mediadecode_ffmpeg/video/

mod.rs

//! `mediadecode::VideoStreamDecoder` impl with HW + SW fallback.
//!
//! [`FfmpegVideoStreamDecoder`] starts on the hardware path: an inner
//! [`crate::VideoDecoder`] that auto-probes VideoToolbox / VAAPI /
//! NVDEC / D3D11VA. When every HW backend fails — at `open` time
//! (no backend opens) or mid-stream ([`crate::Error::AllBackendsFailed`]
//! from `send_packet` / `receive_frame` / `send_eof`) — we transparently
//! fall back to a **software** `ffmpeg::decoder::Video` opened from the
//! same `Parameters`.
//!
//! Two HW-exhaustion shapes feed the same fallback, distinguished by an
//! **explicit origin** the `AllBackendsFailed` carries
//! ([`crate::error::FallbackOrigin`]) — *not* by whether its rescued
//! `unconsumed_packets` is empty (both shapes can be empty: a probe-era
//! failure on the first packet has no prior history, exactly like every
//! post-commit failure):
//!
//! * **Probe-era** (pre-first-frame, [`crate::error::FallbackOrigin::Probe`]):
//!   the inner decoder buffered every packet it consumed and surfaces them in
//!   `unconsumed_packets`. We **replay exactly those** through the SW decoder
//!   (lossless — no frame was delivered yet), then route the still-unconsumed
//!   current packet (the one the inner decoder failed on / refused) to SW
//!   ourselves. This is the original pre-runtime-fallback behaviour and is
//!   unchanged.
//! * **Post-commit** (after the first frame, the inner probe is gone,
//!   [`crate::error::FallbackOrigin::PostCommit`]): a runtime HW-decode failure
//!   — e.g. VideoToolbox choking on H.264 High 4:2:2 10-bit — is reclassified
//!   to `AllBackendsFailed` by the inner decoder with an **empty**
//!   `unconsumed_packets` (the probe buffer no longer exists). Here we
//!   **degrade and continue** rather than reconstruct: open the SW decoder with
//!   an empty replay set and let it **resync at the next keyframe**. Fed forward
//!   packets from the failure point, the SW decoder naturally produces nothing
//!   until that keyframe, then decodes normally from there. The bounded span
//!   from the failure point to the next keyframe is dropped — an accepted,
//!   **loudly logged** gap (a single `tracing::warn!`), not a silent one. The
//!   indexing pipeline this serves prefers a small logged gap over the
//!   error-prone mid-stream-reconstruction state machine a lossless replay
//!   would require (see findit-studio/mediadecode#12). The *bounded*-ness is
//!   **enforced, not assumed**: a post-commit fallback enters a degraded-resync
//!   mode that holds until a **keyframe-anchored** resync — the SW decoder
//!   delivering a frame *after* a keyframe was fed to it across the gap. (Gating
//!   on a keyframe, not on *any* frame, matters because a lenient codec will
//!   decode a lone P-frame from the dropped span into a concealed frame; that
//!   must not count as a resync, or the one-GOP bound isn't truly enforced.) If
//!   EOF is reached while the mode is still pending — no keyframe ever arrived
//!   across the gap and the whole tail was lost — `receive_frame` escalates with
//!   a distinct [`VideoDecodeError::PostCommitNeverResynced`] (and a
//!   `tracing::error!`) rather than surfacing a clean end-of-stream that would
//!   swallow the tail silently. So the gap is either bounded-and-logged (a real
//!   keyframe resync happened) or reported-at-EOF (it never did) — never
//!   silent-and-unbounded.
//!
//!   The post-commit path retains and reconstructs **zero** frames: it opens SW
//!   cold, forwards only the failure arm's current packet (or EOF), and lets SW
//!   resync naturally. It never populates the replay-frame queue, so the
//!   replay/conversion machinery the probe-era path uses cannot touch it.
//!
//! The probe-era replay happens before the new packet (or the next
//! `receive_frame` poll) is processed, so a probe-era HW exhaustion on a
//! non-seekable input loses no compressed data. The post-commit path
//! intentionally accepts the next-keyframe gap.
//!
//! After the transition the decoder stays on SW for the rest of its
//! life — there's no probe-back-to-HW logic; once we've decided the
//! stream isn't HW-decodable, that decision is sticky.
//!
//! Frames produced by either path are converted via
//! [`crate::convert::av_frame_to_video_frame`] so the consumer sees
//! the same `mediadecode::VideoFrame<PixelFormat, VideoFrameExtra,
//! FfmpegBytes>` shape regardless of which backend produced it.

use std::collections::VecDeque;

/// Maximum number of frames the SW fallback replay path will buffer
/// while draining the new SW decoder during packet/EOF replay.
/// Replaying many compressed packets through SW can produce hundreds
/// of decoded frames before the fallback commits; with no cap the
/// resident memory grows unbounded (e.g. 4K frames at ~12 MB each ×
/// 100s of frames). 64 frames is enough room to absorb every
/// realistic codec's reorder/lookahead window without becoming a
/// resource sink.
const SW_REPLAY_FRAME_CAP: usize = 64;

use derive_more::{IsVariant, TryUnwrap, Unwrap};
use ffmpeg_next::{Packet, codec::Parameters, frame};
use mediadecode::{
  Received, Sent, Timebase,
  decoder::{ScaledOutputCapability, VideoStreamDecoder},
  frame::VideoFrame,
  packet::VideoPacket,
};

use crate::{
  Backend, DecoderLimits, Error, Ffmpeg, Frame, VideoDecoder, boundary,
  convert::{self, ConvertError},
  decoder::{build_codec_context, try_clone_parameters},
  error::FallbackFailed,
  extras::{VideoFrameExtra, VideoPacketExtra},
  frame::alloc_av_video_frame,
};

/// Which decode path a video session takes — the choice
/// [`CarrierVideoStreamDecoder::open_as`] is given.
///
/// # The arms differ in what they PERMIT, not only in where they start
///
/// [`Auto`](Self::Auto) is a preference: it starts on hardware and is
/// free to end on software, at open or mid-stream. The other two are
/// **pins**, and a pin that a mid-stream failure could quietly undo
/// would not be one — so a session opened on either of them stays on
/// the path it was opened on for its whole life, and a hardware failure
/// that `Auto` would degrade through is reported instead.
///
/// That is the difference the two consumers of this door need. A
/// determinism comparison decodes *one stream* both ways and compares
/// the pixels; a run that silently swapped paths halfway would compare
/// nothing and say it had. An operator turning hardware off for a lane
/// over a driver that produces wrong pixels needs it to stay off.
///
/// # Observability is unchanged
///
/// [`is_hardware`](CarrierVideoStreamDecoder::is_hardware) and
/// [`is_software`](CarrierVideoStreamDecoder::is_software) read where a
/// session **is**, which stays a live reading — under
/// [`Auto`](Self::Auto) it can still change once, and under the pins it
/// answers what was pinned because nothing can move it.
///
/// This type deliberately grows **no** `is_*` predicates of its own,
/// where most vocabularies in this crate do. They would spell the
/// decoder's two questions a second time with a different meaning —
/// `path.is_software()` is *what was asked for* and
/// `decoder.is_software()` is *where it ended up*, and under
/// [`Auto`](Self::Auto) those genuinely differ. A caller that needs to
/// branch on the choice it made already holds the value and can
/// `match` it.
#[derive(Clone, Copy, Debug, PartialEq, Eq, Hash)]
pub enum DecodePath {
  /// Probe the platform's hardware backends in order and fall back to
  /// software — at open, and again on a mid-stream hardware failure.
  ///
  /// What [`CarrierVideoStreamDecoder::open`] has always done, and what
  /// it still does.
  Auto,
  /// **This hardware backend, or nothing.** No other backend is probed
  /// and software is never opened.
  ///
  /// A backend that cannot be opened for the stream fails the
  /// [`open_as`](CarrierVideoStreamDecoder::open_as) call. A backend
  /// that opens and then fails to decode surfaces
  /// [`Error::AllBackendsFailed`] from the send or receive road that
  /// met it, carrying that backend and what it said — the same error
  /// [`Auto`](Self::Auto) treats as its cue to degrade, reported here
  /// because degrading is what this arm declines.
  Hardware(Backend),
  /// **Software, with no probe at all.**
  ///
  /// Opens `libavcodec`'s own decoder for the stream directly. There is
  /// no hardware in this session to fail, so there is nothing for it to
  /// fall back from — the terminal state [`Auto`](Self::Auto) reaches
  /// by degrading, entered on purpose.
  Software,
}

/// `mediadecode::VideoStreamDecoder` impl with transparent HW → SW
/// fallback.
pub struct CarrierVideoStreamDecoder<C: crate::FfmpegCarrier> {
  state: DecodeState,
  /// The path this session was opened on — see [`DecodePath`].
  ///
  /// Read for exactly one question, [`Self::may_open_software`]: whether
  /// a hardware exhaustion is this session's cue to degrade or its cue
  /// to report. Kept as the whole choice rather than reduced to that
  /// bit so a session can say what it *is*, not only what it allows.
  path: DecodePath,
  /// Codec parameters retained so we can open a software
  /// `ffmpeg::decoder::Video` if the HW probe exhausts.
  parameters: Parameters,
  /// HW-side scratch frame (filled by [`VideoDecoder::receive_frame`]).
  hw_scratch: Frame,
  /// SW-side scratch frame (filled by `ffmpeg::decoder::Video::receive_frame`).
  sw_scratch: frame::Video,
  /// Frames produced while draining the SW decoder during fallback
  /// replay (see [`Self::fall_back_to_sw`]). The trait's
  /// `receive_frame` delivers from this queue before pulling new
  /// frames from the SW decoder. Empty in steady-state operation.
  sw_replay_frames: VecDeque<frame::Video>,
  /// Resource ceilings for the frames this decoder exports, and for the
  /// `AVCodecContext`s it opens — HW candidates, the SW fallback, and
  /// any decoder a later probe advance builds all get the same number.
  limits: DecoderLimits,
  /// `true` once `send_eof` has been called on the active decoder.
  /// Used to propagate EOF to the SW decoder when fallback fires
  /// during the drain phase — without this, codecs that hold tail
  /// frames at EOF would hang waiting for an EOF they already saw on
  /// the HW path.
  eof_sent: bool,
  /// `true` between a **post-commit** fallback firing and a *keyframe-anchored*
  /// resync (the SW decoder delivering a frame **after** a keyframe was fed to
  /// it across the gap). A post-commit fallback opens SW cold and drops the
  /// bounded span up to the next keyframe; the promise is that the span is
  /// *bounded* — SW resyncs at that keyframe. This flag makes the promise
  /// enforced rather than assumed: while it is set we have no proof SW ever
  /// recovered from a real keyframe. It is cleared only when SW delivers a frame
  /// *and* [`Self::degraded_keyframe_seen`] is set (a lone concealed P-frame a
  /// lenient codec emits from the gap does **not** clear it); if EOF is reached
  /// while it is still set the loss is escalated (a distinct loud error) rather
  /// than silently swallowing the whole tail. Probe-era fallbacks never set it —
  /// they replay losslessly and produce frames immediately.
  degraded_resync_pending: bool,
  /// `true` once a **keyframe** packet has been successfully fed to the SW
  /// decoder while [`Self::degraded_resync_pending`] is set — i.e. a real resync
  /// anchor crossed the gap. The pending flag clears only on a delivered SW
  /// frame *after* this is set, so a concealed non-keyframe frame (a lenient
  /// codec decoding a lone P-frame from the dropped span) cannot masquerade as a
  /// resync and prematurely clear the guard. Set alongside `enter`/cleared with
  /// the pending flag.
  degraded_keyframe_seen: bool,
  /// Packets fed to the SW decoder since the post-commit fallback fired while
  /// [`Self::degraded_resync_pending`] is set — i.e. across the unresolved
  /// resync gap. Reported in the escalation message so the lost span is
  /// quantified ("N packets, no keyframe found"). Reset whenever the flag
  /// clears or on `flush`.
  degraded_packets_since_fallback: u64,
  /// Source-stream time base, used to label produced frames.
  time_base: Timebase,
  /// The lane this decoder captures into. A marker: the carrier
  /// appears in the frames it produces, not in its own state.
  /// `true` when the scratch frame holds a decoded frame whose
  /// conversion has **not committed** — see
  /// [`CarrierAudioStreamDecoder::scratch_pending`](crate::audio::CarrierAudioStreamDecoder)
  /// for the reasoning, which is the same on both roads.
  ///
  /// **This decoder has two scratches and can change which one is
  /// current, so the seat is enforced rather than merely recorded.**
  /// While it is set, `send_packet` and `send_eof` answer
  /// [`Sent::MustDrain`]: both are the roads that commit a
  /// hardware-to-software fallback, and a fallback under a parked frame
  /// would leave the retry reading the *other* scratch — delivering a
  /// stale frame, or refusing permanently and stranding a decoded one.
  /// Refusing makes the retry's state the state that parked it **by
  /// construction**, which is a stronger guarantee than remembering
  /// which road produced it.
  ///
  /// **The discipline is unchanged; only its spelling moved.** It was
  /// `VideoDecodeError::FramePending`, and the escape was already
  /// documented as "call `receive_frame`, or `flush` to abandon it" —
  /// which is to say it was back pressure wearing an error's clothes.
  /// Now it says so, and a caller can act on it without inspecting a
  /// backend-specific error type. The subtitle decoder keeps the same
  /// seat one road over, spelled the same way.
  scratch_pending: bool,
  _carrier: core::marker::PhantomData<C>,
}

/// Hardware-decode seam behind [`DecodeState::Hw`]. In production this is
/// the real [`VideoDecoder`]; tests substitute a fake to drive the
/// post-commit fallback path without a live GPU. Mirrors the subset of
/// `VideoDecoder`'s surface the wrapper drives on the HW path.
pub(crate) trait HwInner: Send {
  /// See [`VideoDecoder::send_packet`].
  fn send_packet(&mut self, packet: &Packet) -> Result<Sent, Error>;
  /// See [`VideoDecoder::receive_frame`].
  fn receive_frame(&mut self, frame: &mut Frame) -> Result<Received, Error>;
  /// See [`VideoDecoder::send_eof`].
  fn send_eof(&mut self) -> Result<Sent, Error>;
  /// See [`VideoDecoder::flush`]. Returns `Result` for a uniform seam even
  /// though the inherent method is infallible.
  fn flush(&mut self) -> Result<(), Error>;
  /// Downcast to the concrete [`VideoDecoder`] when this seam is the real
  /// HW decoder, so [`FfmpegVideoStreamDecoder::hardware_inner`] can keep
  /// exposing it. Returns `None` for a test fake.
  fn as_video_decoder(&self) -> Option<&VideoDecoder>;

  /// Whether a packet submitted **now** would be recorded for replay.
  ///
  /// The probe keeps a rescue history so that a decoder which exhausts
  /// every backend can hand the caller everything FFmpeg consumed since
  /// open. It records by `av_packet_ref`, and
  /// [`AllBackendsFailed::into_unconsumed_packets`] hands those
  /// recordings out as owned, **mutable** `Packet`s — which is why the
  /// view lane must not share its carrier's storage into a submission
  /// that could be recorded. See
  /// [`CarrierVideoStreamDecoder::send_packet_impl`].
  fn records_submissions(&self) -> bool;

  /// See [`VideoDecoder::scaled_output_capability`].
  ///
  /// Defaulted to the refusal so a test fake — which has no
  /// VideoToolbox road behind it, and therefore no stage — answers
  /// honestly without having to say so.
  fn scaled_output_capability(&self) -> ScaledOutputCapability {
    ScaledOutputCapability::Unsupported
  }

  /// See [`VideoDecoder::request_scaled_output`]. Defaulted to the
  /// refusal, for the same reason as above.
  fn request_scaled_output(&mut self, size: (u32, u32)) -> ScaledOutputCapability {
    let _ = size;
    ScaledOutputCapability::Unsupported
  }

  /// See [`VideoDecoder::cancel_scaled_output`]. Defaulted to nothing,
  /// because a seat that never accepts a request has none to withdraw.
  fn cancel_scaled_output(&mut self) {}
}

impl HwInner for VideoDecoder {
  #[inline]
  fn records_submissions(&self) -> bool {
    self.is_probing()
  }

  #[inline]
  fn send_packet(&mut self, packet: &Packet) -> Result<Sent, Error> {
    VideoDecoder::send_packet(self, packet)
  }
  #[inline]
  fn receive_frame(&mut self, frame: &mut Frame) -> Result<Received, Error> {
    VideoDecoder::receive_frame(self, frame)
  }
  #[inline]
  fn send_eof(&mut self) -> Result<Sent, Error> {
    VideoDecoder::send_eof(self)
  }
  #[inline]
  fn flush(&mut self) -> Result<(), Error> {
    VideoDecoder::flush(self);
    Ok(())
  }
  #[inline]
  fn as_video_decoder(&self) -> Option<&VideoDecoder> {
    Some(self)
  }
  #[inline]
  fn scaled_output_capability(&self) -> ScaledOutputCapability {
    VideoDecoder::scaled_output_capability(self)
  }
  #[inline]
  fn request_scaled_output(&mut self, size: (u32, u32)) -> ScaledOutputCapability {
    VideoDecoder::request_scaled_output(self, size)
  }
  #[inline]
  fn cancel_scaled_output(&mut self) {
    VideoDecoder::cancel_scaled_output(self);
  }
}

/// Internal: which backend is currently driving the decode.
enum DecodeState {
  /// Hardware-backed decoder (auto-probe). May transition to `Sw` on
  /// `AllBackendsFailed`. Boxed behind [`HwInner`] so tests can inject a
  /// fake HW decoder.
  Hw(Box<dyn HwInner>),
  /// Software decoder. Terminal state.
  Sw(SwDecoder),
}

/// A software decoder and the callback state its codec context points
/// at.
///
/// The state carries the allocator judge's byte budget and the
/// `get_format` declination; it has to outlive the `AVCodecContext`
/// that references it, which is why it is a field here rather than a
/// value dropped at the end of `open_sw_decoder`.
///
/// `Deref` so that every call site keeps talking to the decoder and
/// only the construction changed — this pairing is a lifetime fact, not
/// a new abstraction.
pub(crate) struct SwDecoder {
  decoder: ffmpeg_next::decoder::Video,
  /// Declared **after** the decoder: fields drop in declaration order,
  /// so the codec context is freed before the state it points at.
  _callback_state: Box<crate::ffi::CallbackState>,
}

impl SwDecoder {
  /// The callback state this decoder's codec context points at.
  ///
  /// Handed out as a raw pointer so an error closure can consult it
  /// while the decoder itself is mutably borrowed — every software send
  /// / receive / EOF failure on this road goes through
  /// [`crate::decoder::software_exit`] with it, so a frame the
  /// allocator judge refused surfaces named instead of as the `EINVAL`
  /// libavcodec also uses for corrupt input.
  ///
  /// `Deref` alone was not enough: it exposes the decoder and hides the
  /// state, so every call site kept wrapping raw and the budget refusal
  /// had no way out on the whole software road — including the replay
  /// and cold-fallback helpers, which drop the state when they finish.
  pub(crate) fn state(&self) -> *const crate::ffi::CallbackState {
    &*self._callback_state
  }
}

impl core::ops::Deref for SwDecoder {
  type Target = ffmpeg_next::decoder::Video;
  fn deref(&self) -> &Self::Target {
    &self.decoder
  }
}

impl core::ops::DerefMut for SwDecoder {
  fn deref_mut(&mut self) -> &mut Self::Target {
    &mut self.decoder
  }
}

/// What the cold SW decoder is fed on a **post-commit** degrade transition,
/// named by the failure arm so the three shapes stay mutually exclusive (a
/// current packet and EOF are never forwarded together). The post-commit path
/// retains no replay frames, so this is the *only* thing handed to the new SW
/// decoder at fallback time. See [`FfmpegVideoStreamDecoder::degrade_to_sw`].
enum PostCommitInput<'a> {
  /// `send_packet` arm: forward this current packet — the one the HW decoder
  /// refused (so it was never in any replay set). If it is a keyframe it is the
  /// resync anchor.
  Packet(&'a Packet),
  /// `receive_frame` arm: a frame-time failure has no current packet to forward.
  FrameTime,
  /// `send_eof` arm: EOF was pending on the HW path; re-forward it to the cold
  /// SW so tail-delaying codecs don't hang.
  Eof,
}

impl<C: crate::FfmpegCarrier + crate::CarrierOps> CarrierVideoStreamDecoder<C> {
  /// Opens a decoder for the given codec parameters with the default
  /// HW backend probe order. If the HW probe can't open any backend,
  /// falls back to a software `ffmpeg::decoder::Video` immediately —
  /// `open` only returns `Err` when both paths fail.
  ///
  /// Subsequent mid-stream `AllBackendsFailed` from the HW path
  /// triggers the same SW fallback (with rescued packets replayed).
  ///
  /// `limits` bounds what one decoded frame may cost. It is taken here
  /// rather than through a builder because half of it —
  /// [`DecoderLimits::max_pixels`] — is written into every
  /// `AVCodecContext` this decoder opens, and a context's ceiling
  /// cannot be moved after `avcodec_open2`. That includes the contexts
  /// opened later, by a mid-stream fallback or a probe advance: the
  /// limits are retained for exactly that reason.
  pub(crate) fn open_impl(
    parameters: Parameters,
    time_base: Timebase,
    limits: DecoderLimits,
  ) -> Result<Self, Error> {
    Self::open_as_impl(parameters, time_base, limits, DecodePath::Auto)
  }

  /// [`Self::open_impl`], with the decode path chosen rather than
  /// probed. `DecodePath::Auto` is the constructor above, verbatim.
  pub(crate) fn open_as_impl(
    parameters: Parameters,
    time_base: Timebase,
    limits: DecoderLimits,
    path: DecodePath,
  ) -> Result<Self, Error> {
    // ffmpeg-next's `Parameters` carries an optional `owner: Rc<dyn Any>`
    // (when constructed from `stream.parameters()` it points back at
    // the demuxer's `AVStream`). Upstream marks the type `Send`
    // anyway, which is unsound the moment a non-`None` owner is in
    // play — moving such a value across threads moves the `Rc`. We
    // sidestep this by always storing a deep-cloned `Parameters`
    // (`avcodec_parameters_copy` produces an owner-free copy), so
    // the `FfmpegVideoStreamDecoder`'s `Send` reachability never
    // depends on the caller's owner discipline.
    //
    // Use `try_clone_parameters` instead of `Parameters::clone` —
    // ffmpeg-next's `clone` calls `Parameters::new()` which can
    // return a `Parameters` whose inner pointer is null on OOM
    // (`avcodec_parameters_alloc` returns null without indication);
    // the subsequent `avcodec_parameters_copy` against that null
    // destination is C UB. Our checked helper surfaces the OOM as
    // an error instead.
    let owned_parameters = try_clone_parameters(&parameters, limits.max_codec_parameter_bytes())?;
    let hw_scratch = Frame::empty()?;
    let sw_scratch = alloc_av_video_frame()?;
    let state = match path {
      DecodePath::Auto => match VideoDecoder::open_with_frame_limits_timed(
        try_clone_parameters(&owned_parameters, limits.max_codec_parameter_bytes())?,
        limits,
        time_base,
      ) {
        Ok(hw) => DecodeState::Hw(Box::new(hw)),
        Err(Error::AllBackendsFailed(_)) => {
          // Open-time HW exhaustion: no rescued packets (open didn't
          // see any). Just open SW directly from our owned copy.
          let sw = open_sw_decoder(&owned_parameters, limits, Some(time_base))?;
          DecodeState::Sw(sw)
        }
        Err(other) => return Err(other),
      },
      // **The named backend, and no probe order at all.** Nothing is
      // tried before it and nothing after it, which is what makes the
      // arm a pin: an open that fails is the answer, where `Auto` would
      // have read the same failure as a reason to look elsewhere.
      DecodePath::Hardware(backend) => {
        DecodeState::Hw(Box::new(VideoDecoder::open_with_limits_timed(
          try_clone_parameters(&owned_parameters, limits.max_codec_parameter_bytes())?,
          backend,
          limits,
          time_base,
        )?))
      }
      // The software decoder, opened on purpose rather than reached by
      // degrading. `DecodeState::Sw` is terminal, so this session has
      // nothing to keep it on its path but the shape of the state
      // machine itself.
      DecodePath::Software => {
        DecodeState::Sw(open_sw_decoder(&owned_parameters, limits, Some(time_base))?)
      }
    };
    Ok(Self {
      state,
      path,
      parameters: owned_parameters,
      hw_scratch,
      sw_scratch,
      sw_replay_frames: VecDeque::new(),
      eof_sent: false,
      degraded_resync_pending: false,
      degraded_keyframe_seen: false,
      degraded_packets_since_fallback: 0,
      time_base,
      limits,
      scratch_pending: false,
      _carrier: core::marker::PhantomData,
    })
  }

  /// Returns `true` when this decoder has fallen back to the software
  /// path. `false` while still on the HW probe (the initial state).
  #[cfg_attr(not(tarpaulin), inline(always))]
  pub(crate) const fn is_software_impl(&self) -> bool {
    matches!(self.state, DecodeState::Sw(_))
  }

  /// Returns `true` while the HW probe is still active.
  #[cfg_attr(not(tarpaulin), inline(always))]
  pub(crate) const fn is_hardware_impl(&self) -> bool {
    matches!(self.state, DecodeState::Hw(_))
  }

  /// Whether this session can currently honor a
  /// [`Self::request_scaled_output_impl`] request. See
  /// [`ScaledOutputCapability`] for the determinism trade a caller
  /// takes on by requesting one.
  ///
  /// **[`ScaledOutputCapability::Supported`] on exactly one road: a
  /// live VideoToolbox session on an Apple target.** There, a
  /// `VTPixelTransferSession` sits between the decoded hardware frame
  /// and `av_hwframe_transfer_data` and resizes the `CVPixelBuffer` on
  /// the GPU, so the fitted picture is what crosses to the CPU — see
  /// [`crate::vtscale`] for the design, and
  /// [mediadecode#55](https://github.com/findit-studio/mediadecode/issues/55)
  /// for the ruling that chose it. Everything else answers
  /// `Unsupported`, and each refusal has its own reason rather than a
  /// shared shrug:
  ///
  /// - **A session that has degraded to software.** The stage is the
  ///   hardware road's; this answer follows the session, so it flips to
  ///   `Unsupported` the moment a fallback commits, and a caller that
  ///   asks again learns it.
  /// - **The other hardware backends.** [`Backend::Vaapi`],
  ///   [`Backend::Cuda`] and [`Backend::D3d11va`] are wired in source
  ///   (`Backend::av_hwdevice_type`, `probe_order`) but cannot be
  ///   compiled, run or verified on a non-Linux, non-Windows host, and
  ///   each has a native scaling seam of its own that this crate has
  ///   not built: NVDEC/CUVID in-decode scaling
  ///   ([#56](https://github.com/findit-studio/mediadecode/issues/56)),
  ///   VAAPI VPP
  ///   ([#57](https://github.com/findit-studio/mediadecode/issues/57)),
  ///   the D3D11 Video Processor
  ///   ([#58](https://github.com/findit-studio/mediadecode/issues/58)).
  ///   Filed rather than fabricated.
  /// - **Software.** See [`Self::request_scaled_output_impl`] for the
  ///   software road's own, separate refusal.
  ///
  /// What the VideoToolbox road did **not** get is decode-time
  /// scaling, and the distinction is worth keeping: inter prediction
  /// needs full-resolution reference frames, so every road decodes full
  /// size internally. What this seam saves is the GPU→CPU crossing and
  /// the CPU frame at the end of it — roughly thirtyfold on a 4K stream
  /// fitted to a 512-class box. A caller-owned `VTDecompressionSession`
  /// would save the same crossing and no more, which is why it stays
  /// #55's standing future enhancement rather than this release's work.
  ///
  /// A pure query: calling it requests nothing and changes nothing
  /// about what [`Self::receive_frame`] delivers.
  #[cfg_attr(not(tarpaulin), inline(always))]
  pub(crate) fn scaled_output_capability_impl(&self) -> ScaledOutputCapability {
    match &self.state {
      DecodeState::Hw(hw) => hw.scaled_output_capability(),
      DecodeState::Sw(_) => ScaledOutputCapability::Unsupported,
    }
  }

  /// Requests that this session emit pictures at `size` from the next
  /// frame on. See [`Self::scaled_output_capability_impl`] for which
  /// road can honor it at all.
  ///
  /// # A parked frame refuses
  ///
  /// While [`Self::scratch_pending`] holds a decoded picture whose
  /// conversion did not commit, this refuses. That frame is already
  /// decided — the retry delivers it from the scratch without
  /// consulting the stage — so accepting a new size would promise an
  /// extent the very next frame cannot have. Drain it and ask again;
  /// the same escape every other seat guarded by that flag offers.
  ///
  /// The refusal carries the same meaning as every other: the session
  /// returns to full coded size, so any request already standing is
  /// withdrawn. The parked picture keeps the extent it was decoded at.
  ///
  /// # When a mid-stream request takes effect
  ///
  /// On the **next** picture [`Self::receive_frame`] produces. The
  /// stage is consulted per frame, on the way out of the hardware
  /// decoder and before the GPU→CPU download, so a request never
  /// reaches back to a picture already decoded and never waits longer
  /// than the one being decoded now.
  ///
  /// # The two refusals this seat mints itself
  ///
  /// Neither is an error, and each **returns the session to full coded
  /// size**, dropping any request already standing — what the trait
  /// says this answer means, and the only reading a caller can act on
  /// without risking a second resample of an already-fitted picture:
  ///
  /// - **A zero extent.** A zero-extent picture is not a smaller
  ///   picture.
  /// - **An upscale.** The stage exists to move fewer bytes across the
  ///   GPU→CPU bus; enlarging moves more, and inventing detail the
  ///   decoder did not produce is the caller's business rather than a
  ///   decode session's. An *equal* size is not an upscale: it is
  ///   accepted, and the stage simply has nothing to do.
  ///
  /// # The software road's refusal has its own, different shape
  ///
  /// Worth naming rather than folding into "no backend does this yet":
  /// FFmpeg's software decoders have no *general* decode-time scaling
  /// seam. The one option that comes close — `AVCodecContext.lowres`
  /// (the CLI's `-lowres`) — falls short on three separate counts, any
  /// one of which would disqualify it as this seam's software answer:
  ///
  /// 1. **Narrow codec coverage.** `lowres` is wired only into the
  ///    legacy MPEG-family decoders (MPEG-1/2/4 part 2, H.263) that
  ///    still carry the low-resolution IDCT machinery it depends on.
  ///    HEVC, AV1 and VP9 — the codecs a modern HDR pipeline actually
  ///    decodes — implement no `lowres` support at all.
  /// 2. **The one codec that is wired is broken.** `lowres` on H.264
  ///    (also nominally covered) has been non-functional for years —
  ///    the decoder does not honor it correctly — so even the "old
  ///    family" half of the promise does not hold across the board.
  /// 3. **It is not a resize, it is reduced reconstruction.** Where it
  ///    does work, `lowres` decodes at a coarser IDCT precision
  ///    (`1<<lowres`), skipping reconstruction detail rather than
  ///    decoding in full and scaling the result — later inter frames
  ///    drift from a reference the decoder itself degraded, which is a
  ///    different (and worse) contract than "the same picture, smaller".
  ///
  /// So the software road's answer is not "unimplemented" the way the
  /// other hardware backends' is — it is "full-size decode, then the
  /// fused conform walk downstream", by design, on every codec this
  /// crate decodes in software.
  #[cfg_attr(not(tarpaulin), inline(always))]
  pub(crate) fn request_scaled_output_impl(&mut self, size: (u32, u32)) -> ScaledOutputCapability {
    // **A parked frame is already decided, so it may not be
    // re-promised.** [`Self::scratch_pending`] means a picture came out
    // of the decoder and its conversion did not commit; the retry
    // delivers *that* frame from the scratch without going back through
    // the stage, so it will arrive at whatever extent it already has.
    // Accepting a new size here would answer `Supported` and then hand
    // the caller a frame the new request never touched — the exact
    // silent mismatch [`Self::scaled_output_capability_impl`]'s promise
    // exists to rule out. Refusing changes nothing, which is the
    // contract for a refusal, and the caller's escape is the one this
    // seat already documents everywhere else: drain the frame, then ask
    // again.
    if self.scratch_pending {
      tracing::debug!(
        requested_width = size.0,
        requested_height = size.1,
        "mediadecode-ffmpeg: scaled-output request refused while a decoded frame is parked; \
         the session returns to full size — drain it and ask again"
      );
      // **A refusal means the same thing here as anywhere else.**
      // Returning `Unsupported` while an earlier request stayed armed
      // would leave the caller resampling pictures this session went on
      // fitting — the very double-scale the word exists to prevent. So
      // the standing request goes, and with it what was built for it.
      // The parked picture keeps the extent it was decoded at, which is
      // the same rule an *acceptance* has always carried.
      if let DecodeState::Hw(hw) = &mut self.state {
        hw.cancel_scaled_output();
      }
      return ScaledOutputCapability::Unsupported;
    }
    match &mut self.state {
      DecodeState::Hw(hw) => hw.request_scaled_output(size),
      DecodeState::Sw(_) => ScaledOutputCapability::Unsupported,
    }
  }

  /// Borrow the inner [`VideoDecoder`] when this decoder is still on the
  /// real HW path. Returns `None` after the SW fallback has fired (or, in
  /// tests, when the HW seam is a fake rather than a real decoder).
  #[cfg_attr(not(tarpaulin), inline(always))]
  pub(crate) fn hardware_inner_impl(&self) -> Option<&VideoDecoder> {
    match &self.state {
      DecodeState::Hw(hw) => hw.as_video_decoder(),
      DecodeState::Sw(_) => None,
    }
  }

  /// Returns the time base associated with the source stream.
  #[cfg_attr(not(tarpaulin), inline(always))]
  pub(crate) const fn time_base_impl(&self) -> Timebase {
    self.time_base
  }

  /// Whether this session may open a software decoder in answer to a
  /// hardware exhaustion.
  ///
  /// **The one place the pin is enforced**, consulted by all three
  /// roads that can meet [`Error::AllBackendsFailed`] — the two send
  /// arms and the receive arm. It is one predicate rather than three
  /// conditions because the pin is one promise: a session opened on
  /// [`DecodePath::Hardware`] ends on hardware or ends in an error, and
  /// a road that forgot to ask would break that promise silently,
  /// which is the failure mode a caller cannot see.
  ///
  /// [`DecodePath::Software`] answers `true` and it costs nothing:
  /// `DecodeState::Sw` is terminal, so no hardware exhaustion can
  /// reach a road that asks. Answering for it by state rather than by
  /// pin would make the predicate say something it does not mean.
  #[cfg_attr(not(tarpaulin), inline(always))]
  const fn may_open_software(&self) -> bool {
    !matches!(self.path, DecodePath::Hardware(_))
  }

  /// Internal: **probe-era** transition from HW to SW. Replays the rescued
  /// packets (the inner decoder's buffered history, already accepted by the HW
  /// probe but not yet decoded) through the new SW decoder so the stream resumes
  /// seamlessly. No frame was delivered on the HW path yet, so replaying the
  /// history is lossless.
  ///
  /// Only the probe-era branches drive this. The **post-commit** path does
  /// *not* — it retains and reconstructs zero frames, opening SW cold via
  /// [`Self::degrade_to_sw`] and resyncing at the next keyframe instead of
  /// replaying. (That is why this method's replay/drain machinery — and the
  /// finding that the in-transaction drain doesn't cover later frame
  /// *conversion* — cannot affect the post-commit path: it never produces a
  /// post-commit replay frame to convert.)
  ///
  /// **Transactional**: drained replay frames accumulate in a local
  /// queue; we only commit them to `self.sw_replay_frames` and switch
  /// `self.state` to `Sw` after the replay (and EOF re-forwarding, if
  /// needed) succeed. On failure, the SW decoder, the local frame
  /// queue, and (where reachable) any consumed packets are dropped —
  /// `self` is left in its prior state.
  ///
  /// **EOF-aware**: when EOF was already accepted on the HW path
  /// (`self.eof_sent`), the new SW decoder also receives `send_eof()`
  /// after replay. Without this, codecs that delay tail frames hang
  /// forever in the drain phase.
  ///
  /// **EAGAIN-aware**: if SW's `send_packet` returns EAGAIN during
  /// replay, drain produced frames into the local queue and retry.
  ///
  /// `eof_pending` is passed as a **local** argument rather than read from
  /// `self.eof_sent`: callers must not mutate `self.eof_sent` before this
  /// transaction commits (see [`VideoStreamDecoder::send_eof`]), so the
  /// in-transaction SW EOF re-forward keys off the local flag and `self`'s
  /// EOF state is updated only after a clean commit.
  fn fall_back_to_sw(
    &mut self,
    unconsumed_packets: std::vec::Vec<ffmpeg_next::Packet>,
    eof_pending: bool,
  ) -> Result<(), Error> {
    tracing::info!(
      packets_replayed = unconsumed_packets.len(),
      eof_pending,
      "mediadecode-ffmpeg: HW probe exhausted, falling back to software decode",
    );
    // Wrap the internal worker so any failure path returns the
    // rescued packets to the caller via `Error::FallbackFailed`.
    // Without this, non-seekable streams (live feeds, pipes) would
    // lose every compressed byte the HW path had consumed when a
    // fallback transition fails partway.
    match self.fall_back_to_sw_inner(&unconsumed_packets, eof_pending) {
      Ok(()) => Ok(()),
      Err(source) => Err(Error::FallbackFailed(FallbackFailed::new(
        Box::new(source),
        unconsumed_packets,
      ))),
    }
  }

  /// Worker for [`Self::fall_back_to_sw`]. Returns the rescued packets
  /// untouched on the borrowed slice; the wrapper takes ownership of
  /// them and surfaces them in `FallbackFailed` if this returns Err.
  fn fall_back_to_sw_inner(
    &mut self,
    unconsumed_packets: &[ffmpeg_next::Packet],
    eof_pending: bool,
  ) -> Result<(), Error> {
    let mut sw = open_sw_decoder(&self.parameters, self.limits, Some(self.time_base))?;
    // Bound before the decoder is mutably borrowed, so the error
    // closures below can still consult it.
    let sw_state = sw.state();
    let mut local_replay: VecDeque<frame::Video> = VecDeque::new();
    // Helper: drain SW into the local replay queue, capped at
    // `SW_REPLAY_FRAME_CAP`.
    //
    // Error discipline: stop the drain **only** on the transient
    // backpressure signals EAGAIN / EOF (the decoder has no more output for
    // now). Every other `ffmpeg_next::Error` — e.g. `InvalidData` from a
    // corrupt replayed packet — is a real decode failure and is propagated,
    // so a non-recoverable error surfaces as `FallbackFailed` (carrying the
    // replay packets) instead of being silently swallowed and the fallback
    // committed over corruption.
    fn drain_into(
      sw: &mut ffmpeg_next::decoder::Video,
      state: *const crate::ffi::CallbackState,
      local_replay: &mut VecDeque<frame::Video>,
    ) -> std::result::Result<(), Error> {
      loop {
        let mut tmp = alloc_av_video_frame()?;
        match sw.receive_frame(&mut tmp) {
          Ok(()) => {
            if local_replay.len() >= SW_REPLAY_FRAME_CAP {
              tracing::error!(
                cap = SW_REPLAY_FRAME_CAP,
                "mediadecode-ffmpeg: SW fallback replay produced more frames than the \
                 replay cap allows; aborting fallback (no frames dropped — they're \
                 still in the SW decoder's internal queue and will be released when \
                 it drops)",
              );
              return Err(Error::Ffmpeg(ffmpeg_next::Error::Other {
                errno: libc::ENOMEM,
              }));
            }
            local_replay.push_back(tmp);
          }
          // EAGAIN / EOF: no more output for now — stop draining, success.
          Err(ffmpeg_next::Error::Other { errno }) if errno == ffmpeg_next::error::EAGAIN => {
            break;
          }
          Err(ffmpeg_next::Error::Eof) => break,
          // Any other error is a genuine decode failure on a replayed
          // packet — surface it so it is not masked as a clean fallback.
          Err(other) => return Err(crate::decoder::software_exit(state, other)),
        }
      }
      Ok(())
    }

    for pkt in unconsumed_packets {
      let mut attempts: u32 = 0;
      loop {
        match sw.send_packet(pkt) {
          Ok(()) => break,
          Err(ffmpeg_next::Error::Other { errno }) if errno == ffmpeg_next::error::EAGAIN => {
            drain_into(&mut sw, sw_state, &mut local_replay)?;
            attempts += 1;
            if attempts > 16 {
              return Err(Error::Ffmpeg(ffmpeg_next::Error::Other {
                errno: ffmpeg_next::error::EAGAIN,
              }));
            }
          }
          Err(other) => return Err(crate::decoder::software_exit(sw_state, other)),
        }
      }
    }
    // Re-forward EOF if the HW path already saw it. SW EOF can also
    // return EAGAIN until prior output is drained — mirror the
    // packet-replay loop.
    if eof_pending {
      let mut attempts: u32 = 0;
      loop {
        match sw.send_eof() {
          Ok(()) => break,
          Err(ffmpeg_next::Error::Other { errno }) if errno == ffmpeg_next::error::EAGAIN => {
            drain_into(&mut sw, sw_state, &mut local_replay)?;
            attempts += 1;
            if attempts > 16 {
              return Err(Error::Ffmpeg(ffmpeg_next::Error::Other {
                errno: ffmpeg_next::error::EAGAIN,
              }));
            }
          }
          Err(other) => return Err(crate::decoder::software_exit(sw_state, other)),
        }
      }
    }
    // Final drain BEFORE commit — the transactional commit boundary. The
    // EAGAIN-triggered drains above only fire when SW exerts backpressure mid
    // replay; a SW decoder that ACCEPTS every replayed packet (and the EOF)
    // without one then surfaces a non-transient error — `InvalidData` from a
    // corrupt replayed packet, or any other decode failure — only on the *next*
    // `receive_frame`. Without this drain that error would land after the
    // commit (frames appended, `state` flipped to `Sw`, rescued packets
    // dropped) and reach the caller as a plain decode failure, not
    // `FallbackFailed` — breaking probe-era recovery on non-seekable input.
    // Draining to EAGAIN/EOF here forces any such error to surface now, so it is
    // wrapped as `FallbackFailed` (retaining the rescued packets) and the
    // decoder stays on HW — nothing is committed. (Only the probe-era path
    // reaches this; the post-commit path degrades via `degrade_to_sw` and never
    // replays, so it has no drained frames to commit or convert.)
    drain_into(&mut sw, sw_state, &mut local_replay)?;
    // Commit: only after replay, any EOF forwarding, AND the final drain
    // succeeded do we move the new SW decoder and queue into `self`.
    self.sw_replay_frames.append(&mut local_replay);
    self.state = DecodeState::Sw(sw);
    Ok(())
  }

  /// **Post-commit** degrade-and-continue transition: open the SW decoder
  /// **cold** and forward only the failure-arm's input, retaining and
  /// reconstructing **zero** frames. This is the whole post-commit path: open
  /// SW, forward the current packet (or EOF), degrade-track — nothing is drained
  /// into `sw_replay_frames`, so there is no replayed frame to convert later and
  /// no terminal-drain transaction to reason about. SW naturally produces no
  /// frame until the next keyframe arrives across the gap, then decodes normally;
  /// the failure-point→next-keyframe span is the accepted, logged drop.
  ///
  /// **Transactional (SW-open only)**: `self.state` flips to `Sw` *only after*
  /// `open_sw_decoder` and the input forward succeed. On any failure the new SW
  /// decoder is dropped and the decoder is left on its prior HW state, the error
  /// surfaced as [`Error::FallbackFailed`] (with an empty rescue set — a
  /// post-commit failure never carries unconsumed packets). With no replay-frame
  /// retention there is nothing else to roll back.
  ///
  /// On a clean commit it enters degraded-resync mode (see
  /// [`Self::enter_degraded_resync`]); if the forwarded current packet is itself
  /// a keyframe, the resync anchor is recorded immediately
  /// ([`Self::note_degraded_keyframe`]).
  ///
  /// # `eof_pending`
  ///
  /// Whether the session's end-of-stream has already been **committed**,
  /// and so must be re-forwarded into the cold decoder. Carried as a
  /// local argument for the same two reasons the probe-era road carries
  /// it (see [`Self::fall_back_to_sw`]): it is read from `eof_sent`
  /// before anything is mutated, so a fallback that fails leaves no
  /// half-truth behind — and one question deserves one mechanism on
  /// both fallback roads.
  ///
  /// It is **not** expressed by selecting [`PostCommitInput::Eof`],
  /// even though that arm forwards the same call. That enum is named by
  /// the *failure arm* — which road raised the exhaustion — and the
  /// `warn!` each site emits says so; borrowing the EOF arm for a
  /// frame-time failure would make it lie about where the failure came
  /// from.
  fn degrade_to_sw(&mut self, input: PostCommitInput<'_>, eof_pending: bool) -> Result<(), Error> {
    match self.degrade_to_sw_inner(input, eof_pending) {
      Ok(()) => Ok(()),
      // **A budget refusal is not a fallback failure.** It travels
      // unwrapped, and the spelling was chosen rather than inherited:
      //
      // * `FallbackFailed` means the fallback *machinery* could not
      //   complete, and its contract is to hand back the unconsumed
      //   packets so a caller can re-drive them. On this road that set
      //   is empty by construction — the probe buffer is gone and no
      //   replay frames are retained — so the envelope carries no
      //   recovery affordance at all, only a label.
      // * And the label is the wrong one. Re-driving is the natural
      //   response to a fallback failure, and re-driving a budget
      //   refusal under the same limits refuses identically. Naming it
      //   a fallback failure invites an action that cannot succeed,
      //   while `FrameBudgetExceeded` names the one that can: raise
      //   the ceiling, or accept the refusal.
      //
      // So it keeps the same spelling here as on every other road. One
      // fact, one name.
      Err(budget @ Error::FrameBudgetExceeded(_)) => Err(budget),
      // Everything else really is the machinery failing, and keeps the
      // envelope — empty rescue set and all, which is what a
      // post-commit failure has to hand back.
      Err(source) => Err(Error::FallbackFailed(FallbackFailed::new(
        Box::new(source),
        std::vec::Vec::new(),
      ))),
    }
  }

  /// Worker for [`Self::degrade_to_sw`]. Opens SW cold, forwards the arm's input,
  /// and on success commits + enters degraded-resync mode. Returns `Err` (and
  /// commits nothing) if SW cannot open or the forward fails.
  fn degrade_to_sw_inner(
    &mut self,
    input: PostCommitInput<'_>,
    eof_pending: bool,
  ) -> Result<(), Error> {
    // The invariant [`PostCommitInput`] documents, stated where it can
    // be checked: a current packet and an end-of-stream are never
    // forwarded together. The send road cannot violate it — its own
    // gate refuses every packet once `eof_sent` is committed — so this
    // records the coupling rather than defending against it.
    debug_assert!(
      !(matches!(input, PostCommitInput::Packet(_)) && eof_pending),
      "a current packet and a committed EOF must never be forwarded together",
    );
    let mut sw = open_sw_decoder(&self.parameters, self.limits, Some(self.time_base))?;
    // Captured before the decoder is borrowed for the forward, and
    // before it can be dropped on the error road: this temporary
    // decoder owns the callback state, so a `judge_buffer` refusal
    // recorded during either forward below dies with it unless the
    // reason is collected here. That was the last software road still
    // wrapping libavcodec's `EINVAL` raw.
    let state = sw.state();
    let mut forwarded_keyframe = false;
    let mut forwarded_packet = false;
    match input {
      PostCommitInput::Packet(pkt) => {
        // The HW decoder REFUSED this packet, so it was never decoded; forward
        // it to the cold SW. A failure here surfaces (it is not silently
        // dropped) and rolls back to HW.
        sw.send_packet(pkt)
          .map_err(|e| crate::decoder::software_exit(state, e))?;
        forwarded_keyframe = pkt.is_key();
        forwarded_packet = true;
      }
      // Neither of these forwards a packet; the end-of-stream below is
      // the only thing they can hand the cold decoder.
      PostCommitInput::FrameTime | PostCommitInput::Eof => {}
    }
    // **The end of the stream is re-forwarded here, on every arm that
    // has one, and that is the fix rather than an extra.**
    //
    // The cold decoder knows nothing: it was opened a moment ago, from
    // codec parameters alone. If the session had already been told the
    // stream ended and this new decoder is not, it answers `EAGAIN` to
    // every drain — which reaches the caller as
    // [`Received::NeedsInput`], an instruction to send another packet.
    // On a session whose end is committed there is no legal way to obey
    // that: both send gates refuse. The caller loops, or quietly
    // accepts a truncated tail, until `flush`.
    //
    // It used to be reachable only through the `Eof` failure arm, so
    // the frame-time road — a post-commit exhaustion raised *while
    // draining*, after EOF was accepted — opened cold and stayed cold.
    // A cold decoder has no buffered output, so this cannot answer
    // `EAGAIN` itself.
    if eof_pending {
      sw.send_eof()
        .map_err(|e| crate::decoder::software_exit(state, e))?;
    }
    // Commit: only after a clean open + forward.
    self.state = DecodeState::Sw(sw);
    self.enter_degraded_resync();
    if forwarded_keyframe {
      // The refused current packet was itself the resync anchor.
      self.note_degraded_keyframe(true);
    }
    if forwarded_packet {
      self.count_degraded_packet();
    }
    Ok(())
  }

  /// Enter post-commit degraded mode after a post-commit fallback commits: the
  /// SW decoder opened cold and the span up to the next keyframe is being
  /// dropped. We hold this mode until SW proves a *keyframe-anchored* resync
  /// (a delivered frame after a keyframe was fed — see
  /// [`Self::note_degraded_keyframe`] / [`Self::resync_on_frame`]) and the EOF
  /// escalation in [`VideoStreamDecoder::receive_frame`]. Called only on the
  /// post-commit path, only after a clean commit. Resets the keyframe-seen anchor
  /// and the gap counter.
  #[inline]
  fn enter_degraded_resync(&mut self) {
    self.degraded_resync_pending = true;
    self.degraded_keyframe_seen = false;
    self.degraded_packets_since_fallback = 0;
  }

  /// Record that a packet fed to the SW decoder across an unresolved post-commit
  /// gap was a **keyframe** — the resync anchor. Only a frame delivered *after*
  /// this clears the pending flag, so a lenient codec's concealed P-frame can't
  /// masquerade as a resync. A no-op outside degraded mode, or for a
  /// non-keyframe.
  #[inline]
  fn note_degraded_keyframe(&mut self, is_key: bool) {
    if self.degraded_resync_pending && is_key {
      self.degraded_keyframe_seen = true;
    }
  }

  /// Count one packet fed to the SW decoder while a post-commit resync is still
  /// unproven, so the EOF escalation can quantify the lost tail. A no-op once
  /// SW has resynced (the flag is clear).
  #[inline]
  fn count_degraded_packet(&mut self) {
    if self.degraded_resync_pending {
      self.degraded_packets_since_fallback = self.degraded_packets_since_fallback.saturating_add(1);
    }
  }

  /// A SW frame was delivered. Clear post-commit degraded mode **only if** a
  /// keyframe was fed across the gap ([`Self::degraded_keyframe_seen`]) — that is
  /// a real keyframe-anchored resync, so the dropped span is now the promised
  /// *bounded* gap. A frame delivered with no keyframe yet (a concealed P-frame
  /// from the dropped span) leaves the guard set, so the one-GOP bound stays
  /// enforced and the EOF escalation still fires if no keyframe ever arrives.
  /// Idempotent; a no-op outside degraded mode (steady state, probe-era replay).
  #[inline]
  fn resync_on_frame(&mut self) {
    if self.degraded_resync_pending && self.degraded_keyframe_seen {
      self.clear_degraded_resync();
    }
  }

  /// Unconditionally reset post-commit degraded-mode state. Used where the gap
  /// is moot regardless of resync proof: a `flush` (seek/reset re-anchors the
  /// stream) and the cleanup after an EOF escalation has already fired (so a
  /// follow-up poll sees plain EOF, not a repeated escalation). The
  /// frame-delivery path uses the keyframe-gated [`Self::resync_on_frame`]
  /// instead.
  #[inline]
  fn clear_degraded_resync(&mut self) {
    self.degraded_resync_pending = false;
    self.degraded_keyframe_seen = false;
    self.degraded_packets_since_fallback = 0;
  }

  /// The one place a delivered frame is committed.
  ///
  /// Every road that hands a frame to the caller passes through here —
  /// the hardware scratch, the software scratch, both replay-queue
  /// entries, and the retry of a parked frame — so the bookkeeping a
  /// delivery owes cannot be attached to some of them and forgotten on
  /// others. It was: a parked software frame delivered on the retry
  /// road skipped [`Self::resync_on_frame`], so the last recovered
  /// frame of a degraded stream could leave the resync guard standing
  /// and turn a clean EOF into a false
  /// [`PostCommitNeverResynced`].
  fn commit_delivery(
    &mut self,
    frame: VideoFrame<mediadecode::PixelFormat, VideoFrameExtra, C::Buffer>,
    dst: &mut VideoFrame<mediadecode::PixelFormat, VideoFrameExtra, C::Buffer>,
  ) {
    // The seat is free once a carrier exists for what it held.
    self.scratch_pending = false;
    // A delivered frame is what clears a keyframe-anchored resync. A
    // no-op on every road that never entered degraded mode, which is
    // why it can be unconditional here.
    self.resync_on_frame();
    *dst = frame;
  }

  /// Where this session is. See
  /// [`SessionPhase`](crate::decoder::SessionPhase).
  ///
  /// The wrapper never sees a probe — that lives inside the hardware
  /// seam, which derives its own — so only the committed pair is
  /// reachable from here.
  const fn phase(&self) -> crate::decoder::SessionPhase {
    if self.eof_sent {
      crate::decoder::SessionPhase::Draining
    } else {
      crate::decoder::SessionPhase::Streaming
    }
  }

  /// Reads a drain answer against the session's own committed end.
  ///
  /// Routes a settled end through the post-commit gap check.
  ///
  /// **The `NeedsInput`-past-the-end reading moved out of here.** It
  /// used to be this method's own comparison against `eof_sent` — one
  /// more road deriving the session's phase for itself, which is the
  /// habit [`SessionPhase`](crate::decoder::SessionPhase) ended. The
  /// classifier makes that reading now, for every road at once, and
  /// what is left here is the part that is genuinely this wrapper's:
  /// an end is not clean if a post-commit gap never closed.
  fn settle(&mut self, status: Received) -> Result<Received, VideoDecodeError> {
    match status {
      Received::Ended => self.ended(),
      other => Ok(other),
    }
  }

  /// The end of the stream, read against a post-commit gap that never
  /// closed.
  ///
  /// One place, because there are now two spellings that reach it — the
  /// substrate's `AVERROR_EOF` and a settled [`Received::NeedsInput`]
  /// past a committed end — and a lost tail must escalate on both. The
  /// flag is cleared as it fires so a caller draining to the end sees
  /// the escalation once and the plain end afterwards.
  fn ended(&mut self) -> Result<Received, VideoDecodeError> {
    if !self.degraded_resync_pending {
      return Ok(Received::Ended);
    }
    let packets_lost = self.degraded_packets_since_fallback;
    tracing::error!(
      packets_lost,
      "mediadecode-ffmpeg: post-commit HW->SW fallback never resynced before EOF — \
       {packets_lost} packets fed to the software decoder produced no frame (no \
       keyframe found across the gap); the stream tail from the fallback point was \
       lost",
    );
    self.clear_degraded_resync();
    Err(VideoDecodeError::PostCommitNeverResynced(
      PostCommitNeverResynced::new(packets_lost),
    ))
  }

  /// Internal: convert the active scratch frame into a
  /// `mediadecode::VideoFrame` and write into `dst`.
  fn deliver_frame(
    &mut self,
    dst: &mut VideoFrame<mediadecode::PixelFormat, VideoFrameExtra, C::Buffer>,
  ) -> Result<Received, VideoDecodeError> {
    let av_frame = match &mut self.state {
      DecodeState::Hw(_) => unsafe { self.hw_scratch.as_inner_mut().as_ptr() },
      DecodeState::Sw(_) => unsafe { self.sw_scratch.as_ptr() },
    };
    // SAFETY: the scratch frame is live — either just filled by the
    // inner decoder's `receive_frame`, or left holding a frame whose
    // conversion did not commit. Convert takes what it needs out of it,
    // so the scratch can be reused once this has committed.
    let converted = unsafe {
      convert::av_frame_to_video_frame_as::<C>(av_frame, self.time_base, self.limits.frame())
    };
    match converted {
      Ok(new_frame) => {
        self.commit_delivery(new_frame, dst);
        Ok(Received::Frame)
      }
      Err(e) => {
        // Park only what another attempt could survive.
        self.scratch_pending = e.parks_in_decode();
        Err(VideoDecodeError::Convert(e))
      }
    }
  }
}

#[cfg(test)]
impl<C: crate::FfmpegCarrier + crate::CarrierOps> CarrierVideoStreamDecoder<C> {
  /// Build a decoder around an injected HW seam, bypassing the real probe.
  /// Lets tests drive the post-commit fallback path with a [`HwInner`] fake
  /// instead of a live GPU. The SW fallback still opens the **real**
  /// `ffmpeg::decoder::Video` from `parameters`, so a fallback in these tests
  /// genuinely decodes.
  pub(crate) fn from_hw_inner_for_test(
    hw: Box<dyn HwInner>,
    parameters: Parameters,
    time_base: Timebase,
  ) -> Result<Self, Error> {
    Self::from_hw_inner_for_test_as(hw, parameters, time_base, DecodePath::Auto)
  }

  /// [`Self::from_hw_inner_for_test`], with the session's
  /// [`DecodePath`] named.
  ///
  /// The seam a **pinned** session's mid-stream behaviour is driven
  /// through: a pin's promise is about what happens when the hardware
  /// fails after opening, and the only way to reach that on a machine
  /// whose GPU works is to inject a seam that fails on demand. See
  /// `a_hardware_pin_reports_a_mid_stream_exhaustion_instead_of_degrading`.
  pub(crate) fn from_hw_inner_for_test_as(
    hw: Box<dyn HwInner>,
    parameters: Parameters,
    time_base: Timebase,
    path: DecodePath,
  ) -> Result<Self, Error> {
    let limits = DecoderLimits::default();
    let owned_parameters = try_clone_parameters(&parameters, limits.max_codec_parameter_bytes())?;
    Ok(Self {
      state: DecodeState::Hw(hw),
      path,
      parameters: owned_parameters,
      hw_scratch: Frame::empty()?,
      sw_scratch: alloc_av_video_frame()?,
      sw_replay_frames: VecDeque::new(),
      eof_sent: false,
      degraded_resync_pending: false,
      degraded_keyframe_seen: false,
      degraded_packets_since_fallback: 0,
      time_base,
      limits,
      scratch_pending: false,
      _carrier: core::marker::PhantomData,
    })
  }

  /// Whether `send_eof` has been committed on the active decoder. Lets the
  /// rollback tests assert that a failed EOF fallback restores (never
  /// half-mutates) `eof_sent`.
  pub(crate) const fn eof_sent_for_test(&self) -> bool {
    self.eof_sent
  }

  /// Whether a post-commit fallback is awaiting a keyframe-anchored resync.
  /// Lets the escalation tests observe the degraded-resync state machine.
  pub(crate) const fn degraded_resync_pending_for_test(&self) -> bool {
    self.degraded_resync_pending
  }

  /// Whether a keyframe has been fed to the SW decoder across the unresolved
  /// post-commit gap (the resync anchor). Lets the keyframe-gating test confirm
  /// a concealed P-frame does not set it (so the resync clear stays blocked).
  pub(crate) const fn degraded_keyframe_seen_for_test(&self) -> bool {
    self.degraded_keyframe_seen
  }

  /// Whether the post-commit path retained any replay frames — must always be
  /// empty for a post-commit fallback (it retains zero). Lets the finding-1
  /// dissolution test assert no replay frame was ever queued.
  pub(crate) fn sw_replay_frames_is_empty_for_test(&self) -> bool {
    self.sw_replay_frames.is_empty()
  }

  /// Packets fed to SW across an unresolved post-commit resync gap. Lets the
  /// counter test confirm packets crossing the gap from the `send_packet` arm
  /// are tallied (and cleared on resync).
  pub(crate) const fn degraded_packets_since_fallback_for_test(&self) -> u64 {
    self.degraded_packets_since_fallback
  }
}

impl<C: crate::FfmpegCarrier + crate::CarrierOps> CarrierVideoStreamDecoder<C> {
  /// The fault a submission after end-of-stream earns on this face.
  ///
  /// **Censused from the empty-seat road rather than invented.** With
  /// the seat free, a post-EOF `send_packet` or a repeated `send_eof`
  /// reaches libavcodec, which answers `AVERROR_EOF`, and all four
  /// roads through this wrapper — hardware and software, packet and
  /// EOF — surface it as exactly this value. The gates below short
  /// out to the same one so a parked seat cannot change *which* answer
  /// a caller gets, only how quickly. `the_post_eof_fault_is_the_one_the_substrate_gives`
  /// pins the two against each other.
  ///
  /// Deliberately **not** a new `VideoDecodeError` arm. The subtitle
  /// seam had to mint `AfterEof` because `avcodec_decode_subtitle2` has
  /// no state machine to refuse for it; this face already has an answer
  /// for the condition, and a second spelling for one fault on one
  /// surface is the disease this release is curing.
  fn after_eof() -> VideoDecodeError {
    VideoDecodeError::Decode(Error::Ffmpeg(ffmpeg_next::Error::Eof))
  }

  pub(crate) fn send_packet_impl(
    &mut self,
    packet: &VideoPacket<VideoPacketExtra, C::Buffer>,
  ) -> Result<Sent, VideoDecodeError> {
    // **The end of the stream outranks the parked seat, and the order
    // is the whole point.**
    //
    // `Sent::MustDrain` is a promise: drain the output and this same
    // offer becomes acceptable. Past end-of-stream that promise is
    // false — draining empties the seat and the retry still faults,
    // until `flush`. Checking the seat first made the wrapper answer
    // `MustDrain` for a submission nothing could ever accept, which is
    // the same fault-under-back-pressure inversion the subtitle seam
    // carried: a caller obeying the contract loops, drains, re-offers,
    // and is refused anyway.
    //
    // It is reachable: `send_eof` is accepted and sets `eof_sent`, a
    // delayed tail frame comes out of the decoder, its carrier
    // allocation fails parkably, and the seat is taken on a session
    // that is already over.
    if !self.phase().accepts_input() {
      return Err(Self::after_eof());
    }
    // **Nothing is sent while a frame is parked.** Both send roads can
    // commit a hardware-to-software fallback, and a fallback under a
    // parked frame would leave the retry reading the other scratch. See
    // [`Self::scratch_pending`]. Nothing was consumed, so this is back
    // pressure and the packet is still the caller's to re-offer — which
    // is true precisely because the stream is not over, checked above.
    if self.scratch_pending {
      return Ok(Sent::MustDrain);
    }
    let phase = self.phase();
    // Scoped submission: the rebuilt `AVPacket` never leaves this call,
    // which is what lets the view lane share its buffer with libavcodec
    // rather than copy into it. See `boundary::with_ffmpeg_video_packet`.
    let limits = self.limits.packet_limits();
    // **The route depends on what this decoder does with what it is
    // sent.** While the hardware probe is open it `av_packet_ref`s
    // every accepted packet into a rescue history, and
    // `AllBackendsFailed::into_unconsumed_packets` hands those out as
    // owned, mutable `Packet`s — so a shared body would escape this
    // call as a live mutable alias of a carrier the caller may still be
    // reading. Inside that window the body is copied; once the probe
    // has committed, nothing is recorded and the send is zero-copy
    // again. The software road never records.
    let route = match &self.state {
      DecodeState::Hw(hw) if hw.records_submissions() => crate::carrier::BodyRoute::Copy,
      _ => crate::carrier::BodyRoute::Submission,
    };
    boundary::with_ffmpeg_video_packet::<C, _>(packet, limits, route, |av_pkt| {
      match &mut self.state {
        DecodeState::Hw(hw) => match hw.send_packet(av_pkt) {
          // The seam already classified libavcodec's back pressure, so
          // both states travel on unchanged.
          Ok(status) => Ok(status),
          Err(Error::AllBackendsFailed(p)) => {
            // **A pinned hardware session reports rather than degrades.**
            // See [`Self::may_open_software`]: this is the exhaustion
            // `DecodePath::Auto` reads as its cue to open software, and
            // the pin's whole content is that it is not that cue here.
            // Reported with the payload intact, so the caller keeps the
            // backend, its error, and any rescued packets.
            if !self.may_open_software() {
              return Err(VideoDecodeError::Decode(Error::AllBackendsFailed(p)));
            }
            // Route on the EXPLICIT origin, never on whether `rescued` is empty (a
            // probe-era first-packet cap trip is *also* empty).
            if p.origin().is_post_commit() {
              // Post-commit: DEGRADE AND CONTINUE. No lossless mid-stream
              // reconstruction — the SW decoder opens cold, retains zero replay
              // frames, and resyncs at the next keyframe. The current packet (the
              // one HW REFUSED) is forwarded to that cold SW: if it is the resync
              // keyframe SW decodes from it, otherwise SW drops it until a keyframe
              // arrives. The bounded span from here to that keyframe is dropped — a
              // loudly logged gap (see the `warn!`), not a silent one.
              tracing::warn!(
                backend = ?p.attempts().last().map(|(b, _)| *b),
                pts = ?av_pkt.pts(),
                "mediadecode-ffmpeg: HW decode failed post-commit; falling back to \
                 software, resyncing at next keyframe — a bounded span of frames \
                 may be dropped at this boundary",
              );
              // Transactional SW-open + current-packet forward; degrade-tracking
              // (incl. keyframe-anchor recording) happens inside on a clean commit.
              // A failure surfaces `FallbackFailed` and stays on HW.
              // A clean degrade forwarded this very packet into the
              // cold software decoder, so it was consumed.
              // `false`: this road is unreachable once the end is
              // committed — `send_packet_impl`'s first gate refuses
              // every packet past `eof_sent` — so there is no EOF to
              // re-forward, and forwarding one alongside a packet is
              // the pairing [`PostCommitInput`] forbids.
              return self
                .degrade_to_sw(PostCommitInput::Packet(av_pkt), false)
                .map(|()| Sent::Accepted)
                .map_err(VideoDecodeError::Decode);
            }
            // Probe-era: replay the inner decoder's buffered history (lossless —
            // no frame was delivered yet), then forward the still-unconsumed
            // current packet to SW.
            let rescued = p.into_unconsumed_packets();
            // `eof_pending` is the committed EOF state — never pre-mutated here.
            let eof_pending = self.eof_sent;
            self
              .fall_back_to_sw(rescued, eof_pending)
              .map_err(VideoDecodeError::Decode)?;
            // Forward the new (still-unconsumed) current packet to the
            // freshly-opened SW decoder — the HW decoder REFUSED it, so it was not
            // in the replay set. A failure here surfaces (it is not silently
            // dropped), and back pressure from the fresh decoder is reported as
            // such rather than mistaken for one: the fallback committed either
            // way, and the caller re-offers the packet.
            if let DecodeState::Sw(sw) = &mut self.state {
              let st = sw.state();
              if let Err(e) = sw.send_packet(av_pkt) {
                return crate::decoder::software_send(st, e, phase)
                  .map_err(VideoDecodeError::Decode);
              }
            }
            Ok(Sent::Accepted)
          }
          Err(other) => Err(VideoDecodeError::Decode(other)),
        },
        DecodeState::Sw(sw) => {
          let st = sw.state();
          if let Err(e) = sw.send_packet(av_pkt) {
            // Funnel, then gate. **Nothing below runs on back pressure**,
            // which is the point of returning here rather than falling
            // through: a packet libavcodec did not take must not be
            // counted across the resync gap or recorded as a keyframe
            // anchor, or a caller's honest re-offer would double-count
            // it.
            return crate::decoder::software_send(st, e, phase).map_err(VideoDecodeError::Decode);
          }
          // A keyframe fed across an unresolved post-commit gap is the resync
          // anchor; record it so the next delivered frame can clear the guard.
          self.note_degraded_keyframe(av_pkt.is_key());
          // Count packets crossing an unresolved post-commit resync gap so the
          // escalation at EOF can report how much tail was lost.
          self.count_degraded_packet();
          Ok(Sent::Accepted)
        }
      }
    })
    .map_err(|e| VideoDecodeError::Decode(Error::PacketBuild(e)))?
  }

  pub(crate) fn receive_frame_impl(
    &mut self,
    dst: &mut VideoFrame<mediadecode::PixelFormat, VideoFrameExtra, C::Buffer>,
  ) -> Result<Received, VideoDecodeError> {
    // Deliver any frames produced during SW fallback replay before
    // pulling new ones from the SW decoder. This is the queue
    // populated by `fall_back_to_sw` when SW returned EAGAIN during
    // packet replay — a **probe-era** path only (the post-commit path retains
    // no replay frames), so `resync_on_frame` here is a no-op (probe-era never
    // enters degraded mode).
    // **Peeked, not popped.** A replayed frame is the rescue history's
    // only copy: popping it before the conversion committed lost it to
    // any allocation failure, which is the one thing this queue exists
    // to prevent. It leaves the queue when a carrier exists for it.
    if let Some(replayed) = self.sw_replay_frames.front() {
      // SAFETY: `replayed` is a live AVFrame owned by this queue;
      // convert takes what it needs out of it.
      let converted = unsafe {
        convert::av_frame_to_video_frame_as::<C>(
          replayed.as_ptr(),
          self.time_base,
          self.limits.frame(),
        )
      };
      let new_frame = match converted {
        Ok(new_frame) => new_frame,
        Err(e) if e.parks_in_decode() => return Err(VideoDecodeError::Convert(e)),
        // A frame nothing can carry is dropped rather than re-offered
        // forever — the same rule the scratch seat follows.
        Err(e) => {
          self.sw_replay_frames.pop_front();
          return Err(VideoDecodeError::Convert(e));
        }
      };
      self.sw_replay_frames.pop_front();
      self.commit_delivery(new_frame, dst);
      return Ok(Received::Frame);
    }
    // A frame whose conversion did not commit is converted again before
    // the decoder is asked for another — see [`Self::scratch_pending`].
    // The scratch still holds it, and `deliver_frame` reads whichever
    // scratch the current state uses.
    if self.scratch_pending {
      return self.deliver_frame(dst);
    }
    let phase = self.phase();
    loop {
      match &mut self.state {
        DecodeState::Hw(hw) => match hw.receive_frame(&mut self.hw_scratch) {
          Ok(Received::Frame) => {
            // The frame is out of the decoder's queue from here; the
            // seat is what keeps it if the conversion cannot commit.
            self.scratch_pending = true;
            return self.deliver_frame(dst);
          }
          // The hardware seam already classified the two flow signals.
          // They still pass the session's own end: see [`Self::settle`].
          Ok(status) => return self.settle(status),
          Err(Error::AllBackendsFailed(p)) => {
            // The pin, on the receive road — see
            // [`Self::may_open_software`] and the identical gate on the
            // two send roads.
            if !self.may_open_software() {
              return Err(VideoDecodeError::Decode(Error::AllBackendsFailed(p)));
            }
            // HW exhausted at frame-time. There is no current packet here.
            // Route on the explicit origin.
            if p.origin().is_post_commit() {
              // Post-commit: DEGRADE AND CONTINUE — open SW cold (no current
              // packet to forward, no replay frames retained) and resync at the
              // next keyframe, dropping the bounded span up to it. Loud single
              // `warn!` marks that accepted gap. A clean commit enters degraded
              // mode; a SW-open failure surfaces `FallbackFailed` and stays HW.
              tracing::warn!(
                backend = ?p.attempts().last().map(|(b, _)| *b),
                "mediadecode-ffmpeg: HW decode failed post-commit at frame-time; \
                 falling back to software, resyncing at next keyframe — a bounded \
                 span of frames may be dropped at this boundary",
              );
              // **The committed end travels with the fallback.** Read
              // before anything mutates, exactly as the probe-era road
              // below reads it. Without it the cold decoder answers
              // `EAGAIN` forever on a session no send can feed.
              let eof_pending = self.eof_sent;
              self
                .degrade_to_sw(PostCommitInput::FrameTime, eof_pending)
                .map_err(VideoDecodeError::Decode)?;
              // Nothing to deliver yet — fall through to the loop; the next
              // iteration takes the Sw arm and pulls from the cold SW decoder.
              continue;
            }
            // Probe-era: replay the buffered history (lossless).
            let rescued = p.into_unconsumed_packets();
            // `eof_pending` is the committed EOF state — never pre-mutated here.
            let eof_pending = self.eof_sent;
            self
              .fall_back_to_sw(rescued, eof_pending)
              .map_err(VideoDecodeError::Decode)?;
            // If the replay produced any drained frames, return one
            // immediately — preserves stream order vs. whatever the
            // SW decoder will produce next.
            // **Peeked, not popped** — the second delivery path onto
            // this queue, and it owes the same discipline as the first
            // (see the head of `receive_frame_impl`). The replay queue
            // is the rescue history's only copy of these frames, so a
            // conversion that cannot commit must leave the head where
            // it is rather than advance past it.
            if let Some(replayed) = self.sw_replay_frames.front() {
              // SAFETY: `replayed` is a live AVFrame owned by this
              // queue; convert takes what it needs out of it.
              let converted = unsafe {
                convert::av_frame_to_video_frame_as::<C>(
                  replayed.as_ptr(),
                  self.time_base,
                  self.limits.frame(),
                )
              };
              let new_frame = match converted {
                Ok(new_frame) => new_frame,
                Err(e) if e.parks_in_decode() => return Err(VideoDecodeError::Convert(e)),
                // A frame nothing can carry is dropped rather than
                // re-offered forever.
                Err(e) => {
                  self.sw_replay_frames.pop_front();
                  return Err(VideoDecodeError::Convert(e));
                }
              };
              self.sw_replay_frames.pop_front();
              self.commit_delivery(new_frame, dst);
              return Ok(Received::Frame);
            }
            // Fall through to the loop; next iteration takes the Sw arm.
          }
          Err(other) => return Err(VideoDecodeError::Decode(other)),
        },
        DecodeState::Sw(sw) => {
          // Convert inline (rather than via `deliver_frame`, which borrows all
          // of `self`) so only the disjoint fields `sw_scratch` / `time_base`
          // are touched alongside the `self.state` borrow `sw` holds.
          let st = sw.state();
          match sw.receive_frame(&mut self.sw_scratch) {
            Ok(()) => {
              // The frame is out of the decoder's queue from here; the
              // seat is what keeps it if the conversion cannot commit.
              self.scratch_pending = true;
              // SAFETY: the scratch frame is live (just filled by
              // `receive_frame`); convert takes what it needs out of
              // it, so the scratch can be reused once this commits.
              let converted = unsafe {
                convert::av_frame_to_video_frame_as::<C>(
                  self.sw_scratch.as_ptr(),
                  self.time_base,
                  self.limits.frame(),
                )
              };
              let new_frame = match converted {
                Ok(new_frame) => new_frame,
                Err(e) => {
                  self.scratch_pending = e.parks_in_decode();
                  return Err(VideoDecodeError::Convert(e));
                }
              };
              // SW produced a frame. The commit point clears degraded mode only
              // if a keyframe was fed across the gap — a real keyframe-anchored
              // resync, so the dropped span is the promised bounded gap. A
              // concealed P-frame (no keyframe yet) does not clear it (see
              // `resync_on_frame`).
              self.commit_delivery(new_frame, dst);
              return Ok(Received::Frame);
            }
            // Funnel first — so a recorded budget refusal is named
            // rather than laundered — read as a status second (`EAGAIN`
            // is `NeedsInput`, `Eof` is `Ended`, and the errno stops
            // inside this crate either way), and settled against the
            // session's own end third.
            //
            // That last step is where a post-commit resync that never
            // closed becomes [`VideoDecodeError::PostCommitNeverResynced`]
            // instead of a clean end that would swallow the tail — and
            // it now catches the end however the codec spelled it. See
            // [`Self::settle`] and [`Self::ended`].
            Err(e) => {
              let status =
                crate::decoder::software_receive(st, e, phase).map_err(VideoDecodeError::Decode)?;
              return self.settle(status);
            }
          }
        }
      }
    }
  }

  pub(crate) fn send_eof_impl(&mut self) -> Result<Sent, VideoDecodeError> {
    // The same two gates in the same order, for the same reason: a
    // repeated end-of-stream past a committed one is refused however
    // much is drained, so answering back pressure would be a promise
    // this face cannot keep. See [`Self::after_eof`].
    if !self.phase().accepts_input() {
      return Err(Self::after_eof());
    }
    // As `send_packet`: EOF can commit a fallback too, and the escalation
    // it may raise reads the resync standing a parked frame has not yet
    // had the chance to clear. Nothing was recorded, so drain and signal
    // again.
    if self.scratch_pending {
      return Ok(Sent::MustDrain);
    }
    let phase = self.phase();
    let outcome = match &mut self.state {
      DecodeState::Hw(hw) => match hw.send_eof() {
        // The seam classified libavcodec's back pressure already.
        Ok(status) => Ok(status),
        Err(Error::AllBackendsFailed(p)) => {
          // The pin, on the EOF road — see [`Self::may_open_software`].
          // Returned rather than folded into `outcome`: the commit below
          // fires only on `Ok(Sent::Accepted)`, so the two roads agree,
          // and leaving early keeps the fallback body at the nesting it
          // was written at.
          if !self.may_open_software() {
            return Err(VideoDecodeError::Decode(Error::AllBackendsFailed(p)));
          }
          // EOF is pending for this transaction, so the SW decoder must also
          // receive `send_eof` (codecs that delay tail frames hang otherwise).
          // We pass that intent locally rather than pre-setting `self.eof_sent`:
          // a fallback that fails returns `FallbackFailed` and stays on HW, and a
          // half-mutated `self.eof_sent = true` would then make a *later*
          // fallback inject an EOF into SW even though this `send_eof` errored.
          // `self.eof_sent` is committed only after the whole operation succeeds
          // (the `outcome` check below), keeping the fallback all-or-nothing.
          if p.origin().is_post_commit() {
            // Post-commit: DEGRADE AND CONTINUE — open SW cold, re-forward EOF
            // (no current packet, no replay frames). The cold SW produces no
            // frame from EOF alone, so the drain-to-EOF in `receive_frame`
            // escalates (`PostCommitNeverResynced`) unless a later keyframe-fed
            // poll resyncs first. A clean commit enters degraded mode; a SW-open
            // failure surfaces `FallbackFailed` and stays HW.
            tracing::warn!(
              backend = ?p.attempts().last().map(|(b, _)| *b),
              "mediadecode-ffmpeg: HW decode failed post-commit at EOF; falling \
               back to software — a bounded span of tail frames may be dropped",
            );
            // Both fallback roads forward the EOF inside their own
            // transaction, so a clean commit means it was recorded.
            // `true`: this *is* the end being sent. `eof_sent` is not
            // committed until the whole operation succeeds, so the
            // intent is passed locally rather than read back.
            self
              .degrade_to_sw(PostCommitInput::Eof, true)
              .map(|()| Sent::Accepted)
              .map_err(VideoDecodeError::Decode)
          } else {
            // Probe-era: replay the buffered history (lossless), re-forwarding
            // EOF inside the transaction.
            let rescued = p.into_unconsumed_packets();
            self
              .fall_back_to_sw(rescued, true)
              .map(|()| Sent::Accepted)
              .map_err(VideoDecodeError::Decode)
          }
        }
        Err(other) => Err(VideoDecodeError::Decode(other)),
      },
      DecodeState::Sw(sw) => {
        let st = sw.state();
        match sw.send_eof() {
          Ok(()) => Ok(Sent::Accepted),
          Err(e) => crate::decoder::software_send(st, e, phase).map_err(VideoDecodeError::Decode),
        }
      }
    };
    // Commit EOF state only when the EOF was actually **taken** — a failed
    // fallback left `self.eof_sent` untouched (restored-by-construction: we
    // never mutated it), so HW stays EOF-not-yet-sent and a retry behaves
    // correctly.
    //
    // **`is_ok()` is not the test any more, and that is not a stylistic
    // change.** `Ok(Sent::MustDrain)` means the decoder did not take the
    // end-of-stream; recording `eof_sent` there would make a later fallback
    // inject an EOF into the software decoder for a signal that was never
    // accepted — the exact half-mutation the local `eof_pending` argument
    // exists to prevent on the failure road.
    if matches!(outcome, Ok(Sent::Accepted)) {
      self.eof_sent = true;
    }
    outcome
  }

  pub(crate) fn flush_impl(&mut self) -> Result<(), VideoDecodeError> {
    // Drop any frames buffered during SW fallback replay before
    // flushing the inner decoder — otherwise a seek/reset would
    // surface stale pre-flush frames on the next `receive_frame`.
    self.sw_replay_frames.clear();
    // And a parked frame belongs to the position being abandoned.
    self.scratch_pending = false;
    // Flush ends the drain phase; the decoder accepts new packets
    // after this, so reset EOF tracking.
    self.eof_sent = false;
    // A flush (seek/reset) re-anchors the stream — any in-flight post-commit
    // resync tracking from before the flush is moot. Clear it so the next EOF
    // doesn't escalate over a now-irrelevant pre-flush gap.
    self.clear_degraded_resync();
    match &mut self.state {
      // The HW seam's `flush` returns `Result` for a uniform trait; the
      // real `VideoDecoder::flush` is infallible (always `Ok`).
      DecodeState::Hw(hw) => hw.flush().map_err(VideoDecodeError::Decode)?,
      DecodeState::Sw(sw) => sw.flush(),
    }
    Ok(())
  }
}

macro_rules! video_lane_face {
  ($($lane:ty),+ $(,)?) => { $(
    impl CarrierVideoStreamDecoder<$lane> {
      /// Opens a video decoder for `parameters`, probing hardware
      /// backends in order and falling back to software.
      ///
      /// [`open_as`](Self::open_as)`(.., DecodePath::Auto)`, which is
      /// what this has always done.
      pub fn open(
        parameters: Parameters,
        time_base: Timebase,
        limits: DecoderLimits,
      ) -> Result<Self, Error> {
        Self::open_impl(parameters, time_base, limits)
      }

      /// Opens a video decoder on a **named decode path**.
      ///
      /// [`DecodePath::Auto`] is [`open`](Self::open) exactly; the
      /// other two arms pin the session to hardware or to software for
      /// its whole life. See [`DecodePath`] for what a pin promises and
      /// what it costs.
      ///
      /// Everything else about the session is unchanged — the same
      /// [`VideoStreamDecoder`] face, the same frames, the same
      /// [`is_hardware`](Self::is_hardware) / [`is_software`](Self::is_software)
      /// readings. The choice is *which decoder is behind them*, which
      /// is what a determinism comparison and a deployment policy each
      /// need and neither could reach.
      ///
      /// # Errors
      ///
      /// [`DecodePath::Hardware`] fails here when the named backend
      /// cannot be opened for the stream — where [`DecodePath::Auto`]
      /// would have gone on to software. [`DecodePath::Software`] fails
      /// only where libavcodec has no decoder for the stream, or the
      /// context cannot be built.
      ///
      /// # Examples
      ///
      /// ```no_run
      /// use mediadecode_ffmpeg::{DecodePath, DecoderLimits, FfmpegVideoStreamDecoder};
      /// # fn f(parameters: ffmpeg_next::codec::Parameters, time_base: mediadecode::Timebase)
      /// # -> Result<(), Box<dyn std::error::Error>> {
      /// // The same stream, decoded without a GPU anywhere in the story.
      /// let decoder = FfmpegVideoStreamDecoder::open_as(
      ///   parameters,
      ///   time_base,
      ///   DecoderLimits::default(),
      ///   DecodePath::Software,
      /// )?;
      /// assert!(decoder.is_software());
      /// # Ok(())
      /// # }
      /// ```
      pub fn open_as(
        parameters: Parameters,
        time_base: Timebase,
        limits: DecoderLimits,
        path: DecodePath,
      ) -> Result<Self, Error> {
        Self::open_as_impl(parameters, time_base, limits, path)
      }

      /// Whether this decoder is currently running on software.
      pub const fn is_software(&self) -> bool {
        self.is_software_impl()
      }

      /// Whether this decoder is currently running on hardware.
      pub const fn is_hardware(&self) -> bool {
        self.is_hardware_impl()
      }

      /// Whether this session can currently emit pictures at a
      /// caller-requested output size. See
      /// [`ScaledOutputCapability`] and
      /// [`Self::request_scaled_output`].
      ///
      /// `Supported` on a live VideoToolbox session on an Apple
      /// target, `Unsupported` everywhere else — including on a
      /// session that has degraded to software, which is why this
      /// reads the session's live state rather than a fact recorded
      /// once at open.
      pub fn scaled_output_capability(&self) -> ScaledOutputCapability {
        self.scaled_output_capability_impl()
      }

      /// Requests that this session emit pictures at `size` (width,
      /// height) from the next frame on, and reports whether the
      /// request was recorded. See
      /// [`VideoStreamDecoder::request_scaled_output`] for the full
      /// contract (never an error) and
      /// [`Self::scaled_output_capability`]'s documentation for which
      /// road can honor one, what a mid-stream request means, and the
      /// zero / upscale refusals this seat mints itself.
      pub fn request_scaled_output(&mut self, size: (u32, u32)) -> ScaledOutputCapability {
        self.request_scaled_output_impl(size)
      }

      /// The hardware wrapper, when one is in use.
      pub fn hardware_inner(&self) -> Option<&VideoDecoder> {
        self.hardware_inner_impl()
      }

      /// The stream timebase every produced timestamp is stamped with.
      pub const fn time_base(&self) -> Timebase {
        self.time_base_impl()
      }
    }

    impl VideoStreamDecoder for CarrierVideoStreamDecoder<$lane> {
      type Adapter = Ffmpeg;
      type Buffer = <$lane as crate::FfmpegCarrier>::Buffer;
      type Error = VideoDecodeError;

      fn send_packet(
        &mut self,
        packet: &VideoPacket<VideoPacketExtra, Self::Buffer>,
      ) -> Result<Sent, Self::Error> {
        self.send_packet_impl(packet)
      }

      fn receive_frame(
        &mut self,
        dst: &mut VideoFrame<mediadecode::PixelFormat, VideoFrameExtra, Self::Buffer>,
      ) -> Result<Received, Self::Error> {
        self.receive_frame_impl(dst)
      }

      fn send_eof(&mut self) -> Result<Sent, Self::Error> {
        self.send_eof_impl()
      }

      fn flush(&mut self) -> Result<(), Self::Error> {
        self.flush_impl()
      }

      fn scaled_output_capability(&self) -> ScaledOutputCapability {
        self.scaled_output_capability_impl()
      }

      fn request_scaled_output(&mut self, size: (u32, u32)) -> ScaledOutputCapability {
        self.request_scaled_output_impl(size)
      }
    }
  )+ };
}

video_lane_face!(crate::View, crate::Owned);

fn open_sw_decoder(
  parameters: &Parameters,
  limits: DecoderLimits,
  pkt_timebase: Option<Timebase>,
) -> Result<SwDecoder, Error> {
  // Use the checked codec-context builder — ffmpeg-next's
  // `Context::from_parameters` calls `Context::new()` which doesn't
  // null-check `avcodec_alloc_context3`'s return value before
  // running `avcodec_parameters_to_context` against it. Under
  // memory pressure that's C-level UB; `build_codec_context`
  // surfaces the OOM as an error instead.
  let (ctx, callback_state) = build_codec_context(parameters, limits, pkt_timebase)?;
  // Opened without forming a bindgen enum from FFmpeg memory: the codec
  // is resolved off a raw `codec_id`, and the medium is proved off a raw
  // `codec_type`. See `crate::decoder::ensure_codec_type`.
  let codec = crate::decoder::find_decoder(parameters)?;
  let opened = ctx.decoder().open_as(codec).map_err(Error::Ffmpeg)?;
  crate::decoder::ensure_video_codec_type(&opened)?;
  Ok(SwDecoder {
    decoder: ffmpeg_next::decoder::Video(opened),
    _callback_state: callback_state,
  })
}

/// Payload for [`VideoDecodeError::PostCommitNeverResynced`].
///
/// A **post-commit** HW->SW fallback degraded the stream (dropping the
/// bounded span up to the next keyframe) but the software decoder
/// reached EOF without ever producing a frame — it never resynced, so
/// the entire tail from the failure point was lost. The "bounded,
/// logged gap" the post-commit path promises did not materialise (no
/// keyframe arrived before EOF), so the loss is surfaced loudly here
/// instead of being silently swallowed as a clean end-of-stream.
#[derive(thiserror::Error, Debug)]
#[error(
  "post-commit HW->SW fallback never resynced before EOF: {packets_lost} packets fed to the \
   software decoder produced no frame (no keyframe found across the gap) — the stream tail \
   from the fallback point was lost"
)]
pub struct PostCommitNeverResynced {
  packets_lost: u64,
}

impl PostCommitNeverResynced {
  /// Constructs a `PostCommitNeverResynced` payload.
  #[inline]
  pub const fn new(packets_lost: u64) -> Self {
    Self { packets_lost }
  }
  /// Packets fed to the software decoder across the unresolved resync
  /// gap.
  #[inline]
  pub const fn packets_lost(&self) -> u64 {
    self.packets_lost
  }
}

/// Error type for [`FfmpegVideoStreamDecoder`] — **faults and the
/// send-side refusal**.
///
/// Every arm here is something that went wrong or something the push
/// face declined. The drain's *needs input* and *ended* are
/// [`Received`] states out of `receive_frame`; they used to arrive as
/// `Decode(Ffmpeg(Other { errno: EAGAIN }))` and `Decode(Ffmpeg(Eof))`,
/// which is to say they had no name at this tier at all.
/// [`Self::PostCommitNeverResynced`] is the deliberate exception on the
/// end-of-stream road: it is not "the stream ended", it is "the stream
/// ended and the tail was lost", which is a fault.
///
/// **Open fault taxonomy, so it is `#[non_exhaustive]`.** New ways to
/// fail are discovered — a backend, a ceiling, a corruption a codec
/// learns to report — and a consumer that meets one it has never heard
/// of should take its generic-fault path. That is exactly what the
/// wildcard arm this attribute forces is for. The two status
/// vocabularies opposite it,
/// [`Sent`](mediadecode::Sent) and [`Received`](mediadecode::Received),
/// are exhaustive for the mirror-image reason: their arms are the
/// substrate's fixed state set, and there the wildcard would be dead
/// weight hiding a state a consumer forgot.
#[derive(thiserror::Error, Debug, IsVariant, Unwrap, TryUnwrap)]
#[unwrap(ref, ref_mut)]
#[try_unwrap(ref, ref_mut)]
#[non_exhaustive]
pub enum VideoDecodeError {
  /// The wrapped decoder (HW or SW) reported an error.
  #[error(transparent)]
  Decode(#[from] Error),
  /// Frame conversion from FFmpeg's native types to mediadecode's
  /// types failed.
  #[error(transparent)]
  Convert(#[from] ConvertError),
  /// A **post-commit** HW->SW fallback degraded the stream but the
  /// software decoder reached EOF without ever producing a frame.
  #[error(transparent)]
  PostCommitNeverResynced(#[from] PostCommitNeverResynced),
}

#[cfg(test)]
mod tests;