apply_patch reports success for no-op update hunks

Summary

apply_patch currently reports success for a no-op update hunk: the CLI exits 0 and prints M <file> even when the target file bytes are unchanged.

This is not about normal hunk mismatch failures. If the old lines do not match the file, apply_patch correctly returns Failed to find expected lines. The problematic case is when the old lines do match, but the replacement computes to identical file contents, causing a no-op update to be reported as a successful modification.

Reproduction

Create modify.txt:

line1
line2

Run this patch:

*** Begin Patch
*** Update File: modify.txt
@@
-line2
+line2
*** End Patch

Current behavior on main

I reproduced this against a clean main worktree at commit a7dcd20d38:

exit:0
stdout:
Success. Updated the following files:
M modify.txt
stderr:
file content:
line1
line2
sha256 before:2751a3a2f303ad21752038085e2b8c5f98ecff61a2e4ebbd43506a941725be80
sha256 after:2751a3a2f303ad21752038085e2b8c5f98ecff61a2e4ebbd43506a941725be80
changed: no

The file content and hash are unchanged, but the command reports a successful modification.

Expected behavior

A patch that produces no actual file changes should not report success as a modification. A clearer behavior is a non-zero exit with:

No files were modified.

Root-cause hypothesis

In codex-rs/apply-patch/src/lib.rs, apply_hunks_to_files derives new_contents for update hunks and records the path as modified after writing it. It does not check whether new_contents == original_contents, so a same-text replacement can be counted as a modification even though the bytes are unchanged.

Local fix branch

I have a focused local fix and regression tests here:

https://github.com/myersguo/codex/tree/codex/reject-noop-apply-patch

Commit:

477d5ba751 Reject no-op apply_patch updates

The fix skips non-rename update hunks whose computed new_contents equals original_contents; if all hunks are no-ops, the CLI returns No files were modified..

Fixed behavior from local branch

exit:1
stdout:
stderr:
No files were modified.
file content:
line1
line2
sha256 before:2751a3a2f303ad21752038085e2b8c5f98ecff61a2e4ebbd43506a941725be80
sha256 after:2751a3a2f303ad21752038085e2b8c5f98ecff61a2e4ebbd43506a941725be80
changed: no

Validation

On the local fix branch:

just fmt
just test -p codex-apply-patch
just fix -p codex-apply-patch
git diff --check

Latest targeted test result:

Summary [2.442s]88 tests run:88 passed,0 skipped

I am opening this issue first per the contribution guidelines, since external code PRs are by invitation only.