kopia/kopia · error

decompression error

Error message

decompression error

What it means

deflateCompressor.Decompress wraps a failure from iocopy.JustCopy while copying decompressed bytes from the flate.Reader to the output. The deflate stream was invalid (corrupt data, bad checksum/trailer) or the output write failed. Corrupt input is the most common cause: flate reports unexpected EOF or corrupt data before flagging it as io errors.

Solutions

1. Verify the blob's stored checksum; re-upload/re-fetch if it does not match — corrupt source is the usual cause.
2. Run an independent flate/gzip integrity test (gzip -t / python zlib) on the raw bytes to confirm corruption.
3. Confirm you use the matching wrapper: library's flate reader expects plain deflate stream, not gzip-wrapped data.
4. Check the output writer (disk space, connection) if the unwrapped error is a write error rather than a decode error.

Example fix

// before
err := comp.Decompress(out, bytes.NewReader(blob))
// after
if err := verifyChecksum(blob, expected); err != nil {
    return refetchBlob(ctx, id) // corrupt source
}
err := comp.Decompress(out, bytes.NewReader(blob))

if len(blob) < 4 { return errors.New("blob too short") }
if sum := sha256.Sum256(blob); !bytes.Equal(sum[:], storedChecksum) {
    return errors.New("blob corrupt: checksum mismatch")
}
err := comp.Decompress(out, bytes.NewReader(blob), true)

if err := comp.Decompress(out, r, true); err != nil {
    if strings.Contains(err.Error(), "decompression error") {
        return fmt.Errorf("deflate stream corrupt or output failed: %w", err)
    }
    return err
}

When it happens

Trigger: Calling Decompress on bytes that are not valid deflate: truncated stream, bit-rotted blob, data compressed with raw deflate vs zlib confusion, or output writer errors mid-copy.

Common situations: Partial uploads stored as complete objects; files edited/truncated after compression; reading with the wrong wrap (raw flate vs gzip) so the decoder chokes on header bytes; full disk while writing large output.

Understand the failure class

Background: Checksum mismatch errors: "checksum verification failed", "digest mismatch", "expected vs actual checksum" — what they mean and how to fix them — this error's family across 41 libraries.

Related errors

• decompression error
• decompression error
• error decompressing
• compression close error
• compression close error

AI-assisted analysis of kopia/kopia@82495e54b5 (2026-09-07). Data as JSON: /api/errors/bdd172691c107969. Report an issue: GitHub.

Appendix: source

Thrown at repo/compression/compressor_deflate.go:74

	if err := w.Close(); err != nil {
		return errors.Wrap(err, "compression close error")
	}

	return nil
}

func (c *deflateCompressor) Decompress(output io.Writer, input io.Reader, withHeader bool) error {
	if withHeader {
		if err := verifyCompressionHeader(input, c.header); err != nil {
			return err
		}
	}

	r := flate.NewReader(input)

	if err := iocopy.JustCopy(output, r); err != nil {
		return errors.Wrap(err, "decompression error")
	}

	return nil
}

View on GitHub (pinned to 82495e54b5)