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fs.readFile(path, cb) and fs.writeFile(path, data, cb) pass the callback to the native parser as `options`. The parsers of readFile, writeFile, appendFile, readdir, readlink, realpath and mkdtemp test is_object(), which is true for a function, and read the options from it. The sync and promise forms go through the same parsers. Node's getOptions() returns the defaults for a function. The four parsers now have an arm that does the same. The JS copies of that rule get the arm too: the win32 realpathSync port, the Dir constructor and fs.promises.writeFile with iterable data. fs.realpath.native(path, fn, cb) now calls cb, as Node does. It called the function in the options slot, and cb never ran.
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Status: the fix is in this PR (#46369). How I reproduced it. With Bun 1.4.2 and a release build of main (9b4284f), and with Node v26.3.0 for comparison: const fs = require("fs");
const { debounce } = require("lodash");
fs.writeFileSync("f.txt", "old");
fs.writeFile("f.txt", "new", debounce(err => console.log("saved", err)));
A script with 64 call shapes (a function that carries option properties, in the |
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Updated 5:00 PM PT - Oct 21st, 2026
✅ @robobun, your commit 885d1aca53fb954d062b0469b53bb096b399ede2 passed in 🧪 To try this PR locally: bunx bun-pr 46369That installs a local version of the PR into your bun-46369 --bun |
WalkthroughFilesystem option handling now treats callable values as absent options in specified APIs. The ChangesFilesystem function-option handling
Suggested reviewers: Priority: ➖ Normal Merge Risk: 🔵 Low · up to
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| Check name | Status | Explanation |
|---|---|---|
| Title check |  |
The title clearly and concisely describes the main change: functions in the options slot now use default options for node:fs APIs. |
| Description check | |
The description explains the problem, implementation, scope, limitations, and verification results in detail. It does not use the template headings exactly, but it provides the required information fo… |
| Linked Issues check | |
Check skipped because no linked issues were found for this pull request. |
| Out of Scope Changes check | |
Check skipped because no linked issues were found for this pull request. |
Full details: Description check
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Explanation
The description explains the problem, implementation, scope, limitations, and verification results in detail. It does not use the template headings exactly, but it provides the required information for what the PR does and how it was verified.
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Actionable comments posted: 1
- 🪄 Fix comments: Fix CodeRabbit comments on this PR
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Inline comments:
Review comments at @src/js/node/fs.ts:
- Around line 952-953: Update the overload detection condition near the Node
compatibility comment to use a falsy check on callback rather than
$isCallable(callback), matching Node’s callback || options behavior. Preserve
the existing reassignment of callback and options so undefined callback
overloads continue to work while truthy non-callable values remain available for
the correct validation error.
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| // node takes `callback || options`: https://github.com/nodejs/node/blob/v26.3.0/lib/fs.js#L2993 | ||
| if (!$isCallable(callback)) { |
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🎯 Functional Correctness | 🟡 Minor | ⚡ Quick win
Use a falsy check so the overload detection matches Node's callback || options.
Node calls makeCallback(callback || options). That swaps the arguments only when callback is falsy. The new check !$isCallable(callback) also swaps them when callback is truthy but not a function.
- Trigger:
fs.realpath.native(p, fn, 5).- Node throws
ERR_INVALID_ARG_TYPEfor5. - Bun moves
fninto the callback slot and calls it with the result.
- Node throws
- Trigger:
fs.realpath.native(p, { encoding: "hex" }, 5).- Both runtimes throw.
- Bun's error reports the options object instead of
5.
The code comment says this branch follows Node, but it does not. Use !callback instead. The overloads covered by the tests keep working:
realpath.native(p, cb)realpath.native(p, undefined, cb)realpath.native(p, fn, cb)
Proposed fix
--- "a/src/js/node/fs.ts"
+++ "b/src/js/node/fs.ts"
@@ -949,11 +949,11 @@
}
} as typeof import("node:fs").realpath);
realpath.native = function realpath(p, options, callback?) {
// node takes `callback || options`: https://github.com/nodejs/node/blob/v26.3.0/lib/fs.js#L2993
- if (!$isCallable(callback)) {
+ if (!callback) {
callback = options;
options = undefined;
}
callback = ensureCallback(callback);
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Carefully review the code before committing. Ensure that it accurately replaces the highlighted code, contains no missing lines, and has no issues with indentation. Thoroughly test & benchmark the code to ensure it meets the requirements.
| // node takes `callback || options`: https://github.com/nodejs/node/blob/v26.3.0/lib/fs.js#L2993 | |
| if (!$isCallable(callback)) { | |
| // node takes `callback || options`: https://github.com/nodejs/node/blob/v26.3.0/lib/fs.js#L2993 | |
| if (!callback) { |
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Review comment at @src/js/node/fs.ts around lines 952 - 953:
Update the overload detection condition near the Node compatibility comment to
use a falsy check on callback rather than $isCallable(callback), matching Node’s
callback || options behavior. Preserve the existing reassignment of callback and
options so undefined callback overloads continue to work while truthy
non-callable values remain available for the correct validation error.
After applying the fix, consider running `coderabbit review --agent` for local
review. Visit https://docs.coderabbit.ai/cli?utm_source=ghpr
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Nothing blocking. The comments below are optional suggestions. There is no need to push a fix for them before merging.
Beyond the inline findings, I also checked the new is_function_options arm in the four Rust parsers for a missing exception check: JSC__JSValue__isCallable is a nothrow type-info query (src/jsc/bindings/bindings.cpp:4249), not a property read, and the arm sits after the string branch in each parser, so encoding-string and plain-object paths keep their previous control flow.
Extended reasoning...
The PR adds a callable check to the native readFile/writeFile/appendFile/readdir/readlink/realpath/mkdtemp argument parsers in src/runtime/node/node_fs.rs and three JS copies in src/js/node/fs.ts and fs.promises.ts, with new tests. It touches no security-sensitive surface. Inline findings cover the realpath.native callback shape and the iterable writeFile flag-argument regression, so a human look is already signaled.
Additional findings (outside the current diff — GitHub can't attach inline comments there):
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🟣
src/js/node/fs.promises.ts— pre-existing: callers of fs.promises.opendir(path, fn) still get a promise that never settles, while Node resolves it with a Dir. The promise form delegates to require("node:fs").opendir at src/js/node/fs.promises.ts:206, and that callback form takes any function in the options slot as the callback (src/js/node/fs.ts:623), so the user's fn is invoked with the Dir and the settle callback is discarded. Fix: make every opendir form treat a function in the options slot as the defaults, e.g. clear a callable options in fs.promises.opendir before delegating (as writeFileAsyncIterator now does at fs.promises.ts:1614), so the promise resolves with the Dir. The PR fixes the sibling opendirSync in the Dir constructor but leaves this form as it is on the base branch. [also at: src/js/node/fs.ts:1083 - pre-existing: callers of fs.promises.opendir(path, fn) still get a promise that never settles, while opendirSync(path, fn) now returns the defaults as in Node.]Why this was flagged
A user calls fs.promises.opendir(dir, someFunction); Node's getOptions() returns the defaults and the promise resolves with a Dir. In Bun, src/js/node/fs.promises.ts:206 calls require("node:fs").opendir(dir, options, settleFromNodeCallback.bind(null, resolve, reject)). In src/js/node/fs.ts:623-626
typeof options === "function"makes callback = options and options = undefined, so the user's function replaces the settle callback. The stat callback at fs.ts:636-639 then invokes the user's function with (null, Dir) and resolve/reject are never called, so the awaited promise hangs forever. The base branch behaves the same way; this PR changes the Dir constructor at src/js/node/fs.ts:1080-1082 so opendirSync(path, fn) now works, but the promise form still routes through the callback wrapper's function test and is not covered by any new test (dir.test.ts only exercises opendirSync).Verification: pre-existing: triggered when a caller passes any function in the options slot of
fs.promises.opendir(path, fn). src/js/node/fs.promises.ts:206 delegates to the callback form, and src/js/node/fs.ts:623-626 makes the user'sfnthe callback and drops the settle callback, so the promise never settles. Both are byte-identical on the base. The PR fixesopendirSync(path, fn)but leaves this sibling untested.
| // node takes `callback || options`: https://github.com/nodejs/node/blob/v26.3.0/lib/fs.js#L2993 | ||
| if (!$isCallable(callback)) { | ||
| callback = options; | ||
| options = undefined; | ||
| } |
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🟡 nit (optional): callers of fs.realpath.native(path, options, nonFunction) now get an ERR_INVALID_ARG_TYPE that reports the options object instead of the bad callback value. The comment cites Node's callback || options, but the code at src/js/node/fs.ts:953 uses !$isCallable(callback), which also replaces a truthy non-function callback (5, {}) with options. Fix: mirror Node's shape, falling back to options only when callback is falsy (if (!callback) { callback = options; options = undefined; }), so ensureCallback throws with the value the caller actually passed, as on the base branch and in Node.
Why this was flagged
A caller passes fs.realpath.native(p, { encoding: "utf8" }, 42). Node runs makeCallback(42 || options) = makeCallback(42) and throws ERR_INVALID_ARG_TYPE with "Received type number (42)". On the base branch $isCallable(options) is false, callback stays 42, and ensureCallback at src/js/node/fs.ts:40 throws the same message. After this diff src/js/node/fs.ts:953 sees callback is not callable, assigns callback = { encoding: "utf8" } and options = undefined, so ensureCallback throws with "Received an instance of Object" and the error no longer names the offending argument. The code comment at src/js/node/fs.ts:952 states the intent is callback || options, which only falls back for a falsy callback, not for every non-callable one.
Verification: Triggers when a caller passes a truthy non-function in the callback slot of fs.realpath.native. /home/claude/bun/src/js/node/fs.ts:953 reads if (!$isCallable(callback)), so ensureCallback(callback) at line 958 throws with the options object rather than the bad callback. On the base branch the condition was $isCallable(options). No test in test/js/node/fs/fs.test.ts pins the shape.
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||
| async function writeFileAsyncIterator(fdOrPath, iterable, optionsOrEncoding, flag, mode) { | ||
| // node's getOptions: a function means no options | ||
| if ($isCallable(optionsOrEncoding)) optionsOrEncoding = undefined; |
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🟡 (optional) Callers of fs.promises.writeFile(path, iterable, fn, "a") now get the file appended to, where Node truncates it; on the base branch the call threw. src/js/node/fs.promises.ts:1614 clears a callable options value, so the call falls into the string/null branch at src/js/node/fs.promises.ts:1627 where flag ??= "w" and mode ??= 0o666 keep whatever the 4th and 5th positional args hold. Those come straight from user args via ...args at src/js/node/fs.promises.ts:324. Node's getOptions returns the defaults and ignores extra positional arguments. Fix: when optionsOrEncoding is a function, also reset flag and mode to their defaults (or stop forwarding positional flag/mode from the public wrapper), so every function-in-slot call gets w/0o666 like the object and string forms.
Why this was flagged
Trigger: fs.promises.writeFile(path, asyncIterable, someFunction, "a") or with a 5th mode argument, via the public fs.promises.writeFile at src/js/node/fs.promises.ts:314-324 which spreads user args into writeFileAsyncIterator(fdOrPath, iterable, optionsOrEncoding, flag, mode). The new line src/js/node/fs.promises.ts:1614 sets optionsOrEncoding = undefined, so the branch at src/js/node/fs.promises.ts:1625-1628 runs and flag ??= "w" leaves the caller's "a" (and mode ??= 0o666 leaves a caller mode) in place; fs.open at src/js/node/fs.promises.ts:1639 then opens in append mode and the ftruncate at src/js/node/fs.promises.ts:1659 is skipped because flagTruncates("a") is false. On the base branch the function fell through both branches, encoding stayed undefined and Buffer.isEncoding threw "Unknown encoding: undefined", so no data was written. Node's getOptions returns defaultOptions for a function and writeFile has no 4th parameter, so Node truncates and writes with mode 0o666.
Verification: The only caller is src/js/node/fs.promises.ts:324, which spreads raw user args into (fdOrPath, iterable, optionsOrEncoding, flag, mode). The new line 1614 sends a function into the string/null branch (line 1625), where flag ??= "w" (1627) keeps the caller's 4th positional arg, so fs.open(fdOrPath, "a", mode) at line 1639 appends. On base, Buffer.isEncoding(undefined) at 1631 threw. The positional-flag leak itself predates the PR.
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Cross-reference: #46460 fixes the callback selection in Two more functions have the same Node rule and depend on the parser arm of this PR, so #46460 leaves them out:
With the parser arm in place, the wrappers need no slot clear: appendFile = function appendFile(path, data, options, callback?) {
callback ||= options;
callback = ensureCallback(callback);
fs.appendFileCb(callback, path, data, options);
},realpath.native = function realpath(p, options, callback?) {
callback = ensureCallback(callback || options);
fs.realpathNativeCb(callback, p, options);
};I ran both lines on a build with an equivalent parser arm. For each function: |
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#46575 reads This PR sends a function to the default options before
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Problem
fs.writeFile(path, data, _.debounce(cb))throwsERR_INVALID_ARG_TYPE: The "flush" argument must be of type boolean. Received function flush. WithFunction.prototype.flag = "w",fs.readFile(path, cb)truncates the file. Node does neither.fs.readFileandfs.writeFilepass the callback asoptions(src/js/node/fs.ts:350, since Fix argument validation with callbacks in node:fs #12412). Four parsers insrc/runtime/node/node_fs.rs, shared by the sync and promise forms, testis_object(), true for a function, and read the options from it.flagas the default in writeFile and appendFile #45645. No user reported it.Fix
is_function_optionsand one do-nothing arm in each parser (readFile,writeFile,appendFile,readdir,readlink,realpath,mkdtemp): a function means the defaults, as in Node'sgetOptions().realpathSyncport, theDirconstructor, iterablefs.promises.writeFile.fs.realpath.native(path, fn, cb)now callscb.test/js/node/fs/fs.test.ts(47 new tests, 37 fail on main),dir.test.ts,fs-promises-writeFile-async-iterator.test.ts. Alsopromises.test.js, vendoredtest-fs-*.optionsclassifier (2), which belongs to node:fs: reject a non-string, non-object options argument like node #41928.Background
getOptions(). Callers ofvalidateObject(rm,cp) reject a function: not in this PR.fs.tswrappers: 27 of 45 differing probe rows stay wrong, sync forms included.Downsides
bench/snippetsshow no difference outside the run-to-run spread. Release text: +147 bytes.Notes
Repro. Node v26.3.0, Bun 1.4.2, main 9b4284f and this branch.
History. Through v1.1.18 the
readFileandwriteFilewrappers removed the callback from the arguments before the native call. #12412 (v1.1.19) changed them tocallback ||= options, which is Node's line, and the callback stayed in the options slot. #16422 (v1.2.0) added theflushcheck that makes the lodash callback throw. The sync and promise forms have read a function as an options object since at least v1.0.0.Reach. The one trigger I found that needs no unusual code is a debounced or throttled callback given directly to
fs.writeFile. The other rows need a function that carries a property namedencoding,flag,mode,signal,flush,withFileTypesorrecursive, or the same name onFunction.prototype. The option reads stop atObject.prototype(src/jsc/bindings/ObjectBindings.cpp), but the chain of a function reachesFunction.prototypefirst.Node source (v26.3.0).
getOptions: https://github.com/nodejs/node/blob/v26.3.0/lib/internal/fs/utils.js#L324-L327. Its callers:readFile,writeFile,appendFile,readdir,readlink,realpath,mkdtemp,watch,opendir,ReadStream,WriteStream, in each form.fs.realpath.nativetakescallback || options: https://github.com/nodejs/node/blob/v26.3.0/lib/fs.js#L2993.mkdirdoes not callgetOptions:mkdirSync(path, fn)readsrecursiveandmodefrom the function in Node and in Bun, and a test pins that.What changes, by class. One script puts a function that carries option properties in the
optionsslot of eachgetOptionscaller, in the sync, callback and promise form: 64 call shapes. Bun 1.4.2 and main print the same 64 rows.fs.tswrappers patched (simulated with a preload)fs.writeFile(p, d, cb)with a function incb.flush:ERR_INVALID_ARG_TYPE, then the file is writtenfs.readFile(p, cb)withflag: "w"oncbor onFunction.prototype:EBADF, then the file is readfs.realpath.native(p, fn, cb):fnis called, thencbis calledfs.appendFileSync(p, d, fn)withfn.flag = "w": the file is truncated, then it is appended tofs.readFile(p, cb)withcb.encoding = "utf8": a string, then a BufferNo row goes from a result to a throw. A second script puts accessors for the option names on the function and counts the shapes that read one: Node 0 of 37, main 23 of 37, this branch 0 of 37.
Entry points. A function in the slot now gives the defaults in each of these:
fs.readFile,fs.writeFile,fs.appendFile: callback forms (the callback in the slot, or a function and then a callback), sync forms,fs.promisesforms, fd forms,FileHandle#readFile.readdirSync,fs.promises.readdir,readlinkSync,fs.promises.readlink,realpathSync,realpathSync.native,fs.promises.realpath,mkdtempSync,fs.promises.mkdtemp,mkdtempDisposableSync,fs.promises.mkdtempDisposable.fs.opendirSync(path, fn)andfs.promises.writeFile(path, iterable, fn)threw. They now use the defaults.typeofis"function": a class, a bound, async or generator function, a Proxy of a function (itsgettrap does not run).Tests. Of the 47 new tests in
fs.test.ts, 37 fail on main. The 10 that pass are 2 controls (an options object still applies,mkdirSyncstill reads a function) and 8 rows in which a JS wrapper or aFileHandlemethod already drops the function before the native call:appendFile,readdir,readlink,realpath,realpath.nativeandmkdtempwith the callback in the slot, andFileHandle#writeFileandFileHandle#appendFile. They are in their own array.dir.test.tsgets 7 tests:opendirSync(path, fn)fails on main, and 6 controls (a number, boolean, bigint or symbol still throwsERR_INVALID_ARG_TYPE) pass on main. Each expected value is what Node v26.3.0 gives for the same call: I ran the same assertions under Node.Shape of the native change. The arm sits after the string test and before the object test, and no existing line changes.
is_object() && !is_callable()in the object test: node:fs: reject a non-string, non-object options argument like node #41928 adds anelsethat throws for a value that is not an object, and the two changes merge without a conflict into a throw for eachfs.readFile(path, cb).is_object()before the string test, soreadFileSync(path, "utf8")pays for it. One classifier for all copies ofgetOptionshas to decide what a number or a boolean does, and that is the change of node:fs: reject a non-string, non-object options argument like node #41928. The fold belongs there.fs.tswrappers: the parser has the rule, andcallback ||= optionsis Node's own line.is_function_optionstestsFinalObjectfirst. A plain object is the usual options value and cannot be callable, so it does not reach the call into JSC (JSC__JSValue__isCallable).#[inline]onparse_encoding_argkeeps it inlined, as on main. Without it the compiler moves it out of line. Measured before the rebase, at ea6a2c7: release text +1,425 bytes without it and +401 with it. Call time does not tell the two builds apart.Measurements. Release x64 builds of main 9b4284f and of this branch, same tree and build directory.
bench/snippets, 7 runs of each build in turn on one CPU, median of the average ns per iteration and the range over the runs:{recursive: true}{recursive: true}x 10{recursive: false}Instructions in the parser, entry to return (
gdbsingle-step of the main thread,BUN_JSC_useJIT=0).perf,valgrindandbloatyare not installed in my container.ReadFile::from_js: no options /undefined/"utf8"ReadFile::from_js:{}/{encoding}ReadFile::from_js:readFileSync(fd, fn)/readFile(fd, cb)WriteFile::from_js_with_default_flag: no options /"utf8"WriteFile::from_js_with_default_flag:{}/{flag}WriteFile::from_js_with_default_flag:writeFileSync(p, d, fn)/writeFile(p, d, cb)Readdir::from_js: no options /"utf8"Readdir::from_js:{}/{withFileTypes}Readdir::from_js:readdirSync(d, fn)Realpath::from_js: no options /"utf8"Realpath::from_js:{}/{encoding}Realpath::from_js:realpathSync(p, fn)ReadFile::from_jsis +7 for the test and +20 because the compiler no longer inlinesget_encodingthere.#[inline]onget_encodingdid not change that, and#[inline(always)]movedparse_encoding_argout of line, so neither is in the diff.fs.readFile(path, cb): option lookups 3 to 0, malloc/free pairs 2 to 0.fs.writeFile(path, data, cb): lookups 5 to 0, pairs 4 to 0. This is not a speedup that a caller can measure. A round trip through the thread pool (12-byte file, one CPU, best of 7 x 50k, 4 runs of each build in turn) takes 5.7 to 6.1 µs on main and 5.5 to 5.7 µs here forreadFile(p, cb), and 8.7 to 9.1 µs and 8.4 to 8.6 µs forwriteFile(p, d, cb). The unchanged"utf8"forms move by about the same amount between the two builds (6.0 to 6.3 and 5.9 to 6.1 µs, 8.5 to 8.7 and 8.2 to 8.5 µs).JSC__JSValue__isCallablefor a plain options object: 0 on the release build and 0 on the debug build (100 calls ofreadFileSync(fd, {encoding})).size): 107,318,912 to 107,319,059, +147 bytes. The six parser symbols (nm -S): +187 bytes. Bundledfs.js+80 bytes,fs.promises.js+81 bytes.Dirconstructor 121 to 124 instructions, none of the 3 runs for an object, a string orundefined.writeFileAsyncIterator243 to 249, 4 run for each call with iterable data. Therealpath.nativewrapper 18 to 18.Not changed here. Each of these differs from Node on main and on this branch.
fs.appendFile(path, data, fn, 5)usesfnas the callback. Node throwsERR_INVALID_ARG_TYPEforcb. The same holds forfs.realpath.native(path, cb, 5).new String("utf8")is read as an encoding (node:fs: reject a non-string, non-object options argument like node #41928).fs.write:fs.writeSync(fd, buffer, fn)writes, Node throws (fs: read offset, length and position as independent slots in write and writeSync #45837). It reads no option from the function and does not callgetOptions.rm,rmdir,cp,fs.promises.watch: Node rejects a function there (validateObject).fs.rmSync(dir, fn)readsrecursiveandforcefrom the function (jsc: reject callables in validate_object to match Node's validateObject #35227, node:fs: validate rm options before the ERR_FS_EISDIR check #39338).statSync,lstatSync,fstatSyncandstatfsSyncwith a function: Node readsbigintfrom it, Bun gives the defaults.Related open PRs. The rules of
getOptionsare in several PRs, and this arm runs before the object branch, so it does not depend on the others:options.signalis validated: node:fs: validate options.signal in readFile, writeFile and watch like node #39274, fs.promises: forward options.signal from the FileHandle wrappers and validate it for iterable data #39272.readdir), fs.promises: accept iterable data in appendFile, like writeFile and node #39186 (writeFileAsyncIterator), and node:fs: fs.Dir reads from a native directory stream #45416, which rewritesfs.Dirand has theDirarm with its own tests. TheDirhunk here goes away in a rebase if node:fs: fs.Dir reads from a native directory stream #45416 lands first.flagas the default in writeFile and appendFile #45645 lists this bug under "Not changed here". It does not depend on this PR.Windows. The
realpathSyncport is JavaScript only on win32, and I cannot run it. TherealpathSync(path, fn)row of the new table is not gated, so the Windows lanes run it.Suites run. Debug build: the new tests,
dir.test.ts,fs-promises-writeFile-async-iterator.test.ts,promises.test.js,cp.test.ts,glob.test.ts,fs-mkdir.test.ts,fs-path-length.test.ts, and 336 vendoredtest/js/node/test/parallel/test-fs-*files, of which 335 pass.test-fs-cp-async-file-url.mjsfails the same way on main in my runner.fs.test.tson the debug build: 662 pass, 8 fail. The 8 are thereaddir ... x 100tests, which exceed their 10 s limit under debug and ASAN in my container, and tests that time out behind them. Release build of this branch:fs.test.ts668 pass, 2 fail. One of the 2 fails the same way on a release build of main (keeps a non-throwing callback in the same place in the event loop), the other passes alone.abort-signal-leak-read-write-file.test.tspasses.tsc -p src/js/tsconfig.jsonandrustfmt --checkpass. This PR adds 0 passing Node tests: Node's suite has no function that carries option properties.no test proof · iteration 0 · platform-specific test(s) that do not run on this machine, deferring to CI, which covers all platforms: test/js/node/fs/fs.test.ts, test/js/node/fs/dir.test.ts