep-clone in the first place — see Send safety in
/// `VideoDecoder::open`).
pub(crate) fn try_clone_parameters(
  src: &codec::Parameters,
  budget: usize,
) -> std::result::Result<codec::Parameters, Error> {
  // Through the bounded clone, like every other parameter copy in this
  // crate — see [`crate::extras::bounded_clone_parameters`] for the
  // rule and why the wholesale `avcodec_parameters_copy` this used to
  // call is gone. This path is attacker-facing: `VideoDecoder::open`
  // takes whatever `stream.parameters()` hands it, straight off a
  // container.
  //
  // `budget` is the **active** ceiling, threaded from the session's own
  // `DecoderLimits` — through the initial ownership clone, the probe
  // state's copy, every probe advance and the software fallback. It
  // used to be the crate default, so a lowered ceiling did not bind
  // here (the clone admitted 16 MiB whatever the caller configured,
  // and only `build_codec_context` downstream refused) and a raised one
  // could not be used at all.
  //
  // The stream index is reported as 0: this helper is handed
  // parameters, not a stream, and inventing a coordinate it cannot
  // know would be worse than admitting it has none.
  crate::extras::bounded_clone_parameters(src, 0, budget).map_err(|e| match e {
    crate::demuxer::DemuxError::ParametersTooLarge(p) => Error::ParametersTooLarge(p),
    crate::demuxer::DemuxError::ParametersCopy(p) => Error::Ffmpeg(*p.source()),
    // A missing or unallocatable destination is the out-of-memory this
    // helper has always reported.
    _ => Error::Ffmpeg(ffmpeg_next::Error::Other {
      errno: libc::ENOMEM,
    }),
  })
}

/// Checked counterpart to `Packet::clone()`: a copy of `src` whose payload
/// nothing else references.
///
/// ffmpeg-next's `clone_from` makes the same two calls, `av_packet_ref`
/// then `av_packet_make_writable`, and ignores both return codes
/// (`src/codec/packet/packet.rs` 287–297 in ffmpeg-next 9.0.0). Under
/// `ENOMEM` its clone is left empty, or still sharing `src`'s buffer,
/// while the caller assumes a copy. Both are checked here.
///
/// **Why both calls.** `av_packet_ref` alone shares `src`'s payload
/// buffer (`libavcodec/packet.c` 461–468 in FFmpeg 9.0.1), and a `Packet`
/// lends `&mut [u8]` through `data_mut` without consulting writability.
/// A shallow clone kept past the call is a second owner of the caller's
/// bytes: the caller's later writes reach it, and `data_mut` on the two
/// hands out aliasing `&mut [u8]` from safe code, on two threads once
/// the packets are sent apart. `av_packet_make_writable` copies a buffer
/// that is not writable — referenced more than once, or read-only
/// (`libavutil/buffer.c` 147–153) — into one of the packet's own
/// (`packet.c` 516–536). Side data is the copy's own already:
/// `av_packet_copy_props` copies each entry (417–429). `opaque_ref` stays
/// shared (413), and ffmpeg-next's safe API does not reach it.
///
/// The copy's payload and side data are `src`'s sizes, so a budget that
/// charged `src` charges the copy the same bytes. On failure the partial
/// clone is dropped, which releases the reference it took.
pub(crate) fn try_clone_packet(src: &Packet) -> std::result::Result<Packet, ffmpeg_next::Error> {
  let mut dst = Packet::empty();
  // SAFETY: dst is a freshly zero-initialized Packet (av_init_packet inside
  // Packet::empty); av_packet_ref initializes its data fields from src's
  // refcounted buffer, or returns AVERROR(ENOMEM) and leaves dst blank
  // (`packet.h` 861–862).
  let ret = unsafe { av_packet_ref(dst.as_mut_ptr(), src.as_ptr()) };
  if ret < 0 {
    return Err(ffmpeg_next::Error::from(ret));
  }
  // SAFETY: dst is the live packet av_packet_ref just filled. On failure
  // av_packet_make_writable leaves it unchanged (`packet.h` 921–922), and
  // dropping it releases the reference it holds.
  let ret = unsafe { av_packet_make_writable(dst.as_mut_ptr()) };
  if ret < 0 {
    return Err(ffmpeg_next::Error::from(ret));
  }
  Ok(dst)
}

/// Sum of `AVPacket.side_data[i].size` across every entry, plus
/// `nb_entries * SIDE_DATA_ENTRY_OVERHEAD` (descriptor + AVBufferRef +
/// allocator bookkeeping per entry). `av_packet_ref` performs a deep
/// copy of side data via `av_packet_copy_props`, so each probe-buffered
/// clone retains every one of these bytes. Charging both keeps
/// `MAX_PROBE_PACKET_BYTES` a true upper bound — without the overhead,
/// many zero-size entries slip past the cap on pure descriptor cost.
///
/// Walks at most `max_entries` entries even when `side_data_elems`
/// reports a larger count. Defense-in-depth against a corrupt or hostile
/// packet whose `side_data_elems` lies about the actual array length:
/// the caller is expected to also reject any packet whose count exceeds
/// the cap (so the inflated clone is never created), but bounding the
/// walk here means a stale or weaponised value can never trigger an
/// unbounded raw-pointer scan from the safe API.
///
/// Reads only the `size` field of each `AVPacketSideData` entry — never
/// touches the bindgen `AVPacketSideDataType` enum, so no UB even if a
/// future FFmpeg adds a side-data type discriminant our build doesn't
/// know.
pub(crate) fn packet_side_data_bytes(packet: &Packet, max_entries: usize) -> usize {
  // SAFETY: AVPacket.side_data is `*mut AVPacketSideData` and
  // side_data_elems is `c_int`; both are raw struct fields safe to read.
  // Field projection (`.size`) does not reconstruct the enum-typed `type_`
  // field, so the bindgen-enum UB hazard does not apply here.
  unsafe {
    let raw = packet.as_ptr();
    let nel = (*raw).side_data_elems;
    let arr = (*raw).side_data;
    if arr.is_null() || nel <= 0 || max_entries == 0 {
      return 0;
    }
    let count = (nel as usize).min(max_entries);
    let mut total = count.saturating_mul(SIDE_DATA_ENTRY_OVERHEAD);
    for i in 0..count {
      let entry = arr.add(i);
      total = total.saturating_add((*entry).size);
    }
    total
  }
}

/// Number of `AVPacketSideData` entries on `packet`. The probe buffer
/// uses this to enforce [`MAX_PROBE_PACKET_SIDE_DATA_ENTRIES`] before
/// cloning, so a packet whose entry count alone would dominate retained
/// memory is rejected up front.
pub(crate) fn packet_side_data_count(packet: &Packet) -> usize {
  // SAFETY: side_data_elems is `c_int`, safe to read; clamp negatives to 0.
  let nel = unsafe { (*packet.as_ptr()).side_data_elems };
  if nel <= 0 { 0 } else { nel as usize }
}

/// Just `EAGAIN` (separate from EOF — the FFmpeg send/receive state machine
/// distinguishes "drain output and retry" from "stream over").
fn is_eagain(e: &ffmpeg_next::Error) -> bool {
  matches!(e, ffmpeg_next::Error::Other { errno } if *errno == ffmpeg_next::error::EAGAIN)
}

/// The probe square the per-pixel cost is measured on.
///
/// 256 divides every chroma subsampling FFmpeg has **and** every
/// alignment libavcodec uses, so the measurement is exact: no plane is
/// rounded up to cover a half-sized dimension, and no row is padded to
/// an alignment boundary. Measured at 257 the same census reads 16.934
/// bytes per pixel instead of 16.000 — that 5.8% is per-*row* padding,
/// a term linear in height rather than in pixels, and it is not part of
/// the per-pixel rate.
pub(crate) const PROBE_PIXELS: usize = 256 * 256;

/// Bytes a [`PROBE_PIXELS`]-pixel picture costs in the **most expensive
/// pixel format this build of libavcodec can describe**.
///
/// # Why the worst case and not the declared one
///
/// The first cut of this ceiling charged the format the *container*
/// declared, and a container's declaration is not an upper bound on
/// anything. It may be unset, it may be wrong, and it may be narrower
/// than what the decoder actually emits — a stream declaring `yuv420p`
/// at 1.5 bytes per pixel whose decoder outputs `rgbaf32` at 16 got a
/// ceiling more than ten times too generous, which is the same hole one
/// layer down from the one it was added to close.
///
/// So the rate is not negotiated with the file at all. Every stream is
/// charged the worst case, and the worst case is **measured**, not
/// tabulated: this build's descriptor list is walked once and each
/// format priced through `av_image_get_buffer_size`, the same function
/// `avcodec_default_get_buffer2` sizes from. A future FFmpeg that adds
/// a wider format is priced correctly without this crate learning its
/// name.
///
/// # The census, at the time of writing
///
/// 267 descriptors, 251 of them CPU formats that price (the rest are
/// hardware surfaces, which carry no CPU bytes and return no size). The
/// maximum is **16.000 bytes per pixel**, reached by eight formats —
/// `gbrapf32be/le`, `rgbaf32be/le`, `rgba128be/le`, `gbrap32be/le`.
/// Next below are the 12-byte `gbrpf32`/`rgbf32` family.
///
/// # What this trades
///
/// Over-refusal for cheap formats, and it is deliberate. At the 512 MiB
/// default the effective ceiling becomes ~33.55 Mpx, so 8K (33.18 Mpx)
/// still decodes in *any* format — including the 16-byte ones, where it
/// really does cost 506 MiB — but a 16K `yuv420p` frame, which would
/// only have cost 199 MB, is refused too. That is the honest shape of a
/// bound that has to hold before the format is known: the deployment
/// answer is to raise `max_frame_bytes`, which is exactly the knob that
/// says how much memory one frame may cost.
///
/// # The residual, stated
///
/// Row alignment adds at most `align x planes x height` bytes on top of
/// this rate — about 1 MB on an 8K frame, 0.2%, and covered by the fact
/// that `max_frame_bytes` is a policy number rather than a hardware
/// limit. It is only significant for degenerate aspect ratios (a
/// one-pixel-wide frame is all padding), which the *pixel* ceiling has
/// always been the wrong shape to bound and which this change neither
/// introduces nor worsens.
pub(crate) fn worst_bytes_per_probe() -> usize {
  /// The census result, taken once. `av_pix_fmt_desc_next` walks a
  /// static table that cannot change during the process.
  static WORST: std::sync::OnceLock<usize> = std::sync::OnceLock::new();
  *WORST.get_or_init(|| {
    /// The measured maximum at the time of writing, and the floor this
    /// census may not fall below. A build whose census comes back
    /// *smaller* than the eight 16-byte formats has failed to walk the
    /// table, not discovered a cheaper world — take the known number
    /// rather than a ceiling built on a failed measurement.
    const KNOWN_WORST_BYTES_PER_PIXEL: usize = 16;

    let mut worst = 0usize;
    let mut desc: *const ffmpeg_next::ffi::AVPixFmtDescriptor = ptr::null();
    loop {
      // SAFETY: `av_pix_fmt_desc_next` walks libavutil's own static
      // descriptor table, taking the previous entry (or null to start)
      // and returning null at the end. It traffics in descriptor
      // pointers, not enums, so it needs no shim.
      desc = unsafe { ffmpeg_next::ffi::av_pix_fmt_desc_next(desc) };
      if desc.is_null() {
        break;
      }
      // **Both of these go through the `c_int` shims**, and this is the
      // place it matters most: the whole point of walking the table is
      // to price formats this build's bindings may not name, and the
      // generated `av_pix_fmt_desc_get_id` hands those ids back as a
      // closed `AVPixelFormat`. Every future format would have become
      // an invalid enum value on the way into the pricing meant to
      // handle it — the census would have been UB on exactly its reason
      // for existing.
      //
      // SAFETY: `desc` is a live entry from libavutil's static table;
      // the id is passed straight back to libavutil as the integer it
      // is, and `av_image_get_buffer_size` returns a negative AVERROR
      // for ids it cannot size rather than misbehaving.
      let id = unsafe { c_shims::av_pix_fmt_desc_get_id(desc) };
      let size = unsafe { c_shims::av_image_get_buffer_size(id, 256, 256, 1) };
      if size > 0 {
        worst = worst.max(size as usize);
      }
    }
    worst.max(KNOWN_WORST_BYTES_PER_PIXEL * PROBE_PIXELS)
  })
}
/// `AVCodecContext.get_buffer2`: the same pixel ceiling, applied where
/// the **aligned** dimensions are knowable.
///
/// # The hole this closes
///
/// `max_pixels` is checked by libavcodec against the frame's *raw*
/// `width * height`. What it then allocates is the **aligned** shape —
/// `avcodec_align_dimensions2` rounds both dimensions up to whatever
/// the codec and the CPU want — and for degenerate aspect ratios those
/// are not the same number at all. Measured on this build:
///
/// | shape | raw | aligned | inflation |
/// |---|---|---|---|
/// | `gray8` 65536x1 | 65,536 px / 64 KiB | 65536x32 = 2,097,152 px / 2 MiB | **32x** |
/// | `gray8` 1x65536 | 65,536 px / 64 KiB | 16x65536 = 1,048,576 px / 2 MiB | 16x |
/// | `yuv420p` 7680x4320 | 33,177,600 px | 7680x4320 | 1.00x |
/// | `gray8` 1024x1024 | 1,048,576 px | 1024x1024 | 1.00x |
///
/// So a one-pixel-tall frame slips 32 times its declared cost past a
/// scalar compared against `w * h`, and no value of that scalar can fix
/// it: bounding the product cannot bound a product whose factors are
/// then rounded up independently. Real pictures inflate by nothing at
/// all, which is why the ceiling looked sound.
///
/// # Why this hook and not `get_format`
///
/// `get_format` was measured first, because it needs no allocation
/// decision and receives the context. It **does not fire on every
/// road**: on this build a one-shot `mjpeg` decode calls it once and a
/// `png` decode calls it *zero* times. Cover art is overwhelmingly
/// mjpeg or png, so half the road this ceiling exists to guard would
/// have been unguarded.
///
/// `get_buffer2` fired on both — it is the allocator, so every frame
/// libavcodec hands back comes through it, and it sees the frame's
/// *real* format rather than a negotiated candidate.
///
/// # No state, so no lifetime to prove
///
/// The composed-`opaque` design was not needed. This callback reads the
/// ceiling from `AVCodecContext.max_pixels` — the field this crate set
/// itself, one number, already carrying the byte ceiling converted at
/// the worst per-pixel rate — and applies it to the aligned dimensions.
/// Same scalar, same meaning, applied where alignment is knowable.
/// `opaque` is untouched, so the hardware path keeps it and there is no
/// allocation whose lifetime has to outlive a C callback.
///
/// Panic discipline is likewise structural rather than asserted: the
/// body allocates nothing, indexes nothing, unwraps nothing, and calls
/// exactly three FFmpeg functions. There is no Rust operation in it
/// that can panic, and an `extern "C"` function aborts rather than
/// unwinding into C in any case.
///
/// # Safety
///
/// Called by libavcodec with a live context and a frame whose `format`,
/// `width` and `height` are set. Delegates every accepted frame to
/// `avcodec_default_get_buffer2`, which is what libavcodec would have
/// called had this hook not been installed.
unsafe extern "C" fn judge_buffer(
  ctx: *mut ffmpeg_next::ffi::AVCodecContext,
  frame: *mut ffmpeg_next::ffi::AVFrame,
  flags: libc::c_int,
) -> libc::c_int {
  // SAFETY: libavcodec passes a live context and frame; both fields are
  // plain integers.
  let (width, height) = unsafe { ((*frame).width, (*frame).height) };

  // **This seat judges cost, and only cost.**
  //
  // `max_pixels` is a *logical* limit on a picture's extent, and
  // libavcodec already enforces it — against the **raw** dimensions, in
  // `ff_set_dimensions` via `av_image_check_size2`, before any frame
  // exists. That is the semantics the caller asked for and the
  // semantics FFmpeg documents, and this callback does not restate it.
  //
  // It used to. R11 added an *aligned*-dimension comparison here
  // against `max_pixels`, because at the time the callback had no
  // accurate byte check and a degenerate shape could slip its real cost
  // past a raw-pixel gate — 65536x1 aligns to 65536x32, thirty-two
  // times the pixels. That instrument is now both **redundant** and
  // **wrong**:
  //
  // * redundant, because since the byte ceiling was threaded in the
  //   footprint below prices the aligned dimensions itself, so the
  //   degenerate shape is refused on its actual cost; and
  // * wrong, because `max_pixels` is `min(the caller's pixel limit,
  //   byte ceiling / worst-bytes-per-pixel)` — so when the caller's
  //   pixel limit was the tighter seat, alignment inflation alone
  //   refused frames satisfying *both* requested limits. A 65536x1
  //   `gray8` frame under `max_pixels = 65536` and a generous byte
  //   budget fits the pixel limit exactly and costs 2 MiB, and was
  //   refused anyway — for arithmetic the caller never asked about.
  //
  // Logical extent is libavcodec's gate on raw dimensions; allocation
  // cost is this one, against the caller's own `max_frame_bytes`. One
  // question each.
  //
  // Audio reaches here too, and used to pass unpriced entirely:
  // `max_samples` bounds the sample *count*, so one sample across eight
  // packed `f64` channels is 64 valid bytes under a 64-byte ceiling and
  // a 2,080-byte allocation — delivered, because the copy-out only ever
  // rechecks the valid bytes.
  //
  // SAFETY: `ctx` and `frame` are live; every field read is a plain
  // integer, and `format` stays an integer throughout.
  // SAFETY: `frame` is live; the field is a plain pointer.
  let hw_frames = unsafe { (*frame).hw_frames_ctx };

  // A hardware frame carries no CPU bytes for this seat to price — its
  // pool is judged where it is declared, in the `get_format` callback —
  // so it is delegated rather than failed closed on an unpriceable
  // format.
  if hw_frames.is_null() {
    // **The caller's own number, read from the seat that carries it.**
    // This used to recover a byte ceiling from `AVCodecContext.max_pixels`,
    // and the recovery was wrong in both directions:
    //
    // * `max_pixels` is `min(pixel ceiling, byte ceiling / worst)`, so
    //   when the *pixel* seat was the tighter of the two it stopped
    //   encoding the byte ceiling at all — and the recovery invented a
    //   smaller one. A 256x256 frame at 16 bytes a pixel under
    //   `max_pixels = 65536` with a 2 MiB byte budget satisfies both of
    //   the caller's limits, costs 1,050,624 bytes, and was judged
    //   against 1,048,576 and refused. The claim that the conflation
    //   was harmless in one direction was simply wrong: it omitted the
    //   footprint's own alignment and slack, which is exactly where
    //   those extra 2,048 bytes live.
    // * and for audio a pixel ceiling has no business being consulted
    //   at all.
    //
    // The audio road briefly recovered from `max_samples` instead,
    // which *is* exact — but two sources of truth for one number is how
    // the first one went wrong. Both media read the seat now.
    //
    // SAFETY: `opaque` holds the `CallbackState` that
    // `build_codec_context` installed and whose owner outlives the
    // context. A null one means a context this crate did not build, and
    // is refused rather than assumed generous.
    let state = unsafe { (*ctx).opaque } as *const CallbackState;
    if state.is_null() {
      return -(libc::EINVAL);
    }
    // SAFETY: non-null per the check above; the field is a plain `u64`.
    let byte_ceiling = u128::from(unsafe { (*state).max_frame_bytes });

    // SAFETY: `frame` is live; both are plain integer fields.
    let (format_raw, nb_samples) = unsafe { ((*frame).format, (*frame).nb_samples) };
    let priced = if width > 0 && height > 0 {
      crate::footprint::video_frame_bytes(format_raw, width, height)
    } else if nb_samples > 0 {
      // **The frame's layout, not the context's.** FFmpeg's
      // `get_buffer2` contract says the callback reads the values on
      // the *frame*, and `avcodec_default_get_buffer2` sizes from them
      // — the context's layout is whatever was last negotiated and can
      // differ outright. A context claiming mono against a frame
      // carrying 255 `dblp` channels at 130,000 samples prices about a
      // megabyte and allocates about 265 MB.
      //
      // Read raw and signed, per the house discipline, and refused
      // rather than floored: a negative count is malformed, and
      // flooring it to zero would price an allocation that is about to
      // happen at nothing.
      // SAFETY: `frame` is live; `ch_layout.nb_channels` is a plain
      // `c_int`.
      let channels = unsafe { (*frame).ch_layout.nb_channels };
      if channels <= 0 {
        return -(libc::EINVAL);
      }
      crate::footprint::audio_frame_bytes(format_raw, nb_samples as usize, channels as usize)
    } else {
      // Neither geometry nor samples: nothing is being allocated that
      // this seat can price, and nothing is claimed.
      Some(0)
    };

    // **The refusal leaves its reason behind.** A `get_buffer2`
    // callback can only answer libavcodec with an errno, and
    // `AVERROR(EINVAL)` is also what libavcodec reports for corrupt
    // input — so a bare refusal here was indistinguishable from a
    // broken file, and only one of those is worth retrying with a
    // larger ceiling. The decoder funnels collect this the same way
    // they collect the `get_format` declination.
    let record = |bytes: u64| {
      use core::sync::atomic::Ordering;
      // SAFETY: `state` was proved non-null above.
      unsafe {
        (*state)
          .declined_frame_bytes
          .store(bytes, Ordering::Relaxed);
        (*state)
          .declined_frame_audio
          .store(width <= 0 && height <= 0, Ordering::Relaxed);
        (*state)
          .frame_budget_declined
          .store(true, Ordering::Release);
      }
      -(libc::EINVAL)
    };
    match priced {
      // Fail closed. An allocation whose size cannot be established is
      // not a small one — the same stance every other judge here takes.
      // Reported as an unbounded cost, which is what an unprovable one
      // is.
      None => return record(u64::MAX),
      // Nothing to buy, so nothing to refuse.
      Some(0) => {}
      // A budget of zero admits nothing, and this is the arm that used
      // to be a skipped guard.
      Some(bytes) if byte_ceiling == 0 => return record(bytes as u64),
      Some(bytes) if bytes as u128 > byte_ceiling => return record(bytes as u64),
      Some(_) => {}
    }
  }

  // SAFETY: delegating to the allocator libavcodec would have used.
  unsafe { ffmpeg_next::ffi::avcodec_default_get_buffer2(ctx, frame, flags) }
}

/// Prices the CPU frame `av_hwframe_transfer_data` would allocate, and
/// refuses it if it is over the ceiling — **before** the transfer runs.
///
/// # Why the hardware road needs its own seat
///
/// [`judge_buffer`] is not a universal choke point, and the census says
/// so on this machine. `ff_get_buffer` calls `hwaccel->alloc_frame`
/// directly and never reaches `get_buffer2` at all: a VideoToolbox
/// h264 decode of a 160x120 clip records **zero** `get_buffer2` calls
/// while producing a hardware frame. And the CPU destination of a
/// download is allocated by `av_hwframe_transfer_data` itself, outside
/// both hooks.
///
/// # What the census settled about the surface itself
///
/// `max_pixels` **does** bite before `alloc_frame`, and this was
/// measured rather than assumed: with `max_pixels = 100`, a 160x120
/// VideoToolbox h264 decode fails at `avcodec_open2` with
/// `Picture size 160x120 exceeds specified max pixel count 100` from
/// `av_image_check_size2`, zero `get_buffer2` calls and no frame. The
/// check lives in `ff_set_dimensions`, which every decoder runs when it
/// learns its dimensions and before any surface pool exists — so the
/// seat `max_pixels` already occupies covers the hardware surface too.
///
/// The residual on that road is the aligned-dimensions gap
/// [`judge_buffer`] closes for software frames, and it applies to
/// **driver-owned GPU memory** rather than to anything this crate
/// carries. What this crate does carry off the hardware road is the CPU
/// frame downloaded here, and that is bounded exactly, by this
/// function.
///
/// # How the price is taken
///
/// The destination format is not chosen by this crate: `dst.format` is
/// `AV_PIX_FMT_NONE` on entry and FFmpeg picks from
/// `av_hwframe_transfer_get_formats`. So the whole candidate list is
/// priced and the **worst** taken — walked as `*const c_int` through
/// the shim, because a driver may offer a format this build's bindings
/// do not name, which is the same discipline the pixel census keeps.
///
/// When the list cannot be obtained the global worst rate stands in;
/// over-refusing is the safe direction for a ceiling.
///
/// # Safety
///
/// `hw_frame` must be a live `*const AVFrame`.
unsafe fn judge_hw_transfer(
  hw_frame: *const ffmpeg_next::ffi::AVFrame,
  limits: crate::FrameLimits,
) -> std::result::Result<(), crate::error::HwTransferTooLarge> {
  // SAFETY: `hw_frame` is live per the contract; the field is a plain
  // pointer.
  let frames_ctx = unsafe { (*hw_frame).hw_frames_ctx };

  // **The allocated extent, not the displayed one.** `AVFrame.width` /
  // `.height` are the *display* dims; what
  // `av_hwframe_transfer_data` allocates is sized from the frames
  // context, and on a cropped stream the two diverge by orders of
  // magnitude — measured on this build, an h264 stream with SPS
  // cropping shows 32x32 display over a 1920x1088 coded surface, a
  // 2040x gap. This crate already had a helper that reads the pool
  // dims, with a doc comment naming this exact trap; the first version
  // of this judge reached past it for `AVFrame.width` anyway.
  //
  // **Fail closed.** No context, no dims, or no priceable candidate
  // means the allocation extent cannot be proved — and an unprovable
  // extent is not a small one. The same stance
  // `estimate_transfer_bytes` takes next door, and for the same reason:
  // falling back to display dims here would restore precisely the hole
  // this judge exists to close.
  if frames_ctx.is_null() {
    // Not a hardware frame at all. `av_hwframe_transfer_data` refuses
    // such a source with `EINVAL` and allocates nothing, so there is no
    // extent to bound here — and answering "too large" would put a
    // ceiling's name on a completely different fault. The existing path
    // reports it accurately.
    return Ok(());
  }
  let Some((width, height)) = (unsafe { hw_frames_ctx_dimensions_raw(hw_frame) }) else {
    // A hardware frame whose pool extent cannot be read. The transfer
    // may well allocate; nothing here can say how much. Charged as
    // unbounded, which is what an unprovable extent is.
    return Err(crate::error::HwTransferTooLarge::new(
      usize::MAX,
      limits.max_frame_bytes(),
    ));
  };

  // **Every candidate folded in, priceable or not.**
  //
  // FFmpeg picks the destination format from this list; this crate does
  // not get to choose. So the bound has to be the maximum over the
  // *whole* list — and the fold used to skip the members libavutil
  // would not size, updating `worst` only on priceable ones and
  // reaching for a fallback only when *nothing* priced. A list holding
  // one cheap priceable format beside one unpriceable format was
  // therefore judged at the cheap price, while FFmpeg remained free to
  // select the one that was ignored.
  //
  // An unpriceable candidate is charged
  // [`crate::footprint::video_frame_bytes_upper_bound`] instead: the
  // same dimension alignment and per-plane overhead at the widest rate,
  // so it dominates whatever that layout would have cost had it been
  // priceable.
  let mut worst: usize = 0;
  let mut judged_any = false;
  if !frames_ctx.is_null() {
    let mut list: *mut libc::c_int = ptr::null_mut();
    // `AV_HWFRAME_TRANSFER_DIRECTION_FROM` is 0 — passed as the integer
    // it is, like every other open C enum on this road.
    // SAFETY: `frames_ctx` is the frame's live `AVHWFramesContext`
    // reference; on success FFmpeg allocates a NONE-terminated list
    // that the caller frees.
    let rc = unsafe { c_shims::av_hwframe_transfer_get_formats(frames_ctx, 0, &mut list, 0) };
    if rc >= 0 && !list.is_null() {
      let none = ffmpeg_next::ffi::AVPixelFormat::AV_PIX_FMT_NONE as libc::c_int;
      let mut p = list;
      loop {
        // SAFETY: FFmpeg guarantees the list is NONE-terminated; reads
        // up to and including the sentinel are in bounds.
        let candidate = unsafe { ptr::read(p) };
        if candidate == none {
          break;
        }
        // **The allocator's arithmetic, not the payload's.** Pricing
        // `av_image_get_buffer_size` at a fixed alignment is what the
        // pixels weigh laid out tightly — for a 16x16 NV12 destination
        // that is 768 bytes against the 1,792 `av_frame_get_buffer`
        // really takes. See [`crate::footprint`].
        let cost = crate::footprint::video_frame_bytes(candidate, width, height)
          .or_else(|| crate::footprint::video_frame_bytes_upper_bound(width, height));
        match cost {
          Some(size) => {
            worst = worst.max(size);
            judged_any = true;
          }
          // Not even the dimension-only bound could be formed, so the
          // extent itself is not a picture. Nothing here will guess.
          None => {
            // SAFETY: `list` is freed exactly once, on every road out.
            unsafe { ffmpeg_next::ffi::av_freep(ptr::addr_of_mut!(list).cast()) };
            return Err(crate::error::HwTransferTooLarge::new(
              usize::MAX,
              limits.max_frame_bytes(),
            ));
          }
        }
        p = unsafe { p.add(1) };
      }
      // SAFETY: `list` was allocated by `av_hwframe_transfer_get_formats`
      // and is freed exactly once here.
      unsafe { ffmpeg_next::ffi::av_freep(ptr::addr_of_mut!(list).cast()) };
    }
  }

  if !judged_any {
    // An empty list, or a query that failed: no candidate was seen at
    // all. Charge the dimension-only bound over the pool extent, which
    // is the most any format this build can emit could cost there.
    let Some(bound) = crate::footprint::video_frame_bytes_upper_bound(width, height) else {
      return Err(crate::error::HwTransferTooLarge::new(
        usize::MAX,
        limits.max_frame_bytes(),
      ));
    };
    worst = bound;
  }

  if worst > limits.max_frame_bytes() {
    return Err(crate::error::HwTransferTooLarge::new(
      worst,
      limits.max_frame_bytes(),
    ));
  }
  Ok(())
}
/// Reads and clears the coded-surface refusal a `get_format` callback
/// left in its state, if it left one.
///
/// Free-standing rather than a method because the reason has to survive
/// on **every** hardware exit, and one of them — the open-time failure
/// path — runs before a decoder exists to ask.
fn ceiling_declination_of(state: *const CallbackState) -> Option<Error> {
  use core::sync::atomic::Ordering;
  if state.is_null() {
    return None;
  }
  // SAFETY: `state` is the live `CallbackState` the caller owns; it is
  // freed only after the codec context it belongs to.
  let (declined, pixels, limit) = unsafe {
    (
      (*state).ceiling_declined.swap(false, Ordering::Acquire),
      (*state).declined_pixels.load(Ordering::Relaxed),
      (*state).declined_limit.load(Ordering::Relaxed),
    )
  };
  declined.then(|| Error::HwSurfaceTooLarge(crate::error::HwSurfaceTooLarge::new(pixels, limit)))
}
/// The software decoders' error funnel.
///
/// Every road that turns a libavcodec decode failure into an `Error`
/// goes through here, so a frame the allocator judge refused comes back
/// named instead of as the `EINVAL` libavcodec also uses for corrupt
/// input. The hardware roads have their own funnel (`hw_exit`); this is
/// its software twin, and the discipline is the same one: **a consumer
/// added helper-by-helper is lost the next time the surrounding code is
/// restructured, so every exit calls one function.**
///
/// # Safety
///
/// `state` must be null or a live `CallbackState` the caller owns.
pub(crate) fn software_exit(state: *const CallbackState, e: ffmpeg_next::Error) -> Error {
  frame_budget_declination_of(state).unwrap_or(Error::Ffmpeg(e))
}

/// **The software road's only way to read an errno — funnel and
/// classify in one call, because the order between them is a law and
/// laws that depend on remembering get broken.**
///
/// Every receive site used to write the two steps out: funnel, then
/// classify. The R1 report called that ordering load-bearing and
/// explained why — a `get_format` declination or an allocator-judge
/// refusal sits in the callback state waiting to be collected, and a
/// classifier that runs first reads the errno libavcodec reported
/// instead of the refusal this crate made, answering `Ended` or
/// `NeedsInput` for a frame that was declined. Then a restructure
/// reordered one road and the law was simply gone, silently, because
/// nothing enforced it.
///
/// So the classifiers are private to this module now and this is the
/// door. A caller cannot classify a raw error because it cannot reach a
/// classifier; the funnel is not something to remember to call first,
/// it is the only thing there is to call.
///
/// # The verdict is minted once and threaded
///
/// **A funnel consumes what it collects.** `take_ceiling_declination`
/// and `take_frame_budget_declination` both *clear* the latch they
/// read, because a refusal reported twice would be a refusal invented
/// once. That makes the verdict a one-shot value, and the rule that
/// follows is the whole of this invariant:
///
/// > The first funnel on a road mints the verdict. Every later step on
/// > that road **threads it**. A site that re-funnels, or that rebuilds
/// > `Error::Ffmpeg(raw)` after a funnel has run, is the bug class —
/// > the second call finds the latch empty and reports the errno the
/// > substrate happened to give over the refusal this crate made.
///
/// Nothing downstream of a funnel reads the raw errno to decide a route:
/// the hardware roads route on the verdict and on the road's own reading
/// of an unnamed one. What the caller is told is always the verdict.
///
/// # Safety
///
/// `state` must be null or a live [`CallbackState`] the caller owns.
pub(crate) fn software_receive(
  state: *const CallbackState,
  e: ffmpeg_next::Error,
  phase: SessionPhase,
) -> Result<Received> {
  receive_status(software_exit(state, e), phase)
}

/// The send road's half of [`software_receive`]. Same law, same door.
///
/// # Safety
///
/// `state` must be null or a live [`CallbackState`] the caller owns.
pub(crate) fn software_send(
  state: *const CallbackState,
  e: ffmpeg_next::Error,
  phase: SessionPhase,
) -> Result<Sent> {
  send_status(software_exit(state, e), phase)
}

/// Reads and clears a software frame-budget refusal left by
/// [`judge_buffer`], as the named error it deserves.
///
/// The software twin of [`ceiling_declination_of`]: the allocator judge
/// can only answer libavcodec with an errno, so the reason lives in the
/// callback state and every decoder funnel collects it.
pub(crate) fn frame_budget_declination_of(state: *const CallbackState) -> Option<Error> {
  crate::ffi::take_frame_budget_declination(state).map(|(bytes, limit, audio)| {
    Error::FrameBudgetExceeded(crate::error::FrameBudgetExceeded::new(
      bytes,
      limit,
      if audio {
        crate::error::FrameMedium::Audio
      } else {
        crate::error::FrameMedium::Video
      },
    ))
  })
}

/// Proves an opened codec context is a **video** one without going
/// through `Opened::video()`.
///
/// `Opened::video()` calls `Context::medium()`, which reads
/// `AVCodecContext.codec_type` as the bindgen `AVMediaType` enum — a
/// value outside this build's discriminant set is UB the moment it is
/// formed, before any comparison can run. The hardware path has always
/// bypassed that API for this reason; this is that bypass, extracted so
/// the second caller reuses it instead of restating it.
///
/// The caller keeps ownership of `opened` on failure, so its `Drop`
/// still releases the codec context.
pub(crate) fn ensure_video_codec_type(opened: &codec::decoder::Opened) -> Result<()> {
  ensure_codec_type(opened, AVMediaType::AVMEDIA_TYPE_VIDEO)
}

/// The general form: proves an opened context has the medium expected,
/// reading `codec_type` as the integer it is.
///
/// `Opened::{video,audio,subtitle}()` all go through
/// `Context::medium()`, so all three carried the same hazard and all
/// three now come through here.
pub(crate) fn ensure_codec_type(
  opened: &codec::decoder::Opened,
  expected: AVMediaType,
) -> Result<()> {
  // SAFETY: `codec_type` is bound as `AVMediaType` (`#[repr(i32)]`),
  // the same size and alignment as `i32`; reading the bytes as `i32`
  // cannot be UB whatever FFmpeg wrote there.
  let codec_type_int: i32 =
    unsafe { ptr::read(ptr::addr_of!((*opened.as_ptr()).codec_type) as *const i32) };
  if codec_type_int != expected as i32 {
    // The same error `Opened::video()` would have produced, without the
    // enum construction.
    return Err(Error::Ffmpeg(ffmpeg_next::Error::InvalidData));
  }
  Ok(())
}

/// Look up the decoder for `parameters` without going through the bindgen
/// `AVCodecID` Rust enum. Reads the codec_id field as raw `u32` via
/// `addr_of!` + `ptr::read` so a value not in our build's discriminant
/// set never invokes UB.
pub(crate) fn find_decoder(parameters: &codec::Parameters) -> Result<Codec> {
  ensure_parameters_non_null(parameters)?;
  // SAFETY: parameters' inner pointer is non-null (checked above);
  // addr_of! projects to the codec_id field; the *const u32 cast is sound
  // because AVCodecID is `#[repr(u32)]` (same size and alignment as u32).
  // Reading as u32 cannot be UB regardless of the value FFmpeg wrote.
  let raw_id: u32 =
    unsafe { ptr::read(ptr::addr_of!((*parameters.as_ptr()).codec_id) as *const u32) };

  // Call C `avcodec_find_decoder` via our local `c_int`-typed shim — we
  // never construct an `AVCodecID` enum from `raw_id`. The C function
  // returns NULL for unknown ids, which we surface as `Error::NoCodec`.
  // SAFETY: avcodec_find_decoder is a pure FFmpeg lookup; passing any
  // c_int is sound (returns NULL for unknown).
  let codec_ptr = unsafe { c_shims::avcodec_find_decoder(raw_id as libc::c_int) };
  if codec_ptr.is_null() {
    return Err(Error::NoCodec(raw_id));
  }
  // SAFETY: codec_ptr is a non-null *const AVCodec into FFmpeg's static
  // codec table; it lives for the duration of the program.
  Ok(unsafe { Codec::wrap(codec_ptr) })
}

/// Drain output frames from a candidate decoder during probe replay,
/// transferring each one from the candidate's HW context to a fresh CPU
/// frame and queueing it. Returns `Ok(())` once the candidate signals
/// EAGAIN/EOF. The transfer happens while the candidate is still alive
/// (its `AVHWFramesContext` is reachable); the resulting CPU frames remain
/// valid after the candidate is committed because they hold their own
/// buffer references with no dependency on the original device context.
fn drain_into_pending(
  decoder: &mut ffmpeg_next::decoder::Video,
  hw_buf: &mut frame::Video,
  pending: &mut VecDeque<frame::Video>,
  pending_bytes: &mut usize,
  max_bytes: usize,
  frame_limits: crate::FrameLimits,
) -> std::result::Result<(), ffmpeg_next::Error> {
  loop {
    match decoder.receive_frame(hw_buf) {
      Ok(()) => {
        // Pre-transfer cap check: if we are already at or over either cap,
        // the candidate is producing more than we can hold. Treat as an
        // explicit candidate failure so `advance_probe` can try the next
        // backend instead of committing a stream with silently-dropped
        // frames in the middle.
        //
        // TODO: at very large frame sizes (8K HDR P010, > ~96 MiB each)
        // even a single retained frame is significant. Future direction:
        // memmap-backed pending frames (write to a temp file or shared
        // memory segment) so the resident set stays bounded even when the
        // byte cap is raised. Out of scope for now.
        if pending.len() >= MAX_PROBE_PENDING_FRAMES || *pending_bytes >= max_bytes {
          tracing::warn!(
            frames = pending.len(),
            bytes = *pending_bytes,
            max_frames = MAX_PROBE_PENDING_FRAMES,
            max_bytes = max_bytes,
            "hwdecode: probe pending cap reached; failing candidate replay"
          );
          // SAFETY: hw_buf is owned and valid; unref of an empty frame is a no-op.
          unsafe { av_frame_unref(hw_buf.as_mut_ptr()) };
          return Err(ffmpeg_next::Error::Other {
            errno: libc::ENOMEM,
          });
        }
        // Pre-transfer size guard: `av_hwframe_transfer_data` will
        // allocate the CPU buffer based on `hw_buf`'s dimensions. If a
        // single frame's worst-case footprint already pushes past the
        // cap, refuse the candidate **before** allocating so RSS does
        // not spike on a frame we'd immediately drop. Uses a width *
        // height * `WORST_CASE_BYTES_PER_PIXEL` upper bound; the
        // post-transfer accounting via `cpu_frame_bytes` below stays in
        // place as a backstop using the actual stride/format.
        let estimated_bytes = match estimate_transfer_bytes(hw_buf) {
          Some(b) => b,
          None => {
            // SAFETY: AVFrame.width/height are c_int reads.
            let (w, h) = unsafe {
              let raw = hw_buf.as_ptr();
              ((*raw).width, (*raw).height)
            };
            tracing::warn!(
              width = w,
              height = h,
              "hwdecode: HW frame dimensions invalid for sizing; failing candidate replay"
            );
            unsafe { av_frame_unref(hw_buf.as_mut_ptr()) };
            return Err(ffmpeg_next::Error::Other {
              errno: libc::ENOMEM,
            });
          }
        };
        let estimated_total = pending_bytes.saturating_add(estimated_bytes);
        if estimated_total > max_bytes {
          // SAFETY: AVFrame.width/height are c_int reads.
          let (w, h) = unsafe {
            let raw = hw_buf.as_ptr();
            ((*raw).width, (*raw).height)
          };
          tracing::warn!(
            pending_bytes = *pending_bytes,
            estimated_bytes,
            width = w,
            height = h,
            max_bytes = max_bytes,
            "hwdecode: pre-transfer size estimate exceeds cap; \
             refusing candidate replay before allocating CPU frame"
          );
          unsafe { av_frame_unref(hw_buf.as_mut_ptr()) };
          return Err(ffmpeg_next::Error::Other {
            errno: libc::ENOMEM,
          });
        }
        // **The same exact judge, on the replay road.** This site
        // already had a pre-transfer *estimate* (`w * h * 8`) against
        // the probe's own pending budget; that stays, and this adds the
        // frame ceiling itself, priced exactly.
        //
        // The refusal is reported through this function's existing
        // `ffmpeg_next::Error` channel rather than the named arm: every
        // error out of a probe-replay drain is collapsed by the caller
        // into "this candidate failed, try the next backend", so a name
        // has no consumer here. The reason is logged so it is not lost.
        // SAFETY: `hw_buf` holds a live decoded HW frame.
        if let Err(e) = unsafe { judge_hw_transfer(hw_buf.as_ptr(), frame_limits) } {
          tracing::warn!(
            bytes = e.bytes(),
            limit = e.limit(),
            "hwdecode: candidate's hw->cpu transfer would exceed the frame ceiling; \
             refusing the candidate before the download"
          );
          // SAFETY: `hw_buf` is owned and valid.
          unsafe { av_frame_unref(hw_buf.as_mut_ptr()) };
          return Err(ffmpeg_next::Error::Other {
            errno: libc::EINVAL,
          });
        }
        let mut cpu = alloc_av_frame()?;
        // SAFETY: hw_buf is a freshly-decoded HW frame;
        // `av_hwframe_transfer_data` allocates pixel buffers on `cpu`.
        // We use `copy_frame_props_minimal` (only `pts`) instead of
        // `av_frame_copy_props` for the same reason as
        // `transfer_hw_frame`: the public `Frame` API does not expose
        // side data / metadata / opaque refs, so deep-copying them per
        // frame is pure cost and an unbounded allocation source on
        // attacker-controlled streams.
        unsafe {
          let r1 = av_hwframe_transfer_data(cpu.as_mut_ptr(), hw_buf.as_ptr(), 0);
          if r1 < 0 {
            return Err(ffmpeg_next::Error::from(r1));
          }
        }
        // Same post-transfer pix_fmt validation as `transfer_hw_frame`.
        // A driver that picks a CPU format outside our supported set
        // would queue an unusable frame here; later, when
        // `try_pop_pending` hands it to the caller, `Frame::row` /
        // `Frame::as_ptr` would return `None`. Refuse the candidate
        // before the queue grows so probing advances to the next
        // backend instead.
        let cpu_raw_fmt: i32 = unsafe { (*cpu.as_ptr()).format };
        let cpu_pix_fmt = crate::boundary::from_av_pixel_format(cpu_raw_fmt);
        if !crate::frame::is_supported_cpu_pix_fmt(&cpu_pix_fmt) {
          tracing::warn!(
            pix_fmt = cpu_raw_fmt,
            "hwdecode: candidate produced unsupported CPU pix_fmt during \
             probe replay; failing candidate"
          );
          return Err(ffmpeg_next::Error::Other {
            errno: libc::EINVAL,
          });
        }
        let pixel_bytes = match cpu_frame_bytes(&cpu) {
          Some(b) => b,
          None => {
            // Unknown pix_fmt or vertically-flipped layout — we cannot
            // bound this frame's contribution against the byte cap, so up
            // to MAX_PROBE_PENDING_FRAMES of them could exhaust memory.
            // Fail the candidate so probing tries the next backend
            // rather than queueing untracked allocations.
            // SAFETY: AVFrame.format is c_int, safe to read.
            let pix_fmt: i32 = unsafe { (*cpu.as_ptr()).format };
            tracing::warn!(
              pix_fmt,
              "hwdecode: cannot size unknown CPU pix_fmt during replay; failing candidate"
            );
            // cpu drops here.
            return Err(ffmpeg_next::Error::Other {
              errno: libc::ENOMEM,
            });
          }
        };
        // Account for side-data bytes that `av_frame_copy_props`
        // will deep-copy from the source HW frame. HDR streams
        // typically carry mastering display + content light level
        // (~50 bytes) and dynamic HDR metadata (~few hundred bytes);
        // pathological side-data could otherwise quietly bypass the
        // pixel-data byte cap.
        // SAFETY: hw_buf is a valid AVFrame; we read scalar fields
        // and pointer arrays without forming a `&AVFrame`.
        let side_data_bytes = unsafe { sum_side_data_bytes(hw_buf.as_ptr()) };
        let new_total = pending_bytes
          .saturating_add(pixel_bytes)
          .saturating_add(side_data_bytes);
        if new_total > max_bytes {
          tracing::warn!(
            pending_bytes = *pending_bytes,
            pixel_bytes,
            side_data_bytes,
            max_bytes,
            "hwdecode: queueing this frame would exceed byte cap; \
             failing candidate replay"
          );
          // cpu drops here without ever paying a metadata deep copy.
          return Err(ffmpeg_next::Error::Other {
            errno: libc::ENOMEM,
          });
        }
        // Cap check passed — copy AVFrame metadata. SAFETY: cpu and
        // hw_buf are both valid AVFrames we own. On failure (OOM
        // during side-data alloc) we propagate so the probe candidate
        // is treated as failed rather than queueing a frame whose
        // metadata silently disappeared.
        unsafe { copy_frame_props_minimal(cpu.as_mut_ptr(), hw_buf.as_ptr()) }?;
        *pending_bytes = new_total;
        pending.push_back(cpu);
      }
      Err(e) if is_transient(&e) => return Ok(()),
      Err(e) => return Err(e),
    }
  }
}

/// Allocated frame dimensions according to `hw_buf.hw_frames_ctx`.
///
/// Per FFmpeg's `libavutil/hwcontext.c::transfer_data_alloc`, the CPU
/// destination of `av_hwframe_transfer_data` is allocated using
/// `AVHWFramesContext.width / .height` (the *allocated* surface size of
/// the HW pool); only afterwards is `dst->width / dst->height` reset to
/// `src->width / src->height` (the *display* size). For cropped or
/// heavily aligned streams the allocated dims can be much larger than
/// the display dims (e.g. coded 8192×8192 surface with a 100×100
/// display crop), so any byte-cap accounting that uses display dims
/// undercounts by `allocated_height / display_height` and lets the
/// real allocation slip past the cap.
///
/// Returns `None` when no `hw_frames_ctx` is attached or the dimensions
/// are non-positive — the caller treats `None` as "cannot prove
/// allocation extent, fail the candidate."
fn hw_frames_ctx_dimensions(frame: &frame::Video) -> Option<(i32, i32)> {
  // SAFETY: `frame` owns a live `AVFrame` for the call.
  unsafe { hw_frames_ctx_dimensions_raw(frame.as_ptr()) }
}

/// Pointer form of [`hw_frames_ctx_dimensions`], for the judges that
/// hold a raw `AVFrame` rather than a wrapper.
///
/// # Safety
///
/// `raw` must be a live `*const AVFrame`.
unsafe fn hw_frames_ctx_dimensions_raw(raw: *const AVFrame) -> Option<(i32, i32)> {
  // SAFETY: AVFrame.hw_frames_ctx is `*mut AVBufferRef`. When non-null,
  // its `data` field points to an `AVHWFramesContext`. We read `.width`
  // and `.height` (both `c_int`) via field projection — neither field is
  // enum-typed, so no bindgen-enum UB hazard.
  unsafe {
    let hw_ctx_ref = (*raw).hw_frames_ctx;
    if hw_ctx_ref.is_null() {
      return None;
    }
    let data = (*hw_ctx_ref).data;
    if data.is_null() {
      return None;
    }
    let frames_ctx = data as *const AVHWFramesContext;
    let w: i32 = ptr::read(ptr::addr_of!((*frames_ctx).width));
    let h: i32 = ptr::read(ptr::addr_of!((*frames_ctx).height));
    if w <= 0 || h <= 0 {
      return None;
    }
    Some((w, h))
  }
}

/// Conservative upper-bound estimate of the bytes
/// `av_hwframe_transfer_data` will allocate when downloading `hw_buf` to
/// a CPU frame. Used by [`drain_into_pending`] as a pre-transfer guard
/// so a candidate replay can refuse a frame whose footprint would
/// exceed the byte budget *without* first paying the allocation.
///
/// Sizes from `hw_buf.hw_frames_ctx` (the allocated dims used by the
/// FFmpeg transfer path) rather than `AVFrame.width / .height` (display
/// dims). On a cropped stream the two can differ by orders of magnitude
/// and using display dims would let the real allocation slip past the
/// cap.
///
/// Returns `None` when `hw_frames_ctx` is missing or its width/height
/// are non-positive — caller treats as candidate failure since we
/// cannot prove the allocation extent. (A SW source frame on the probe
/// replay path is not expected; we don't fall back to display dims
/// because that's the exact attack the cap is meant to prevent.)
fn estimate_transfer_bytes(hw_buf: &frame::Video) -> Option<usize> {
  let (w, h) = hw_frames_ctx_dimensions(hw_buf)?;
  Some(
    (w as usize)
      .saturating_mul(h as usize)
      .saturating_mul(WORST_CASE_BYTES_PER_PIXEL),
  )
}

/// Exact resident size of a CPU frame: sum of `AVFrame.buf[i].size`
/// across every populated buffer.
///
/// `AVBufferRef.size` is documented as "Size of data in bytes" — the
/// real allocated extent FFmpeg used. Reading it directly handles the
/// cropped/aligned case where `AVFrame.height` (display) is smaller
/// than the underlying allocation height (the `AVHWFramesContext`
/// surface size FFmpeg sized the buffer for); a `linesize *
/// plane_height_for(display_height)` formula would undercount in that
/// case.
///
/// Returns `None` only when `linesize[0]` is negative — FFmpeg's
/// vertically-flipped layout. The crate's safe row accessors
/// ([`crate::Frame::row`] / [`crate::Frame::rows`]) already reject
/// negative-stride frames, so queueing one during probe replay would
/// just delay the failure to the consumer; refusing here lets the
/// probe loop advance to the next backend instead.
fn cpu_frame_bytes(frame: &frame::Video) -> Option<usize> {
  // SAFETY: AVFrame.linesize is `[c_int; 8]`; AVFrame.buf is
  // `[*mut AVBufferRef; 8]`; AVBufferRef.size is `usize`. All are
  // primitive reads / pointer dereferences with no enum interpretation.
  unsafe {
    let raw = frame.as_ptr();
    let first_linesize = (*raw).linesize[0];
    // Vertically-flipped (negative linesize) is the only "unsizeable"
    // case we still surface as `None`; everything else can be exactly
    // measured from buf[i].size.
    if first_linesize < 0 {
      return None;
    }
    let mut total: usize = 0;
    for i in 0..(*raw).buf.len() {
      let buf = (*raw).buf[i];
      if buf.is_null() {
        continue;
      }
      total = total.saturating_add((*buf).size);
    }
    Some(total)
  }
}

#[allow(dead_code)]
fn _assert_send() {
  fn check<T: Send>() {}
  check::<VideoDecoder>();
}

#[cfg(test)]
mod tests;