Quote the Index line like the other file-name headers

[youdie006]

formatPatch writes the file name on seven header lines. Six of them run it through quoteFileNameIfNeeded — rename from, rename to, copy from, copy to, --- and +++. Index: (src/patch/create.ts:416) emitted it raw, twelve lines above the --- line that quotes it.

So createPatch can produce a patch that parsePatch cannot read back:

createPatch('x\n--- evil', 'foo\n', 'bar\n')
// Index: x
// --- evil
// ===================================================================
// --- "x\n--- evil"
// ...
parsePatch(that)  // throws: Missing "+++ ..." file header for evil

createPatch('x\n@@ -1,1 +1,1 @@', ...)
// parsePatch throws: Hunk at line 2 contained invalid line ==============

And a quieter version with no throw — createPatch('with\nnewline.txt', …) round-trips oldFileName correctly but yields index === 'with'.

The parse side gets unquoteIfQuoted, which is the inverse this file already applies to rename from / rename to at src/patch/parse.ts:129. Both halves reuse helpers that were already here; nothing new is introduced.

What this argument does not rest on

I checked, and neither GNU diff nor git emits an Index: line at all — it is a CVS/svn-ism — and git apply ignores it. So there is no external reference saying the line should be quoted. The case here is purely that jsdiff quotes this name everywhere else it writes it, and that its own parser chokes on its own output when it doesn't.

Testing

Two tests in the existing headers handling block, deliberately split so each pins one side: the create-side asserts the emitted text, the parse-side parses a hardcoded string.

• Both fail on master (expected 'Index: x\n--- evil…' to equal 'Index: "x\n--- evil"…' and expected '"x\n--- evil"' to equal 'x\n--- evil') and pass here.
• Mutation-checked in both directions, with a rebuild between each since the tests import from libesm/: reverting create.ts alone fails only the create-side test, reverting parse.ts alone fails only the parse-side test.
• Full suite: 318 passing, against 316 at base — my two tests are the whole delta. yarn lint is clean apart from the pre-existing warning on the TODO at parse.ts:188.
• Also ran a round-trip sweep through createPatch/parsePatch/applyPatch over file names containing a space, a quote, a backslash, a tab, é and 한글, plus content lines that mimic diff syntax — no regressions.

One thing I could not run, honestly

I could not get a meaningful number out of the nyc 100% gate. yarn run-mocha passes --require ./runtime, and on Node 20, 22 and 25 that fails before any test runs — the path has no extension for the ESM resolver, and runtime.js uses require inside a "type": "module" package. This happens on a clean checkout too, so it is not from this change; I worked around it locally with a .cjs shim to run the suite at all. Under nyc the instrumentation then never attaches and every file reports 0%, base included, so the figure is meaningless rather than failing.

The structural argument in the meantime: this changes two existing lines and adds no branch, both call pre-existing helpers, and each changed line is proven executed because reverting it turns exactly one test red. unquoteIfQuoted's false branch is already taken by every ordinary Index: foo patch. Worth confirming on whatever Node version yarn test actually runs for you — and you may want that --require line to point at a .cjs file regardless.

Left alone

The trailing-space file name quirk ('tail ' → 'tail') is real but is the case you explicitly deferred to a major release at parse.ts:188-196.

---

AI assistance disclosure: this patch was found and written with Claude Code. Every quoted output above is verbatim from running it against master and against this branch.

[chatgpt-codex-connector]

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Review	Status	Commit	Review trigger
📝 Code Review	✅ Completed 2026-09-08T14:06:34.445212Z	2de3afd	PR opened

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[chatgpt-codex-connector]

Preserve raw Index filenames from older jsdiff output

For a literal filename such as "report", previous versions of formatPatch emitted Index: "report" while quoting the ---/+++ names as "\"report\"". This unconditional decoding now parses such an older jsdiff patch with index === 'report' but oldFileName === '"report"', so consumers that use index to select the target file can load the wrong file. Restrict unquoting to representations that could have been produced by the new quoting behavior, or reconcile the index with the parsed file headers.

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