decdn-cache 0.1.0

//! Error type for the cache engine.

use std::fmt;

use iroh_blobs::Hash;
use thiserror::Error;

/// Errors surfaced by [`crate::CacheEngine`].
///
/// A variant that wraps a cause names only itself in `Display` and returns the
/// cause from [`std::error::Error::source`]. A `.chain()` walk therefore reaches
/// every cause under a `CacheError`, such as a local disk fault that
/// `decdn_client::classify` looks for, and a chain formatter (`{:#}` or `{:?}` on an
/// `anyhow::Error`) names each cause once. Log a bare `CacheError` with
/// [`CacheError::display_chain`], not with plain `Display`.
#[derive(Debug, Error)]
pub enum CacheError {
    /// The requested blob was not found locally and the configured origin
    /// returned a not-found response.
    #[error("blob {hash} not found in cache or at origin")]
    NotFound {
        /// The hash that was requested.
        hash: Hash,
    },

    /// The origin returned bytes whose BLAKE3 hash did not match what was
    /// requested. The bytes are rejected and are *not* inserted into the store.
    /// A whole-stream failure: the assembled bytes hashed to `actual`, not the
    /// requested `expected`. A mid-stream bao chunk-group verify failure surfaces
    /// as [`Self::VerifyFailed`] instead, so `actual` here is always a real hash.
    #[error("origin returned blob {actual} when {expected} was requested")]
    HashMismatch {
        /// The hash the caller asked for.
        expected: Hash,
        /// The hash the origin's bytes actually produced.
        actual: Hash,
    },

    /// A teed bao verified-stream (#856, ADR 038) failed to decode against the
    /// content root: an interior chunk group did not verify — a corrupt or lying
    /// upstream. Distinct from [`Self::HashMismatch`], which is a whole-stream
    /// hash mismatch; here the failure is at an interior group, so there is no
    /// meaningful whole-blob "actual" hash to report (#915). The bytes are
    /// rejected and are *not* inserted into the store.
    #[error("origin bytes failed bao verification against {expected}")]
    VerifyFailed {
        /// The hash the caller asked for (the content root the stream failed
        /// to verify against).
        expected: Hash,
    },

    /// The origin returned a blob larger than the configured
    /// `max_blob_size_mb`.
    #[error("blob {hash} exceeds max_blob_size of {limit_bytes} bytes")]
    BlobTooLarge {
        /// The hash that was requested.
        hash: Hash,
        /// The configured ceiling, in bytes.
        limit_bytes: u64,
    },

    /// The cache engine was asked to pull a miss but no origin was configured.
    #[error("cache miss for {hash} but no origin is configured")]
    NoOrigin {
        /// The hash that was requested.
        hash: Hash,
    },

    /// The origin backend failed (network error, non-200 status, etc).
    #[error("origin error fetching {hash}")]
    OriginError {
        /// The hash that was requested.
        hash: Hash,
        /// Underlying error from the origin backend.
        #[source]
        source: anyhow::Error,
    },

    /// This node's own iroh-blobs store failed, or a store operation asked for
    /// bytes the store does not hold. Always a local fault: the bytes a sender
    /// feeds into an admit failing to arrive is [`Self::Feed`], and bytes that
    /// fail to verify are [`Self::VerifyFailed`].
    #[error("store error")]
    Store(#[source] anyhow::Error),

    /// The bao stream fed into an admit ended early or failed to read, or its
    /// claimed size cannot belong to the hash. A fault of the sender's delivery,
    /// not of this node's store.
    #[error("admit feed failed")]
    Feed(#[source] anyhow::Error),

    /// A code bug in the cache: a broken internal invariant or a caught panic.
    /// Never an origin fault and never a store fault, so an operator reads it
    /// as a bug to report, not as a backend to check.
    #[error("internal fault")]
    Internal(#[source] anyhow::Error),

    /// The local evicted-hash set is full. Hard cap on the number of
    /// distinct hashes the operator may evict in a single cache lifetime
    /// (issue #279) — protects the in-memory `HashSet` and the on-disk
    /// `evicted.log` from unbounded growth under e.g. an automation
    /// gone-wrong that mass-evicts on every request. Hitting this is
    /// well outside normal usage; the operator should investigate the
    /// caller before raising the cap.
    #[error("evicted-hash set full ({limit} entries); refusing to add more")]
    EvictionLimitExceeded {
        /// The cap that was reached.
        limit: usize,
    },
}

impl CacheError {
    /// Render this error and every cause under it on one line
    /// (`store error: context: root cause`). Use it wherever a bare
    /// `CacheError` reaches a log line or an operator-facing message.
    pub fn display_chain(&self) -> ErrorChain<'_> {
        ErrorChain::new(self)
    }

    /// Walk the source chain of [`Self::OriginError`] looking for an
    /// [`OriginError`]. Returns `Some` when the failure originated in
    /// the HTTP origin's encoding/decompression layer, even after
    /// [`anyhow::Context::with_context`] wrappers were stacked on top —
    /// `anyhow::Error::chain()` exposes every cause in the chain, and
    /// the typed variant we want lives at the root.
    ///
    /// Tests assert on the typed variant via `matches!`; observability
    /// code can switch on the variant rather than parsing `Display`
    /// strings. Returns `None` for non-origin failures or origin failures
    /// that didn't come from `HttpOrigin`'s decoder.
    pub fn origin_error_kind(&self) -> Option<&OriginError> {
        let Self::OriginError { source, .. } = self else {
            return None;
        };
        source
            .chain()
            .find_map(|cause| cause.downcast_ref::<OriginError>())
    }
}

/// `Content-Encoding` values [`crate::origin::HttpOrigin`] knows how to
/// decode. Used as the `encoding` field of
/// [`OriginError::DecompressionFailed`] so that variant is structurally
/// reachable only from genuine decoder failures — making
/// `DecompressionFailed { encoding: "lol", … }` unrepresentable by
/// construction.
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
pub enum SupportedEncoding {
    /// `Content-Encoding: gzip` (and the legacy `x-gzip` alias).
    Gzip,
    /// `Content-Encoding: zstd`.
    Zstd,
}

impl fmt::Display for SupportedEncoding {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        match self {
            Self::Gzip => f.write_str("gzip"),
            Self::Zstd => f.write_str("zstd"),
        }
    }
}

/// Errors raised by the [`crate::origin::HttpOrigin`] decompression layer.
///
/// Surfaced to the engine through the `anyhow::Error` source on
/// [`CacheError::OriginError`] (the `Origin` trait returns `anyhow::Result`).
/// Use [`CacheError::origin_error_kind`] to recover the typed variant —
/// the chain walk works through any number of `with_context` wrappers.
#[derive(Debug, Error)]
pub enum OriginError {
    /// The response advertised a `Content-Encoding` value the origin layer
    /// does not understand. We refuse to silently pass the bytes through —
    /// the BLAKE3 verify in the engine would fail, but with a confusing
    /// "hash mismatch" message rather than a precise "your origin is using
    /// an encoding I don't support" message.
    ///
    /// `encoding` is `Box<str>` rather than `String` to keep the variant
    /// 16 bytes on a 64-bit target — error enums get cloned/passed
    /// around the chain a lot, and the underlying buffer is never
    /// resized after construction.
    #[error("unsupported Content-Encoding: {encoding}")]
    UnsupportedEncoding {
        /// The raw header value the origin sent.
        encoding: Box<str>,
    },

    /// The response carried a non-ASCII `Content-Encoding` byte sequence
    /// that we refuse to interpret. RFC 9110 § 5.6.7 restricts coding
    /// names to ASCII tokens; treating an arbitrary byte sequence as
    /// "no encoding" would silently pass through what may be a real
    /// (mis-cased / mis-typed) compression directive.
    #[error("malformed Content-Encoding header (non-ASCII or unprintable)")]
    MalformedEncoding,

    /// Decompression of the response body failed mid-stream. Either the
    /// origin sent a corrupt payload or an encoding mismatch slipped past
    /// the header check (e.g. `Content-Encoding: gzip` but the bytes are
    /// actually zstd). The `encoding` field is typed (closed set) so
    /// nonsense values are unrepresentable — this variant is reachable
    /// only from the gzip and zstd code paths.
    #[error("failed to decompress {encoding} response body")]
    DecompressionFailed {
        /// The supported encoding whose decoder rejected the body.
        encoding: SupportedEncoding,
        /// Underlying decoder error.
        #[source]
        source: std::io::Error,
    },
}

/// Renders any error and each [`std::error::Error::source`] under it on one
/// line, joined by `": "`. Built by [`CacheError::display_chain`], or by
/// [`ErrorChain::new`] for any other error.
///
/// Each cause appears once only when every link leaves its `source()` out of
/// its own `Display`, as [`CacheError`] does. A link that repeats its source in
/// `Display` repeats that text here.
#[derive(Debug, Clone, Copy)]
pub struct ErrorChain<'a>(&'a (dyn std::error::Error + 'static));

impl<'a> ErrorChain<'a> {
    /// Wrap `err` for one-line chain rendering.
    pub const fn new(err: &'a (dyn std::error::Error + 'static)) -> Self {
        Self(err)
    }
}

impl fmt::Display for ErrorChain<'_> {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        write!(f, "{}", self.0)?;
        let mut cause = self.0.source();
        while let Some(err) = cause {
            write!(f, ": {err}")?;
            cause = err.source();
        }
        Ok(())
    }
}

/// Convenience result alias.
pub type CacheResult<T> = std::result::Result<T, CacheError>;

/// Outcome of a single [`crate::origin::Origin::fetch`] call, classified by
/// whether a retry is appropriate. The cache engine's retry loop drives off
/// this distinction — `Transient` failures are retried up to
/// [`crate::RetryPolicy::max_retries`], `Permanent` failures surface to the
/// caller immediately.
///
/// Both variants carry an [`anyhow::Error`] so adapters keep their
/// `with_context` chains intact. The retry loop, the circuit breaker and the
/// presence probe read the classification. A caller that surfaces the failure
/// drops it with [`Self::into_inner`] and returns the cause as
/// [`CacheError::OriginError`].
#[derive(Debug, Error)]
pub enum OriginPullError {
    /// A retry-eligible failure: HTTP 5xx/408/429, connect/headers/chunk
    /// timeouts, transport-level connection resets, or transient I/O on the
    /// filesystem origin (`Interrupted`, `TimedOut`, `ResourceBusy`, `WouldBlock`).
    #[error(transparent)]
    Transient(anyhow::Error),

    /// A failure that won't be cured by retrying: HTTP 4xx (other than 404,
    /// which maps to [`crate::origin::OriginFetch::NotFound`]),
    /// encoding/decompression problems, size-cap breaches, symlink-escape
    /// or permission-denied on the filesystem origin.
    #[error(transparent)]
    Permanent(anyhow::Error),
}

impl OriginPullError {
    /// True when the variant is `Transient` — the retry loop tests this in
    /// the hot path; keep it `const` so it inlines cleanly.
    pub const fn is_transient(&self) -> bool {
        matches!(self, Self::Transient(_))
    }

    /// Unwrap the underlying `anyhow::Error`, discarding the
    /// transient/permanent classification. Used by the cache engine when
    /// collapsing into [`CacheError::OriginError`] after retries are
    /// exhausted (or on the first permanent failure).
    pub fn into_inner(self) -> anyhow::Error {
        match self {
            Self::Transient(e) | Self::Permanent(e) => e,
        }
    }

    /// Apply `f` to the inner `anyhow::Error` while preserving the
    /// transient/permanent classification. Lets adapters stack
    /// `with_context` (or any other error transform) onto a typed error
    /// without flipping it to the wrong variant.
    pub(crate) fn map_inner<F>(self, f: F) -> Self
    where
        F: FnOnce(anyhow::Error) -> anyhow::Error,
    {
        match self {
            Self::Transient(e) => Self::Transient(f(e)),
            Self::Permanent(e) => Self::Permanent(f(e)),
        }
    }
}

#[cfg(test)]
mod tests;