fix: guard checkDiffFile against mode-only diffs with empty OrigName/NewName

[ktersius]

Problem

The action panics with index out of range [1] with length 1 at diff/diff.go:44 when a pull request contains a file whose only change is a permission/mode bit (no content diff). The bug is present in both v2.0.0 and v2.1.0.

Root cause

checkDiffFile splits OrigName and NewName on / and unconditionally accesses [1]:

parsedFileA := strings.SplitN(parsedDiff.OrigName, "/", 2)
parsedFileB := strings.SplitN(parsedDiff.NewName, "/", 2)
fullPathToA := workspace + "/" + parsedFileA[1]   // line 44 — PANICS
fullPathToB := workspace + "/" + parsedFileB[1]

For a standard git diff, file names always carry an a/ or b/ prefix (or are /dev/null), so SplitN returns two elements. Mode-only diffs have no --- / +++ headers at all. The sourcegraph/go-diff parser therefore returns OrigName = "" and NewName = "" for those entries:

strings.SplitN("", "/", 2)  // [""] — length 1, not 2

Accessing [1] on a single-element slice is a runtime panic.

How to reproduce

Open a PR containing a file whose only change is the execute bit:

git checkout -b repro/mode-change
chmod -x some-file.yaml        # was 100755, becomes 100644
git add some-file.yaml
git commit -m "chore: remove execute bit"
git push origin repro/mode-change
# open PR — the action panics during "Preprocessing diffs..."

The diff block that triggers it (no --- / +++ lines):

diff --git a/some-file.yaml b/some-file.yaml
old mode 100755
new mode 100644

The resulting panic:

panic: runtime error: index out of range [1] with length 1

goroutine 1 [running]:
github.com/launchdarkly/find-code-references-in-pull-request/diff.checkDiffFile(...)
        /app/diff/diff.go:44 +0x46a
github.com/launchdarkly/find-code-references-in-pull-request/diff.PreprocessDiffs(...)
        /app/diff/diff.go:21 +0x9d
main.main()
        /app/main.go:60 +0x4c5

Note: the stat: no such file or directory line printed just before the panic is from a separate, preceding file processed normally — it is not the file that causes the crash.

Fix

Add a length guard before indexing. Mode-only and header-less diffs have no content to scan, so returning ignore = true is the correct behaviour:

if len(parsedFileA) < 2 || len(parsedFileB) < 2 {
    return "", true
}

---

🤖 This PR was created with the assistance of Claude Code