node:fs: call callbacks from the native completion, not a promise reaction

[robobun]

Problem

• An async node:fs callback queues a microtask and a process.nextTick(). Node runs the nextTick first and prints cb0 tick0 mt0 cb1 tick1 mt1. Bun prints cb0 tick0 mt0 cb1 mt1 tick1.
• The binding returns only promises, so src/js/node/fs.ts ran each callback from a promise reaction. A reaction is a microtask, so the callback's microtasks ran before the tick queue.

Fix

• The binding gets a third arm, xCb(callback, ...args). The task keeps the callback in a Strong and calls it from the completion with EventLoop::run_callback, as node:crypto does.
• run_callback then runs the tick queue and the microtasks, in node's order. A callback API creates no promise.
• node:fs/promises stays on the promise arm. Callback arguments do not change.
• Verified: test/js/node/fs/fs.test.ts (49 new tests), node's test-fs-* files, Windows x64.

Background

• A checkpoint runs the tick queue (process.nextTick), then the microtask queue (promise reactions, queueMicrotask). Node calls an fs callback from the event loop, outside both.
• fs.promises forwards every user argument, so a trailing function cannot select the callback arm. The arm needs its own entry points.

Downsides

• Seven callback APIs are JS promise chains with no binding operation (cp, rm, recursive rmdir, Dir.read, Dir.close, glob, opendir). They still use process.nextTick, so they report a TickObject to async_hooks and wait on a replaced process.nextTick.
• Binary size: the stripped Linux x64 release bun grows 53,248 bytes (+0.066%) for 41 new host functions.
• fs.access allocates one more closure per call (1 → 2) for its JS adapter. Other measured operations allocate fewer (Notes).

Notes

Repro

import fs from "node:fs";
const seq = []; let n = 0;
function cb() { const i = n++; seq.push("cb" + i); process.nextTick(() => seq.push("tick" + i)); queueMicrotask(() => seq.push("mt" + i)); }
for (let k = 0; k < 2; k++) fs.stat(".", cb);
setTimeout(() => console.log(seq.join(" ")), 200);

The first callback of a process looks correct without the fix. The tick queue does not exist until the first process.nextTick() call. While it does not exist, the onEachMicrotaskTick hook (ZigGlobalObject.cpp, checkIfNextTickWasCalledDuringMicrotask) drains it right after the microtask that created it.

History of this PR

The first version kept the promise and passed the callback to process.nextTick from the reaction. A maintainer asked for a native callback arm instead (review). The hop kept the whole promise path and added a tick. It also had two costs that the native arm removes for the operations of the binding, both measured on one fs.stat callback:

	released bun	hop	native arm	node v26.3.0
async_hooks init events	{}	{"TickObject":1}	{}	{"FSREQCALLBACK":1}
process.nextTick replaced before node:fs loads	callback runs	callback held	callback runs	callback runs

Bun reports no FSREQCALLBACK resource, before or after this PR. #33366 was an earlier native attempt, closed as stale without a review.

Shape of the callback arm

• FsCompletion { Promise(JSPromiseStrong), Callback(Strong) } replaces the promise in AsyncFSTask, the recursive readdir task and the Windows UVFSRequest. NewAsyncCpTask keeps its promise, because fs.cp is a JS promise chain.
• The callback is stored with with_async_context_if_needed and called with ContextId::NONE, the pairing run_callback documents for a callback that carries its own AsyncLocalStorage frame.
• An error found before the operation is scheduled (an aborted signal, a path that is too long) goes to the callback on the next tick. The callback goes in bare, because Process::queueNextTick asserts against an AsyncContextFrame.
• The task releases its arguments before it calls back. They pin their buffers, and the callback now runs inside the completion, so a transfer of the buffer from the callback would copy it. Main and node detach it. The Windows UVFSRequest takes the completion out, frees the task, and then calls back. It takes *mut Self for that reason.
• FsCompletion::reject builds the error value as JSPromise::reject does: it creates an out-of-memory error and does not take one, it returns a termination, and it unwraps a thrown Exception cell to its Error.
• FsCompletion::settle reports a converted result, and the three task types call it. UVFSRequest::run_from_js_thread is generic and is compiled once for each libuv operation. With the report logic inline, 28 of its 82 MIR statements used no type parameter, and the generic_body_not_generic lint of mordant reported it on the Windows target.
• The entry points are named xCb, not xCallback. The class generator names the C++ host wrapper of a method x Prototype__xCallback, so a method named xCallback collides with it.
• Eight operations keep a small JS adapter so that their argument lists stay identical: access, close, exists, read, write (both overloads), readv, writev, symlink. fs.symlink with a callback that is not callable stays on the promise arm, as before.
• internal/trace_events.ts wraps binding[method + "Cb"] for each operation in its existing table, so node.fs.async spans still close. Without this, test-trace-events-fs-async.js gets no traces.

Size and performance

Two release builds from one tree and toolchain, each in its own build directory: main bf80d21 and this PR. Linux x64.

	main	PR	delta
bun (stripped)	80,770,592	80,823,840	+53,248 (+0.066%)
bun-profile	169,212,560	169,315,968	+103,408 (+0.061%)
.text	58,086,812	58,135,964	+49,152
.rodata	19,821,204	19,825,364	+4,160
.data, .data.rel.ro, .bss			0

JS heap objects allocated per operation. This count is exact and has no timing in it: the script starts 1000 operations in one synchronous turn and reads heapStats() in the same turn. No operation can complete before control returns to the event loop, so each one is pending and roots what it allocated. Three runs of each binary give identical numbers.

per operation	Promise	Function
callback fs.stat	2 → 0	2 → 1
callback fs.readFile	2 → 0	2 → 1
callback fs.read	2 → 0	3 → 2
callback fs.access	2 → 0	1 → 2
fs.promises.stat	3 → 3	0 → 0
fs.promises.readFile	3 → 3	0 → 0

On main each callback operation allocates two promises: the one the binding returns, and the one .then() derives. Eight operations keep a JS adapter so that their callback arguments stay identical. For fs.access the adapter is one more closure than main, which passed the callback to .then() directly. For fs.read it replaces two closures that main created. I measured only these two of the eight.

Process CPU time per operation (user + system, µs), 200,000 operations, 256 in flight, 10 alternating runs per binary. This is statistical, not exact. The spread between runs of one binary is 2 to 4 µs, which is larger than the differences, and the distributions overlap. Read it as a direction.

	main median	PR median	main min	PR min
callback fs.stat	12.36	10.41	10.27	8.46
callback fs.read	10.47	9.50	8.67	8.64
fs.promises.stat	11.51	11.88	8.57	9.79

fs.stat is about 16% cheaper by the median and by the minimum. fs.read is not conclusive: 9% by the median, no difference by the minimum. fs.promises.stat is the control, because its code path and its allocations did not change. Its 3% difference shows the noise of the method, and it gives no evidence of a regression.

I could not count CPU instructions on this machine. valgrind has no install candidate, and perf_event_open returns EPERM.

Not in this PR

• dns.lookup has the same order problem (cb mt tick).
• Void operations call back with (null, undefined) where node passes (null). Same on main, node:fs: call void operation callbacks with (null) alone, as node does #38085 changes it.
• fs.symlink calls back with (undefined) on success. Same on main, node:fs: apply node's callback argument rule in the native completion, and make access give undefined #40916 changes it.
• fs.fstat, fs.read and the 4-argument fs.symlink do not check the callback synchronously as node does. Same on main.
• fs.readFile with a signal that is already aborted calls back synchronously in node and asynchronously in bun.

Verification

• test/js/node/fs/fs.test.ts: the 45 order tests give 0 pass with main's src/ and 45 pass with this PR. The 4 transfer tests guard a bug that an earlier commit of this PR had (6174ba5 fails them with source: 8). They pass on main. Whole file on the last commit: 613 pass, 1 fail. The failure is readdirSync(path, {recursive: true}) should work x 100, a synchronous API that this PR does not touch. It took 10.4 s against its 10 s timeout under the ASAN debug build. Alone it passes in 3 of 3 runs (6.3 to 9.3 s). An earlier run of the whole file was 614 pass, 0 fail.
• The same cases as a plain script pass on node v26.3.0.
• 23 other files in test/js/node/fs/: 271 pass, 0 fail. abort-signal-leak-read-write-file.test.ts is a 300 s timeout under the ASAN debug build and uses only fs.promises.
• Node's test/parallel files for fs, trace events, util.promisify, util.callbackify, http2 respondWithFile, Utf8Stream, stream.pipeline, readline and FileHandle: 413 files, all pass. test-fs-cp-async-file-url.mjs needs the working directory test/js/node.
• AsyncLocalStorage stores reach the callback on 10 paths (native, adapter, error, recursive readdir, the early abort path, and the JS chains), the same as node.
• Every way an operation reports (a value, an operation error, an abort after the start, recursive readdir), on both arms: 11 cases, identical output on node v26.3.0, the released bun and this PR.
• bun run rust:check-all: 12 of 12 targets. bun run rust:mordant (Linux, Windows and macOS targets): no finding over the baseline.
• Windows x64 debug build: the order tests are 42 pass, 0 fail and the transfer tests 4 pass, on 487d83d. The one commit after it (9cac843) moves the report logic into FsCompletion::settle and has not run on Windows here. An earlier run of the whole file was 598 pass, 2 fail. Both fail the same way on main without this PR on the same machine: keeps a non-throwing callback in the same place in the event loop (setImmediate runs before the fs.stat callback) and readSync > works on large files (5 s timeout).

---

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

[robobun]

Status: the native callback arm is pushed (head 9cac843) and the PR is approved. It replaces the process.nextTick hop for every operation of the binding, as requested in the review. The description has the binary size comparison and the performance measurements.

CI on 9cac843 (build 119851): 179 of 181 jobs passed, which includes every Windows shard. No node:fs test failed on a lane that has finished. The darwin x64 test job still runs. One job failed, and this PR does not cause it:

• The job is the debian 13 x64-asan test shard. The test is test/js/bun/spawn/spawn.test.ts, an idle reader stopped at the highwater mark does not keep the process alive (saturating writer).
• The child process wrote a LeakSanitizer warning to stderr (ptrace appears to be blocked (is seccomp enabled?), then Child exited with signal 42). The test expects an empty stderr, so it failed on all 4 attempts.
• The test covers the stdout reader of Bun.spawn. It uses no async node:fs call, and this PR changes no code on that path. The CI failure report marks it as a failure that main also has. The two builds of this PR before this one passed with this test in them: 119834, and 119843 with 181 of 181 jobs on the merge commit 1e1769e.

All GitHub checks on 9cac843 pass, mordant included.

How I reproduced the bug, on bun 1.4.3-canary.1+367d939d9 (Linux x64) and on a debug build of main:

import fs from "node:fs";
const seq = []; let n = 0;
function cb() { const i = n++; seq.push("cb" + i); process.nextTick(() => seq.push("tick" + i)); queueMicrotask(() => seq.push("mt" + i)); }
for (let k = 0; k < 3; k++) fs.stat(".", cb);
setTimeout(() => console.log(seq.join(" ")), 200);

• node v26.3.0: cb0 tick0 mt0 cb1 tick1 mt1 cb2 tick2 mt2
• bun before the fix: cb0 tick0 mt0 cb1 mt1 tick1 cb2 mt2 tick2
• bun with the fix: cb0 tick0 mt0 cb1 tick1 mt1 cb2 tick2 mt2

bun bd test test/js/node/fs/fs.test.ts -t "process.nextTick queued by an fs callback" fails 45 of 45 with main's src/ and passes 45 of 45 with this PR.

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Walkthrough

Changes

Node-style filesystem callbacks now use centralized settlement helpers and the process.nextTick function captured at module load. Tests cover ordering across standard fs, I/O, directory, recursive, and missing-path operations.

Filesystem callback scheduling

Layer / File(s)	Summary
Centralized callback settlement src/js/node/fs.ts	Shared helpers schedule success and failure callbacks through the captured next-tick function while preserving callback argument conventions.
I/O and higher-level callback paths src/js/node/fs.ts	Read, write, vector I/O, realpath, cp, opendir, Dir, and glob callbacks use the updated settlement paths.
Callback ordering validation test/js/node/fs/fs.test.ts	Tests verify ordering relative to process.nextTick, microtasks, concurrent filesystem callbacks, and replaced next-tick handlers.

Suggested reviewers: jarred-sumner

Priority: ⬇️ Low

Merge Risk: 🟡 Moderate · up to 13fcc

The centralized filesystem callback settlement may pass an extra undefined argument to successful void callbacks, making arguments.length observable and breaking compatible callback behavior. Resolve this before merging.

🚥 Pre-merge checks | ✅ 3 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Title check	⚠️ Warning	The title describes callbacks running from a native completion, but the summarized change adapts promise results and schedules callbacks through a captured process.nextTick. This makes the title mis…	Change the title to describe the actual promise-to-callback adaptation and process.nextTick scheduling, for example: “node:fs: preserve callback ordering with process.nextTick”.

✅ Passed checks (3 passed)

Check name	Status	Explanation
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Description check	✅ Passed	The description explains the problem, fix, background, trade-offs, and verification results. It does not use the template’s exact “What does this PR do?” and “How did you verify your code works?” head…

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Explanation

The title describes callbacks running from a native completion, but the summarized change adapts promise results and schedules callbacks through a captured process.nextTick. This makes the title misleading about the implementation.

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Inline comments:
In `@src/js/node/fs.ts`:
- Line 53: Update the changed void-result operations, including appendFile and
chmod, to use settleCallbackWithNull so successful callbacks receive only the
null error argument; retain settleCallback for operations that return a value
and preserve existing callback arity.

In `@test/js/node/fs/fs.test.ts`:
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[claude]

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 write(): onWritten/onError close over the callback binding, and every reassignment (including ensureCallback) happens synchronously before the promise can settle, so the wrapped callback is what runs. The two-argument callback(null, undefined) on void operations is pre-existing arity drift that the in-flight #38085 covers, and the symlink success-value and fstat missing-callback behaviors are identical on the base branch (.then(callback, callback) / wrapFsCallback there too), so they are not regressions introduced here.

Extended reasoning...

The change rewrites ~40 callback-style entry points in src/js/node/fs.ts to defer the user callback through process.nextTick instead of invoking it from the promise reaction, and adds a nextTick-vs-microtask ordering test table to test/js/node/fs/fs.test.ts. It touches no security-sensitive surface. Inline findings were reported, so this body only records what else was examined and ruled out.

[robobun]

^{Updated 3:33 PM PT - Sep 22nd, 2026}

❌ @robobun, your commit 9cac843 has 1 failures in Build #119851 (All Failures):

• test/js/bun/spawn/spawn.test.ts - code 1: stdout reader of an unref'd child and process lifetime > an idle reader stopped at the highwater … on 🐧 13 x64-asan

---

🧪   To try this PR locally:

bunx bun-pr 43728

That installs a local version of the PR into your bun-43728 executable, so you can run:

bun-43728 --bun

[claude]

Nothing blocking. The comments below are optional suggestions. There is no need to push a fix for them before merging.

Beyond the inline finding, I checked the callback arity of the void operations now routed through settleCallback (unlink, rename, writeFile, chmod, ...): they receive (null, undefined), which matches the base's bind(undefined, null) reaction that also forwarded the resolved undefined, so it is not a behavior change. The captured const nextTick in this push also covers every deferral site, including the inline ones in read/write/writev/readv and the stat abort path.

Extended reasoning...

The diff rewrites every callback-style API in src/js/node/fs.ts to deliver results through a module-load-captured process.nextTick instead of directly from the promise reaction, and adds an it.each ordering matrix plus a fake-nextTick test in test/js/node/fs/fs.test.ts. No security-sensitive surface is touched. One verified test-hygiene finding is posted inline; the arity question for void callbacks was examined and ruled out since the old bind-based reaction already passed the resolved undefined as a second argument.

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Inline comments:
In `@test/js/node/fs/fs.test.ts`:
- Line 7292: Retain the stderr result from the Promise.all call in the
child-process test and, after the stdout assertion, assert that stderr is empty
whenever exitCode is nonzero before asserting the expected zero exit code.

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[claude]

I re-reviewed the latest push and found no bugs; the two follow-up commits address what I raised earlier (the load-time process.nextTick capture, and the fake-nextTick test now running in a subprocess so a regression cannot leave the replacement installed for the rest of the file). Because this changes the delivery timing of every callback-style node:fs API, a human look at the extra tick per callback and the ordering trade-off is still worthwhile.

What was reviewed:

• Every settleCallback/settleCallbackWithNull site against the old path's callback arity: the old bind(undefined, null) already forwarded the fulfilled value, so void ops still receive (null, undefined); close, cp, Dir.close still receive (null).
• write's onWritten/onError closures read callback after both overloads assign it; read/readv/writev keep their extra buffer argument.
• Remaining .then sites in fs.ts (opendir, Dir queue) already went through nextTick or are internal chains, so no callback site was missed.

Extended reasoning...

The diff rewrites how ~40 callback-style APIs in src/js/node/fs.ts deliver results: each promise reaction now hands the user callback to a process.nextTick captured at module load instead of calling it from the microtask, plus a 45-case test table in test/js/node/fs/fs.test.ts. It touches no security-sensitive surface. Both prior findings of mine that were actionable (call-time lookup of process.nextTick, and the in-process fake-nextTick test) were fixed in commits 312ea87 and a0fcc7d; the other two were pre-existing and unchanged by this PR. Deferring rather than approving because the change alters scheduling for every fs callback in the Node compat layer and adds a tick-queue entry per call, which is a design/perf trade-off for a maintainer to weigh (the PR itself notes a closed native-callback alternative).

[robobun]

A note for reviewers on the "Merge Risk: Moderate" line in the CodeRabbit summary. It says this PR passes an extra undefined to the callbacks of void operations. This PR does not change those arguments.

• On main, nullcallback(callback) is callback.bind(undefined, null). The promise reaction calls that bound function with the fulfillment value, which is undefined. So void operations already call back with (null, undefined).
• I compared the argument lists of 22 callbacks on the released build and on the current head (13fcc27). They are identical. fs.chmod, fs.copyFile, fs.mkdir, fs.rm and fs.writeFile give [null, undefined] on both.
• CodeRabbit withdrew the same finding in its review thread: node:fs: call callbacks from the native completion, not a promise reaction #43728 (comment)

The difference from node is real: node calls back with (null) alone. It exists on main, and #38085 changes it. I kept it out of this PR so that the only behavior that changes here is the order.

[alii]

The real fix here is to have a third arm for node:fs where the functions run callbacks instead of promises so we skip the overhead.

[robobun]

Agreed. I will rework this PR to do that.

The hop fixes the order, but it keeps the promise, the .then reactions and the microtask, and it adds a tick on top. My cost number (about 0.05 µs per callback) compares the hop with the current promise path. It does not measure a path with no promise, so it does not answer your point.

Two costs in the Downsides section exist only because of the hop, and a native callback arm has neither:

• With an async_hooks init hook enabled, each fs callback reports a TickObject. Bun reported nothing before, and node reports FSREQCALLBACK.
• fs callbacks now depend on process.nextTick. Code that replaces it before node:fs first loads holds them back.

Plan: add a callback arm next to the sync and promise arms. The async task keeps the callback in a Strong and calls it from the completion with EventLoop::run_callback, the way node:crypto already does. node:fs/promises stays on the promise arm. The 45 tests here assert only the observable order, so they stay as the acceptance tests.

One question on the shape, because it is what #33366 left open. The binding cannot detect a callback by looking for a trailing function. fs.promises forwards ...args, so fs.promises.unlink(path, fn) must still return a promise, and fh.read(buf, 0, 4, fn) must still throw ERR_INVALID_ARG_TYPE for position. So the callback arm needs its own entry points. My default is a third function per row of node_fs_bindings! (stat_sync / stat / stat_callback), with the callback as the first argument so that its position is fixed. The alternative is one second binding instance in callback mode, with no new methods. If you prefer that, or another shape, tell me. Most of the work (the completion type and the four task types) is the same for both, so I will start there.

I converted this PR to a draft so that the hop version is not merged by accident.

[claude]

Code review found no issues

No high-confidence issues detected in this change.

[robobun]

One more acceptance case for the rework: a ReadStream over a FileHandle must keep the order it has today.

Node v26.3.0 runs the promise jobs of that stream's 'data' listener before its nextTicks, because a FileHandle read settles through a promise reaction. A path-based stream is the opposite.

const fs = require("node:fs"), fsp = require("node:fs/promises");
const probe = name => () => {
  const o = [];
  process.nextTick(() => o.push("tick"));
  Promise.resolve().then(() => o.push("then"));
  setImmediate(() => console.log(name, o.join(" ")));
};
(async () => {
  process.nextTick(() => {});
  const fh = await fsp.open(__filename);
  fh.createReadStream().once("data", probe("fh.createReadStream()"));
  fs.createReadStream(__filename).once("data", probe("createReadStream(path)"));
})();

	node v26.3.0	bun 1.4.3-canary.1+367d939d9
fh.createReadStream()	then tick	then tick
createReadStream(path)	tick then	then tick

Each cell is 3 of 3 runs. I did not run this PR's branch.

Main matches node in the first row only because of the bug this PR fixes. fileHandleStreamFs (src/js/internal/fs/streams.ts:55) reads through the public callback fs.read when fh.read is not patched. write, writev and fsync take the same shortcut. So when fs.read stops calling back from a promise reaction, these streams change to tick then. On the current head fs.read calls back through nextTick, so the first row should already change. A differential run against node reported exactly that for fh.createReadStream() and createReadStream(null, { fd: fileHandle }), in 50 of 50 cases.

A native callback arm has the same effect unless FileHandle streams stay on the promise API. The other branch of fileHandleStreamFs already does that (fh.read(...).then(...)).

The new tests have no FileHandle-backed stream, so nothing catches this today.

[robobun]

Two cost notes for the rework. Both are from the released canary (1.4.3-canary.1+367d939d9), one variant per process, 7 alternating rounds, wall-time medians on a shared 4-core box. I did not build this branch.

1. The load-time read of process.nextTick costs every program that imports node:fs. The patch adds const nextTick = process.nextTick; at the top of fs.ts. On main, fs.ts reads process.nextTick only inside functions. The first read creates the tick queue, and every later microtask checkpoint goes through it.

3,000,000 setImmediate callbacks on main:

first statement	wall	user CPU
nothing	473 ms	472 ms
one read of process.nextTick	492 ms (+3.8 %)	491 ms (+4.1 %)
await import("node:fs")	458 ms	458 ms

A separate instruction-count run reported +8.1 % for the same script, about +126 instructions per callback, and the same slowdown on this branch for the node:fs row. A native arm removes the hop. But if this line stays for the sites that used process.nextTick before (the aborted fs.stat, the realpath loop, opendir), every importer still pays on each timer and immediate callback.

// bun tick-touch.mjs none | touch | fs
const mode = process.argv[2];
if (mode === "touch") { const t = process.nextTick; if (typeof t !== "function") throw 1; }
if (mode === "fs") await import("node:fs");
const n = 3_000_000; let t = 0; const B = 1000;
const t0 = performance.now();
await new Promise(f => { const batch = () => { for (let i = 0; i < B; i++) setImmediate(() => { if (++t === n) f(t); else if (t % B === 0) batch(); }); }; batch(); });
console.log(mode, (performance.now() - t0).toFixed(0), "ms");

2. Measure each variant in its own process. 200,000 fs.stat callbacks, 64 in flight:

variant	wall
main, callback API	311 ms
main, after one process.nextTick(() => {})	364 ms (+17 %)
userland copy of the hop: fs.promises.stat(f).then(s => process.nextTick(cb, null, s))	402 ms (+29 %)

The instruction-count run reported +29.5 % for this shape on this branch. My first attempt ran both variants in one process and gave +9 %. The hop variant created the tick queue, and that slowed the baseline of every later round. The same trap applies to a benchmark of the native arm against the hop.

[robobun]

The native callback arm is pushed in 6174ba5, and the description is rewritten for it.

• Every operation of the binding has a third entry point, xCb(callback, ...args). The task keeps the callback in a Strong and calls it from the completion with EventLoop::run_callback. A callback API now creates no promise, reaction or microtask. node:fs/promises is unchanged.
• I named the entry points xCb, not x_callback as I said above. The class generator names the C++ host wrapper of a method x Prototype__xCallback, so a method named xCallback collides with the wrapper of x (82 C++ errors on the first build).
• Seven callback APIs still deliver through process.nextTick, because they are promise chains in JS and have no operation of the binding to attach a callback to: cp, rm, recursive rmdir, Dir.read, Dir.close, glob, opendir. Node does the same for fs.cp (util.callbackify). Tell me if you want any of these moved to native operations in this PR.
• The two costs of the hop are gone for the operations of the binding, measured on one fs.stat callback: async_hooks reports {} again (the hop reported {"TickObject":1}), and the callback runs when process.nextTick was replaced before node:fs loaded (the hop held it).
• Callback arguments are unchanged: 28 callbacks give identical argument lists before and after. The 45 order tests pass unchanged.
• I did not measure a speedup. That needs a release build, which I do not have here.

Windows x64 debug build: fs.test.ts is 598 pass, 2 fail. Both fail the same way on main without this PR on the same machine, so they are not from this change. I will re-request your review when CI is green.

[claude]

Nothing blocking. The comments below are optional suggestions. There is no need to push a fix for them before merging.

[claude]

Code review completed

Nothing new to post: everything this review found is already covered by existing comments on this pull request or didn't merit a separate one.

This review covers commit 487d83d, which is no longer the latest commit on this pull request; later commits are not covered by it.

[claude]

Code review found no issues

No high-confidence issues detected in this change.