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The worker thread resolved the entry point against the live FileSystem.top_level_dir, which process.chdir() rewrites. A chdir() between new Worker() and the thread's start changed which module loaded. WebWorker::create() now copies the cwd and spin() resolves from that copy through Transpiler::resolve_entry_point_from().
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Updated 8:20 AM PT - Sep 8th, 2026
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Reproduced on Bun 1.4.2 and 1.4.3 (Linux x64) and on a debug build of main at a3e0ab6. From a directory with const workers = [];
for (let i = 0; i < 4; i++) workers.push(new Worker("./worker.js"));
process.chdir("other");
// 1.4.3: workers 2 to 4 load other/worker.js every run (node:worker_threads: the same)
// node 26: all four load ./worker.js
CI: in builds 112841 and 112926 the new tests pass on every lane, Windows and ASAN included. The one test that fails on every retry is |
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WalkthroughChangesEntry-point resolution now accepts an explicit source directory. Web workers capture the directory at construction and use it during preload and entry-point resolution. Tests cover directory changes between worker construction and startup. Worker entry-point resolution
Suggested reviewers: Merge Risk: 🔵 Low · up to Workers now retain their construction-time directory when resolving relative entry points, but a long working directory can leave stale absolute-entry resolution misses cached, and the new scheduling-sensitive regression test may not reliably detect the original behavior. These bounded issues should be addressed before relying on the change broadly. 🚥 Pre-merge checks | ✅ 4✅ Passed checks (4 passed)
✨ Finishing Touches 💡 1🛠️ Fix failing CI checks 💡
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Actionable comments posted: 4
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Inline comments:
In `@src/bundler/transpiler.rs`:
- Line 493: Update the path-length guard in the entry-point handling logic to
use the absolute branch’s dirname(entry_point)-based length when entry_point is
absolute, and the source_dir-based length only for relative paths. Preserve the
existing cache invalidation behavior while computing each branch’s bound
independently.
In `@src/jsc/web_worker.rs`:
- Around line 347-352: Add regression coverage for relative preloads by creating
a fixture that uses a relative preload, constructing the worker, then calling
process.chdir() before execution. Assert the preload still resolves relative to
the captured cwd, independently of the worker entry-point test.
In `@test/js/web/workers/worker-entry-point.test.ts`:
- Line 76: Replace the parameterized test.each call for the “process.chdir()
right after new Worker()” case with describe.each, moving kind into the describe
scope and retaining a single test within each generated case.
- Around line 97-99: Add a deterministic startup barrier or test hook around the
pre-chdir start calls in the worker test so all four workers are confirmed
initialized before process.chdir("other") executes. Preserve the existing
before/after worker assertions while ensuring no pre-chdir worker can resolve
early.
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src/bundler/transpiler.rssrc/jsc/web_worker.rstest/js/web/workers/worker-entry-point.test.ts
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I reviewed this and didn't find any bugs — the cwd capture and parameter threading look correct, and the tests are deterministic under the fix. A human look is still worthwhile given it changes cross-thread worker entry-point resolution and the description names four overlapping open PRs (#41837, #41471, #36584, #40372) that a maintainer should coordinate this with.
What was reviewed:
resolve_entry_point→resolve_entry_point_fromrefactor: every prior read ofself.fs().top_level_dirin the retry/cache-bust path is now the passedsource_dir, and the old entry point delegates unchanged.cwd: Box<[u8]>ownership: owned copy (not a borrow of the in-place-rewritten buffer),Send, freed byWebWorker's Drop; both call sites (preloads increate, main specifier inspin) thread it through.- Tests: both
Workerandworker_threads.Workercovered, error events wired to resolve, assertion order stderr→stdout→exitCode; result is fully determined by the synchronous capture, so not timing-dependent once fixed.
Extended reasoning...
Overview
The PR fixes a race where a Worker's relative entry-point specifier was resolved on the worker thread against the live process cwd, so a process.chdir() between new Worker("./x.js") and the worker thread reaching spin() could load a different file. The fix has three parts: (1) Transpiler::_resolve_entry_point and its cache-bust retry now take an explicit source_dir parameter, with a new public resolve_entry_point_from(source_dir, entry_point) seam and the existing resolve_entry_point delegating to it with the live top_level_dir; (2) WebWorker::create captures a Box<[u8]> copy of the parent VM's top_level_dir at construction time into a new cwd field, and both resolve_entry_point_specifier call sites (preloads on the parent thread, main specifier in spin() on the worker thread) pass it through; (3) tests spawn four workers, chdir, spawn a fifth, and assert the first four resolve pre-chdir and the fifth post-chdir, for both Web Worker and node:worker_threads.
Security risks
None identified. No user-controlled data reaches new sinks — the only new data is a copy of the process's own cwd, and the resolver already handled that value. No auth, crypto, network, or path-traversal surface changes. The unsafe blocks added/touched carry SAFETY comments matching the existing pattern ("parent is the calling thread's live VM"), and the new one just dereferences parent to read top_level_dir() — identical to the surrounding code.
Level of scrutiny
Moderate. The Rust change is small and largely mechanical parameter-threading; resolve_entry_point's existing callers are unchanged by construction. The cross-thread piece is the load-bearing part: cwd is created on the parent thread and read on the worker thread, but it's an immutable owned Box<[u8]> inside a ThreadSafeRefCounted struct, so it's Send, has a single owner, and is freed by the struct's Drop — no manual lifecycle wiring needed. The comment explaining why it must be a copy (chdir rewrites the backing buffer in place) is exactly the non-obvious "why" the review guide asks for. What pushes this past auto-approve is (a) it's a behavior change in worker module resolution, (b) the PR itself names four open PRs touching the same neighborhood that a maintainer should sequence, and (c) unsafe cross-thread code, however small, benefits from a second pair of eyes.
Other factors
The tests follow harness conventions closely (tempDir, bunExe/bunEnv, describe.concurrent, test.each over the variant matrix, error events wired to resolve, stderr/stdout asserted before exit code, pipes drained via Promise.all inside runWorkerFixture). With the fix applied the assertion is fully deterministic — the cwd is captured synchronously in the constructor — so the test is not racy going forward. The unrelated 30 s timeout added to the existing LSAN test sits in tension with test/CLAUDE.md's "do not set a timeout" rule, though it's justified with a concrete measurement (LSAN's exit check alone consumes the default budget on debug ASAN) and the root CLAUDE.md does allow "rare outlier" per-test timeouts; a human can decide whether that's acceptable. The PR description's self-review explicitly scopes out three sibling live-cwd reads (Bun.build entry points, worker tsconfig.json/.env discovery) with reasons — that's the "if a site is intentionally excluded, say so" pattern REVIEW.md asks for. No CODEOWNERS cover the changed paths.
…es with describe.each
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Problem
new Worker("./w.js")thenprocess.chdir("other")loads./w.jsorother/w.jsdepending on timing. On 1.4.3, four workers created back to back and then achdir: workers 2 to 4 load the wrong file every run.node:worker_threadstoo, unlike Node.spin()insrc/jsc/web_worker.rsresolves the specifier on the worker thread withTranspiler::resolve_entry_point, which reads the liveFileSystem.top_level_dir.process.chdir()rewrites that buffer.Fix
WebWorker::create(), which runs inside the constructor on the parent thread, copies the cwd.spin()resolves the specifier from that copy through a newTranspiler::resolve_entry_point_from(source_dir, entry_point).resolve_entry_pointdelegates to it, so other callers do not change.errorevent (node:worker_threads: improve error messages, support environmentData, emit worker event #18768 moved it there for that reason). Preloads use the same cwd.test/js/web/workers/worker-entry-point.test.ts(two new tests, both fail on 1.4.3), plus theworkers/,worker_threadsandcompile/Worker*suites.Bun.buildentry points, the worker'stsconfig.jsonand.envlookups) and the test budget below.Background
FileSystem(src/resolver/lib.rs) is a process-global singleton. Itstop_level_dirstarts as the startup cwd. Entry points (bun ./file.js,new Worker("./file.js")) resolve against it.VirtualMachine, then resolves and loads the entry point, afternew Worker()has returned and in parallel with the parent.Notes
w.mjsandother/w.mjsthat postimport.meta.url:new Worker()in a process spends about 2 ms in one-time setup after the thread is spawned, so that thread usually wins. Every later constructor returns in well under the thread's startup time and loses.create(): that is what the code did before node:worker_threads: improve error messages, support environmentData, emit worker event #18768, which moved it to the worker thread so that a specifier that does not resolve is reported through theerrorevent and exit code 1 (as in Node) instead of a synchronous throw. Capturing the cwd keeps that and removes the one piece of mutable process state the result depended on.top_level_diris copied into aBox<[u8]>, not borrowed: after the firstchdirit points intotop_level_dir_buf, which the nextchdiroverwrites in place.data:,blob:, absolute paths,file:URLs and embedded (--compile) entry points do not read the cwd and are unchanged.Bun.build({ entrypoints: ["./a.ts"] })resolves its entry points on the bundle thread throughTranspiler::resolve_entry_point(src/bundler/bundle_v2.rs), so achdirright after the call has the same race. That is a different API whoseoutdirandroothandling reads the cwd too, and it overlaps Keep the process's working directory in one place, bun_core::cwd #40372.resolve_entry_point_fromis the seam a follow-up would use.top_level_dirwhile it builds its transpiler: the roottsconfig.jsoninconfigure_linkerand.envdiscovery inrun_env_loader(src/bundler/transpiler.rs). Those pick the worker's transpiler settings, not which module loads, and the right base for them is arguably the startup project root (what the main thread used), not any later cwd. That question belongs with Keep the process's working directory in one place, bun_core::cwd #40372, so this PR does not change them.blob:entry at construction), process.chdir: publish an immutable cwd slice so no thread sees a torn cwd #36584 (torn cwd reads across threads), Keep the process's working directory in one place, bun_core::cwd #40372 (onebun_core::cwd; it keeps entry points on the live cwd, so this race stays relevant after it).test/js/web/workers/{worker-entry-point,worker,worker_blob}.test.ts,test/js/node/worker_threads/worker_threads.test.ts,test/bundler/bundler_compile.test.ts -t Worker.worker.test.ts"terminate() while fs.readFile completions keep arriving" times out at 5 s on this machine with and without the change.[human-review] gate passed · iteration 1 · 3 files touched
fails on main (without fix)
passes on PR (with fix)
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