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rdiff — Wire compatibility

rdiff reproduces librsync's on-disk formats exactly, so its files are interchangeable with the upstream C rdiff in both directions.

What we match

  • Delta format — magic 0x72730236, the full librsync command set: inline LITERAL (1..64), LITERAL with 1/2/4/8-byte length, and the 16 COPY variants (0x45..0x54) selecting 1/2/4/8-byte offset and length, terminated by END (0x00). Integers are big-endian, minimal width.
  • Signature format — magic | block_len | strong_len (three big-endian uint32) followed by weak:uint32 | strong[strong_len] per block.
  • Weak checksum — librsync's Rollsum with RS_CHAR_OFFSET = 31, digest (s2 << 16) | s1 mod 2^16.
  • Strong checksum — MD4 (magic 0x72730136) or BLAKE2b-256 (magic 0x72730137), truncated to strong_len.
  • Short final block — summed over its actual bytes, never zero-padded.

What we test against

The compat CI job installs the upstream rdiff (librsync) on Linux and runs two cross-implementation round-trips (go test -tags=compat):

Test Path
Go signature → C delta → Go patch TestGoSigCDeltaGoPatch
C signature → Go delta → C patch TestCSigGoDeltaCPatch

Both must reconstruct the new file byte-for-byte. The tests t.Skip when rdiff is not on PATH, so the suite stays green locally without librsync installed.

Manual cross-check

rdiff signature old.bin c.sig          # C reference
go run ./cmd/rdiff delta c.sig new.bin go.delta
rdiff patch old.bin go.delta out.bin   # C reference
cmp new.bin out.bin

Known differences

  • A short final basis block is matched only when the new file ends with those bytes; a short block appearing mid-stream is emitted literally. Output is always correct, just occasionally less compact than librsync's.
  • Delta generation currently loads the target file into memory.