ReverseFileDiff on a git binary patch gives a diff that git apply rejects

Reversing a git diff --binary patch with ReverseFileDiff and printing it produces a patch that git apply refuses with "creates incorrect result", while git apply -R of the original patch undoes the same change without complaint.

main.go, in a module requiring go-diff v0.9.0:

package main

import (
	"io"
	"os"

	"github.com/sourcegraph/go-diff/diff"
)

func main() {
	in, _ := io.ReadAll(os.Stdin)
	fd, err := diff.ParseFileDiff(in)
	if err != nil {
		panic(err)
	}
	rev, err := diff.ReverseFileDiff(fd)
	if err != nil {
		panic(err)
	}
	out, err := diff.PrintFileDiff(rev)
	if err != nil {
		panic(err)
	}
	os.Stdout.Write(out)
}

Run from that module's directory (the diff below is git diff --binary of bin.dat going from the two bytes 00 01 to the four bytes 00 01 02 03; the file starts at the four-byte state):

go build -o revdiff .
rm -rf t && mkdir t && cd t && git init -q
printf '\x00\x01\x02\x03' > bin.dat
cat > fwd.diff <<'EOF'
diff --git a/bin.dat b/bin.dat
index bdc955b7b2e610ad5a72302b139a2e6cb325519a..eaf36c1daccfdf325514461cd1a2ffbc139b5464 100644
GIT binary patch
literal 4
LcmZQzWMT#Y01f~L

literal 2
JcmZQz1ONa700IC2

EOF
git apply -R --check fwd.diff && echo "git apply -R: ok"
../revdiff < fwd.diff > rev.diff
cat rev.diff
git apply --check rev.diff

Output:

git apply -R: ok
diff --git b/bin.dat a/bin.dat
index eaf36c1daccfdf325514461cd1a2ffbc139b5464..bdc955b7b2e610ad5a72302b139a2e6cb325519a 100644
GIT binary patch
literal 4
LcmZQzWMT#Y01f~L

literal 2
JcmZQz1ONa700IC2

error: binary patch to 'bin.dat' creates incorrect result (expecting bdc955b7b2e610ad5a72302b139a2e6cb325519a, got eaf36c1daccfdf325514461cd1a2ffbc139b5464)
error: bin.dat: patch does not apply

The reversed patch has the index hashes and the diff --git arguments swapped, but the two literal sections are still in the original order; git diff -R --binary of the same change writes literal 2 before literal 4. Running git apply rev.diff without --check fails the same way and leaves bin.dat unchanged.

Tested on go-diff v0.9.0 and on current master (cf64c62, the same commit), with git 2.50.1.

BTW, this was found by an automated program that writes property-based tests for various open source projects using hegel (but it has been reviewed by hand before reporting). We've also potentially found (but not yet hand validated) 2 other bugs in go-diff. You can see the tests at https://github.com/hegeldev/hegel-zoo/tree/main/targets/go/go-diff. Let us know if you would like us to file the other bugs found and/or contribute the tests. NB the tests are currently LLM generated and probably not yet suitable for inclusion as is, but we're happy to help get them into a better state if you want them.

I couldn't reproduce this on master (cf64c62, which includes #96). Tested with fixtures produced by real git diff --binary output, reversing them via ParseFileDiff -> ReverseFileDiff -> PrintFileDiff, then applying the result for real and comparing the file byte-for-byte against the original blob:

case	`git apply`	result == pre-image
small in-place edit	exit 0	yes
appended tail	exit 0	yes
two separate changes	exit 0	yes
tiny 64-byte file	exit 0	yes

git apply -R on the original patch works in every case too, so the two paths agree.

One thing that looks like a bug but isn't, in case it tripped up the original report: ReverseFileDiff swaps the headers and leaves the GIT binary patch payload byte-identical. That is correct. A git binary patch encodes both directions, and git apply picks the inverse based on the index hashes, which is what the reversal changes. TestReverseFileDiffGitBinarySemantics in diff/reverse_git_test.go asserts exactly this for both literal and delta encodings, with a comment explaining the reasoning.

#96 landed about two weeks before this was filed, and it reworked the header handling this report points at. If the failure was real on v0.9.0, it looks like #96 already resolved it, so this may just need closing. Happy to be wrong, though: if you still see git apply rejecting a reversed binary patch, send the patch and I'll dig in.

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`ReverseFileDiff` on a git binary patch gives a diff that `git apply` rejects #98

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ReverseFileDiff on a git binary patch gives a diff that git apply rejects #98

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methakon commented on Sep 30, 2026

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`ReverseFileDiff` on a git binary patch gives a diff that `git apply` rejects #98