What causes "snappy writer should not fail to finish" in vectordotdev/vector?

Report this as a bug to Vector with the batch/config that triggered it; this is an internal invariant.

snappy writer should not fail to finish

Error message

snappy writer should not fail to finish

What it means

This panic fires when a Snappy encoder's `finish()` returns an error while flushing the compressed stream in `Compressor::into_inner()`. Snappy's streaming encoder only fails on internal stream-state corruption (e.g. a previous write to the underlying writer failed or the buffer was misused), which the sink treats as an unrecoverable invariant breach rather than an expected I/O condition.

Solutions

Report this as a bug to Vector with the batch/config that triggered it; this is an internal invariant.
Check for earlier logged I/O errors in the same sink task that may have left the encoder in a bad state.
As a workaround, switch the sink's compression to gzip/zstd, which report finalize errors via Result paths.

Defensive patterns

Strategy: try-catch

Try / catch

// Panics are not catchable in normal Rust; guard at the process level.
std::panic::catch_unwind(|| compressor.into_inner())

Prevention

Prefer compression codecs that surface errors via Result (gzip, zstd).
Monitor for prior I/O errors in the sink that may corrupt encoder state.
Pin a stable, tested Vector release.

When it happens

Trigger: Calling `Compressor::into_inner()` (or `finish()`) on a compressor built with `Compression::Snappy` whose internal `SnappyEncoder` reports an error when finalizing the frame stream.

Common situations: Almost never hit in practice; would surface during sink shutdown/batch flush if the underlying BytesMut writer was already in a bad state, or after an upstream bug corrupted the encoder.

Understand the failure class

Background: "This is a bug, please report it": internal invariant violations, unreachable panics, and SNH errors explained — this error's family across 47 libraries.

Related errors

AI-assisted analysis of vectordotdev/vector@bdb87aeaa4 (2026-09-16). Data as JSON: /api/errors/2d215b665d6c39b9. Report an issue: GitHub.

Appendix: source

Thrown at src/sinks/util/compressor.rs:50

    pub fn into_inner(self) -> BytesMut {
        match self {
            Writer::Plain(writer) => writer,
            Writer::Gzip(writer) => writer
                .into_inner()
                .expect("BufWriter writer should not fail to finish")
                .finish()
                .expect("gzip writer should not fail to finish"),
            Writer::Zlib(writer) => writer
                .into_inner()
                .expect("BufWriter writer should not fail to finish")
                .finish()
                .expect("zlib writer should not fail to finish"),
            Writer::Zstd(writer) => writer
                .finish()
                .expect("zstd writer should not fail to finish"),
            Writer::Snappy(writer) => writer
                .finish()
                .expect("snappy writer should not fail to finish"),
        }
        .into_inner()
    }

    pub fn finish(self) -> io::Result<BytesMut> {
        let buf = match self {
            Writer::Plain(writer) => writer,
            Writer::Gzip(writer) => writer.into_inner()?.finish()?,
            Writer::Zlib(writer) => writer.into_inner()?.finish()?,
            Writer::Zstd(writer) => writer.finish()?,
            Writer::Snappy(writer) => writer.finish()?,
        }
        .into_inner();

        Ok(buf)
    }
}

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