kopia/kopia · error

unable to prepare content postamble

Error message

unable to prepare content postamble

What it means

Wraps writeRandomBytesToBuffer errors raised while padding the pack with random bytes up to the padding unit before finalizing it. The pack cannot be finalized without the postamble padding, so pack preparation fails.

Solutions

1. Retry the pack write; transient CSPRNG failures usually resolve.
2. Fix the sandbox/seccomp or container profile so getrandom(2)/dev/urandom is reachable.
3. Check host entropy configuration (e.g. haveged/rngd on VMs with poor entropy).

if err := preparePackDataContent(ctx, mp, pp); err != nil {
    if strings.Contains(err.Error(), "postamble") {
        // CSPRNG failure: retry pack preparation
    }
    return err
}

When it happens

Trigger: preparePackDataContent (called by prepareAndWritePackInternal) with sm.paddingUnit > 0 where writeRandomBytesToBuffer fails while appending `sm.paddingUnit - (length % paddingUnit)` random bytes — i.e. the CSPRNG read failed (see 'error getting random bytes').

Common situations: Same root causes as random-byte failures: blocked getrandom(2) in sandboxed/seccomp environments or entropy-source issues on the host.

Related errors

• can't initialize randomness
• error getting random bytes
• can't get random bytes
• did not read exactly
• error generating salt

AI-assisted analysis of kopia/kopia@82495e54b5 (2026-09-07). Data as JSON: /api/errors/6d96ae6adb537631. Report an issue: GitHub.

Appendix: source

Thrown at repo/content/content_manager_lock_free.go:168

		return nil, nil
	}

	if !haveContent {
		// we wrote pack preamble but no actual content, revert it
		pp.currentPackData.Reset()
		return packFileIndex, nil
	}

	if pp.finalized {
		return packFileIndex, nil
	}

	pp.finalized = true

	if sm.paddingUnit > 0 {
		if missing := sm.paddingUnit - (pp.currentPackData.Length() % sm.paddingUnit); missing > 0 {
			if err := writeRandomBytesToBuffer(pp.currentPackData, missing); err != nil {
				return nil, errors.Wrap(err, "unable to prepare content postamble")
			}
		}
	}

	err := sm.appendPackFileIndexRecoveryData(mp, packFileIndex, pp.currentPackData)

	return packFileIndex, err
}

func getPackedContentIV(output []byte, contentID ID) []byte {
	h := contentID.Hash()

	return append(output, h[len(h)-aes.BlockSize:]...)
}

func (sm *SharedManager) writePackFileNotLocked(ctx context.Context, packFile blob.ID, data gather.Bytes, onUpload func(int64)) error {
	ctx, span := tracer.Start(ctx, "WritePackFile_"+strings.ToUpper(string(packFile[0:1])), trace.WithAttributes(attribute.String("packFile", string(packFile))))
	defer span.End()

View on GitHub (pinned to 82495e54b5)