The checksum was not received at the end of the delta data

Error message

The checksum was not received at the end of the delta data

What it means

FinishDelta requires a trailing checksum record in every delta stream. This error means the stream ended without the patcher having received its final checksum, so output integrity could not be verified.

Source

Thrown at tools/rsync/api.go:189

		return err
	}
	self.unconsumed_delta_data = utils.ShiftLeft(self.unconsumed_delta_data, consumed)
	return
}

// Finish applying delta data
func (self *Patcher) FinishDelta() (err error) {
	if err = self.UpdateDelta([]byte{}); err != nil {
		return err
	}
	if len(self.unconsumed_delta_data) > 0 {
		return fmt.Errorf("There are %d leftover bytes in the delta", len(self.unconsumed_delta_data))
	}
	self.delta_input = nil
	self.delta_output = nil
	self.unconsumed_delta_data = nil
	if !self.rsync.checksum_done {
		return fmt.Errorf("The checksum was not received at the end of the delta data")
	}
	return
}

// Create a signature for the data source in src.
func (self *Patcher) CreateSignatureIterator(src io.Reader, output io.Writer) func() error {
	var it func() (BlockHash, error)
	finished := false
	var b [BlockHashSize]byte
	return func() error {
		if finished {
			return io.EOF
		}
		if it == nil { // write signature header
			it = self.rsync.CreateSignatureIterator(src)
			bin.PutUint16(b[:], 0)
			bin.PutUint16(b[2:], uint16(self.Checksum_type))
			bin.PutUint16(b[4:], uint16(self.Strong_hash_type))

View on GitHub (pinned to 6d5d0c4406)

Solutions

Confirm the sender's delta iterator ran to completion (its final checksum write included)
Re-transfer the delta and apply again
If generating deltas yourself, always emit the trailing checksum before finishing

Defensive patterns

Strategy: try-catch

Try / catch

if err := patcher.FinishDelta(); err != nil { do not trust output; restart transfer from a fresh signature }

Prevention

Never use patched output when FinishDelta errors — integrity is unverified
Always let the sender flush the trailing checksum record

When it happens

Trigger: The delta was truncated before the final checksum operation, or a custom delta producer omitted the checksum terminator.

Common situations: Partial network transfer, a sender bug that doesn't emit the checksum, or calling FinishDelta before the last chunk arrived.

Related errors

AI-assisted analysis of kovidgoyal/kitty@6d5d0c4406 (2026-08-27). Data as JSON: /api/errors/ee298f1ce4fe60f8. Report an issue: GitHub.