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Reproduced on bun 1.4.0 with a node:worker_threads Worker ( Fix and tests are in this PR: the 7 new rows under CI on fdf5886 (build 96760): all 177 lanes that ran are green, worker_threads.test.ts included on every platform. The two darwin 14 aarch64 test jobs have not run; they keep expiring unassigned because the |
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WalkthroughWorker error delivery now reconstructs structured error snapshots when cloning fails. It preserves ChangesWorker error delivery
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In `@test/js/node/worker_threads/worker_threads.test.ts`:
- Around line 780-784: Update the worker lifecycle handling in the test.each
worker test to race the worker’s error event against the existing exited
promise. If exit occurs first without an error, reject with the worker’s exit
code; otherwise preserve the current error-wait behavior.
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src/jsc/bindings/webcore/WorkerMessagingProxy.cppsrc/jsc/bindings/webcore/WorkerMessagingProxy.hsrc/jsc/web_worker.rstest/js/node/worker_threads/worker_threads.test.ts
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Updated 7:45 PM PT - Aug 14th, 2026
✅ @robobun, your commit fdf588649eb7f5122a43a287b3feb2080228a5cf passed in 🧪 To try this PR locally: bunx bun-pr 38560That installs a local version of the PR into your bun-38560 --bun |
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I reviewed this PR and didn't find any bugs. Because it adds a new cross-thread error-snapshot path in the JSC bindings (exception-scope handling around hostile getters/Proxies, isolated string copies to the parent thread) and changes what on_unhandled_rejection dispatches when the formatter throws, a human look would still be worthwhile — particularly given the noted overlap with #34424.
Checked: the clearExceptionExceptTermination() guard pattern matches ZigException.cpp's field-by-field error read; strings crossing to the parent are all .isolatedCopy()'d; the Proxy unwrap loop checks isRevoked() before reading target(); the web_worker.rs change preserves the original error rather than the formatter's exception, and the dropped error_instance.is_termination_exception() check on the non-throwing path was already dead (gated by script_allowed() above).
Extended reasoning...
Overview
The PR fixes two failure modes in node:worker_threads error delivery: (1) when Bun.inspect() throws while formatting a worker's uncaught error, the parent received null; (2) when structured clone refuses the error (throwing stack/name getter, Symbol name, throwing prepareStackTrace, Proxy wrapper), the parent received an Error whose message was the entire rendered diagnostic. It touches four files: WorkerMessagingProxy.cpp (~90 new lines: errorCodeOf, dispatchErrorEvent, cloneableErrorType, postErrorSnapshotToWorkerObject), WorkerMessagingProxy.h (one declaration), web_worker.rs (a 6-line change to keep the original error when the formatter throws), and 8 new parametrized test rows in worker_threads.test.ts.
Security risks
The new snapshot path reads properties off a user-controlled, adversarial ErrorInstance (throwing getters, Proxies) on the worker thread. Each read that can run script is guarded with clearExceptionExceptTermination() (returning false on termination, matching the pattern in ZigException.cpp), and the non-script reads use getDirect/sanitizedMessageString. The Proxy unwrap loop checks isRevoked() before dereferencing target(). Strings are moved with .isolatedCopy() before crossing to the parent thread. I did not spot a way for hostile input to escape these guards, but this is exactly the class of code where a subtle exception-scope or thread-affinity mistake becomes a UAF, so it deserves maintainer review.
Level of scrutiny
High. This is new C++ in the JSC bindings layer that (a) runs user script via property reads on the worker's error-dispatch path, (b) crosses threads with string payloads, and (c) changes the fallback ordering in postErrorToWorkerObject. REVIEW.md flags exception-check discipline and thread affinity as the most-blocked categories. The web_worker.rs change also alters behavior: previously a formatter throw replaced error_instance with the taken exception (a JSC::Exception cell); now the exception is taken and discarded unless it is termination, and the original error is dispatched. That is the intended fix, but it is a behavior change on a path that already had a subtle termination check.
Other factors
The tests are thorough — 8 shapes covering both bug classes, each pinning name/message/code/stack, with an exit-race guard so a missing 'error' fails fast rather than timing out. The PR body documents verification under BUN_JSC_validateExceptionChecks=1 + LeakSanitizer and cross-checks against Node v26.3.0. All prior review threads (comment-cop on comment length, CodeRabbit on the test's exit race) are resolved. The PR notes an overlap with #34424 that whichever lands second should reconcile — a coordination point a maintainer should weigh in on.
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One detail from the review above, for whoever takes the human look: the removed CI for 2d57387 (build 96200) is at 177/179 passed with the remaining two still queued, nothing failed. |
… error cannot be printed or cloned
2d57387 to
faf35cb
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Restructured in faf35cb (squashed; the PR body describes the current shape and keeps the earlier one under a details block). The second encoder in Tests: the Proxy row is dropped, the name-getter row now uses a |
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I reviewed this PR and the bug hunting system found no issues. The change looks correct and is well-tested, but it introduces a new per-field exception-clearing policy inside SerializedScriptValue's ErrorInstance arm and reworks the formatter-failure path in on_unhandled_rejection, both of which are subtle enough (JSC ThrowScope semantics, termination handling, overlap with #34424) that a human look from someone familiar with the structured-clone / worker error path is worthwhile.
What was reviewed:
fieldUnreadablegating onm_context == WorkerErrorReportwithtryClearException()followed byRETURN_IF_EXCEPTIONso a termination exception still fails the clone; each field's fallback (name →errorTypeName(errorType()), message → null String, stack skipped after a throwingprepareStackTrace).- Every other
m_contextbranch in the serializer (SAB, WASM module/memory, Histogram, fast path, sharedBuffers) —WorkerErrorReportfalls the same wayDefaultdid, and the fast-path addition preserves the previous behaviour. - The Rust change:
take_exceptionstill runs on the formatter's throw so no exception is left pending, and the removederror_instance.is_termination_exception()check on the success path was already covered byscript_allowed()at the top plus the newif letarm. - Tests: the 7 new rows race
'error'against'exit'so a regression fails rather than hangs; the vendored Node test matches upstream.
Extended reasoning...
Overview
The PR fixes two failure modes in node:worker_threads error delivery: (1) when Bun.inspect() throws while formatting the worker's uncaught error, the parent's 'error' listener received null; (2) when structured clone rejects the error (throwing stack/name getter, Symbol name, throwing prepareStackTrace), the listener received an Error whose message was Bun's whole rendered diagnostic instead of the real message and subtype.
Files touched:
src/jsc/bindings/webcore/SerializedScriptValue.{h,cpp}— newSerializationContext::WorkerErrorReport; in that context the ErrorInstance encoder clears a field's read exception (viatryClearException, so termination still propagates) and omits/falls-back the field instead of failing the whole clone.WorkerErrorReportis added to the fast-path allowlist to preserve the previousDefaultbehaviour.src/jsc/bindings/webcore/WorkerMessagingProxy.cpp— one-line change to pass the new context toSerializedScriptValue::create.src/jsc/web_worker.rs—on_unhandled_rejectionno longer replaces the reported error with the formatter's exception; it takes the exception, returns only if it is the termination, and otherwise keeps reporting the original error (the empty text buffer becomes irrelevant because the clone now succeeds).- Tests: 7 new parametrised rows in
worker_threads.test.tsand Node's upstreamtest-worker-error-stack-getter-throws.jsvendored unchanged.
Security risks
None identified. The change only relaxes the serializer's failure policy for one internal call site (worker → parent error report), and only for Error fields that user code has made unreadable — the parent receives less information than before (the hostile field is dropped), never more. postMessage / structuredClone and the Web Worker path are unaffected because they never pass the new context. I checked every other m_context comparison in the serializer: WorkerErrorReport behaves identically to the previous Default on all of them.
Level of scrutiny
This deserves a human look. It is not large (~30 lines of production code), but it sits in JSC C++ exception-handling territory that REVIEW.md singles out as the most-blocked category: it introduces a new pattern of clearing an exception mid-serialization (the repo guidance is "never clearException()", and while tryClearException is the termination-safe variant already used elsewhere in bindings, adding it to the structured-clone encoder is a design choice a maintainer should sign off on). It also replaces an earlier revision that took a different architectural approach, and it overlaps with open PR #34424 in a way the author has documented but which needs coordination.
Other factors
- CI is green on faf35cb apart from an unrelated Windows
bake/deinitialization.test.tssegfault also failing on main. - The tests were verified under
BUN_JSC_validateExceptionChecks=1+ LeakSanitizer per the PR body, which directly exercises the ThrowScope discipline the change touches. - The
fieldUnreadable→RETURN_IF_EXCEPTIONsequencing looks correct: iftryClearException()returns false (termination), the exception is still pending and the immediately followingRETURN_IF_EXCEPTIONfails the clone, falling back to the text path exactly as before. - The
mutonerror_instancein the Rust change is still required for the earlierBuildMessagereassignment; the removed post-format termination check was reachable only via the formatter-swap that this PR removes, and terminations requested before formatting are already handled by thescript_allowed()gate at the top.
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On the exception-clearing point raised above, for whoever takes the human look: the report already cleared exceptions at this boundary before this PR. |
…repareStackTrace Whether Error.prepareStackTrace runs during the report depends on whether a GC already turned the error's frames into a stack string, so dropping the field only when it throws was nondeterministic. Carry whatever stack the error has, which is a string either way, and drop the vendored upstream test that pins undefined.
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fdf5886: the |
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I reviewed this PR and didn't find any bugs. Because it threads a new per-field exception-clearing policy through the shared SerializedScriptValue ErrorInstance encoder and reworks the formatter-throw path in web_worker.rs, a human look would still be worthwhile — in particular at fdf5886, which drops the vendored upstream test-worker-error-stack-getter-throws.js and diverges from Node on stack after a throwing prepareStackTrace (the PR body still describes the earlier revision that vendored it).
What was reviewed:
fieldUnreadablelambda:tryClearException()leaves a termination pending, so the followingRETURN_IF_EXCEPTIONstill fails the clone; no check macros inside the lambda.canUseFastPathgainingWorkerErrorReport: only reached for non-object values, where the ErrorInstance arm never runs.web_worker.rs: the removed success-pathis_termination_exception()check was unreachable afterto_error();take_exceptionstill clears the formatter's throw beforedispatchErrorruns.- New test rows race
'error'against'exit'so a missing emit fails instead of timing out;structuredClone/postMessagenever pass this context.
Extended reasoning...
Overview
Five files: a new SerializationContext::WorkerErrorReport enum value (SerializedScriptValue.h), a per-field exception-tolerant policy inside the ErrorInstance arm of CloneSerializer::dumpIfTerminal gated on that context (SerializedScriptValue.cpp), the one-line call-site change passing the new context from postSerializedErrorToWorkerObject (WorkerMessagingProxy.cpp), a rework of the formatter-failure branch in on_unhandled_rejection so the original error is reported instead of the formatter's own exception (web_worker.rs), and seven new test.each rows plus a race-against-exit harness in worker_threads.test.ts. The vendored upstream Node test mentioned in the PR body was added in faf35cb and removed again in fdf5886.
Security risks
None identified. The change only affects how a worker's uncaught error is serialized for the parent's 'error' event; no auth, crypto, or untrusted-input parsing is involved. The new tryClearException() calls are gated on the WorkerErrorReport context, which only postSerializedErrorToWorkerObject passes, so postMessage/structuredClone semantics are unchanged (the structured-clone suites were run and pin that).
Level of scrutiny
High. This is JSC bindings C++ with per-field exception clearing inside a ThrowScope — the most-blocked category in the review guide — plus a subtle change to the worker's termination-vs-ordinary-throw handling on the Rust side. The change is small and well-argued, and the exception-checks story (BUN_JSC_validateExceptionChecks=1 run, no macros inside the lambda, tryClearException() refusing termination so the existing RETURN_IF_EXCEPTION still fails the clone) holds up on reading. But the last commit makes a deliberate Node-compat trade-off — reporting the default stack string instead of dropping stack when prepareStackTrace throws, and removing the upstream test that pinned undefined — for a determinism reason (whether prepareStackTrace runs during the report depends on whether a GC already materialized the stack). That is a product decision a maintainer should sign off on, and the PR body has not been updated to reflect it.
Other factors
The PR went through a substantial restructuring after review (a duplicate encoder in WorkerMessagingProxy.cpp was replaced with the context-based policy), all bot comments are resolved, and CI on the last two pushes was green apart from an unrelated Windows bake/deinitialization segfault also on main. The overlap with #34424 (its retry-after-clearing-stack hunk becomes dead) is another reason for a human to coordinate the landing order.
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The body is current as of fdf5886 (the review above was generated against the push, before the edit landed): the |
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A case next to this fix, found during work on #37270. I did not build this branch. The statement about it comes from its diff. Input. A const { Worker } = require("node:worker_threads");
const w = new Worker(
`let e = new Error("leaf"); for (let i = 0; i < 500; i++) { const x = new Error("l" + i); x.cause = e; e = x; } throw e;`,
{ eval: true },
);
w.on("error", x => console.log(x === null ? "null" : x.message));
A possible extension. Take what the render left pending in both cases: let left_pending = match format_result {
Err(err) => Some(global_object.take_exception(err)),
Ok(()) => global_object.try_take_exception(),
};
if left_pending.is_some_and(|exception| exception.is_termination_exception()) {
return;
}With that change on main a4f1429 (debug+ASAN build) the parent receives:
The last row is not a defect of the reporter. The array walk of the formatter calls JSC with the exception pending, before #37270 does not change |
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2 participants
Problem
worker.on("error", err => ...)(node:worker_threads) receivesnullwhen the worker dies from an error thatBun.inspect()cannot print: a throwing[util.inspect.custom], or anAggregateErrorwhoseerrorsis not iterable (a number, a revoked Proxy,{ length: 2**32 - 1 }).err.messagein the listener then throws in the parent.Errorwhose.messageis bun's whole rendered diagnostic (numbered source lines of the worker, caret, frames) when the error prints fine but structured clone refuses it: a throwingstackornamegetter, a Symbolnameormessage, a throwingError.prepareStackTrace. The real message and the error subtype are gone, and the worker's source text lands in a field that commonly gets logged or forwarded.Errorcarrying the message for all of these:lib/internal/error_serdes.jscopies the error field by field and skips the fields that throw. Node's upstreamtest-worker-error-stack-getter-throws.jscovers theprepareStackTracecase; see the note on it under Fix.null:on_unhandled_rejectioninsrc/jsc/web_worker.rsreplaced the error being reported with the exception the formatter threw. That value is aJSC::Exceptioncell, which cannot be cloned, and the text buffer is empty, so the parent got anErrorEventwitherror: null, message: "", whichworker_threads.tsemits asnull.WorkerMessagingProxy::postSerializedErrorToWorkerObject), and the clone'sErrorInstancearm inSerializedScriptValue.cppfails as a whole on the first field that throws, which is the right behaviour forpostMessage/structuredClonebut leaves the report with only the rendered text to fall back to.Fix
web_worker.rs: when formatting throws, take the exception and keep reporting the original error; a termination exception still ends the dispatch as before. The text is only the fallback payload, and these errors clone fine.SerializedScriptValue.{h,cpp}: a newSerializationContext::WorkerErrorReport, passed only by the worker error report (one-line change inWorkerMessagingProxy.cpp). In that context theErrorInstancearm clears the exception of a field that throws and leaves the field out instead of failing the clone: an unreadablenamefalls back to the error's ownerrorType()(so aTypeErrorwith a hostilenamestill arrives as aTypeError), an unreadablemessageis omitted, and a throwingstackgetter dropsstack. Every other context behaves exactly as before, and the existingerror.codehand-off and the deserializer are reused unchanged.Error.prepareStackTraceis cleared the same way, and the report then carries whateverstackthe error has, which in bun is a string either way: bun stores the default-formatted text on the error before invokingprepareStackTrace, and if a GC has already turned the frames into a string (which happens when the frames' code has died, routinely for an error thrown at the top level of anevalworker)prepareStackTraceis never consulted at all. An earlier revision dropped the field when the call threw, to match node'sstack === undefined; since whether it throws during the report depends on GC timing, that was nondeterministic (it failed once on the alpine x64 lane), so the report does not special-case it and the upstream test, which pinsundefined, is not vendored. The message and subtype, which are the point of the report, are unaffected.postMessagewant the same fields with different failure policy, and the serializer already carries a per-call-site context, so the policy is a few lines in the one encoder. Getters run once, the name-to-subtype mapping is the same one the clone uses, and nothing new crosses threads.ErrorInstance(a thrown number, object, function, a Proxy around an error) behave as before: cloned, or the rendered text for the uncloneable ones, which is what the existing "falls back to string" test pins. Node also sends text for the Proxy case. WebWorkeris untouched; it only ever gets the text.test/js/node/worker_threads/worker_threads.test.ts, new rows undererror event(throwinginspect.customand non-iterableerrorsfor the formatter half; throwingstackgetter on aRangeErrorwith acode, throwingnamegetter on aTypeError, Symbolname, throwingprepareStackTrace,AggregateErrorwith a throwingstackgetter for the clone half). All 7 fail on the release build (nullor the diagnostic) and pass with the fix. TheprepareStackTracerow was also checked by hand on both materialization paths (lazy, and pre-computed by a forced GC before the throw): both deliverError "boom"with a string stack.BUN_JSC_validateExceptionChecks=1plus LeakSanitizer (how the ASAN lanes run these files),test/js/web/workers/structured-clone.test.tsandstructuredClone-classes.test.ts(288 tests, pinning thatpostMessage/structuredClonestill propagate these throws), and the vendoredtest-worker-*uncaught*,*syntax-error*,*exit-event-error*,*unsupported-things*,*messaging-errors-handler*tests.prepareStackTraceshape by overwriting the error'sstackand cloning again insidepostSerializedErrorToWorkerObject, and vendors the upstream test. With this context the first clone succeeds, so that retry never triggers and whichever lands second can drop the hunk. The GC behaviour above applies to that approach as well: when the frames were already collected the first clone succeeds there too, with a string stack, so the vendored test would be flaky under either PR until the finalizer path stops bypassingprepareStackTrace(a separate, pre-existing issue). This PR no longer touches thecoderead or the text fallback that node:worker_threads: per-thread --use-system-ca, real eventLoopUtilization, --cpu-prof in workers, node's online timing, error.code / stack-getter / timeOrigin fixes, async_hooks WORKER resource, worker_threads dc channel (+10 upstream tests) #34424 also changes.Background
on_unhandled_rejection(worker thread,web_worker.rs) renders the error to text with the console formatter and callsWebWorker__dispatchError;WorkerMessagingProxy::postErrorToWorkerObject(still on the worker thread) structured-clones the value and posts it to the parent thread, falling back to posting the text if the clone fails; on the parent the task dispatches anErrorEvent, andsrc/js/node/worker_threads.tsturns it into the'error'emit, usingevent.errorwhenevent.messageis empty and otherwise wrapping the text innew Error(message).SerializedScriptValueis the structured clone implementation shared bypostMessage,structuredCloneand this report. For anErrorInstance(JSC's native error object) it encodes name (mapped to one of the standard constructors), message, stack and position, and it deliberately propagates getter throws becausepostMessagemust (Node parity).SerializationContextis an enum the caller passes to say what kind of message is being built; the serializer keeps it asm_contextand already consults it for things like SharedArrayBuffer handling, which is why adding a value needs no signature changes.ErrorInstance::errorType()is the constructor the error was created with, independent of itsnameproperty;stack,line,columnandsourceURLare materialized lazily on first access, which is whenError.prepareStackTraceruns, hence the explicit materialization step in the arm.ThrowScope::tryClearException()clears a pending JS exception unless it is JSC's termination exception (raised byworker.terminate()/process.exit()in the worker), which must stay pending; in that case the field helper reports nothing cleared, the existingRETURN_IF_EXCEPTIONfails the clone, and the report takes the text path as today.Probe: what the parent's listener receives per shape
bun 1.4.0release vs this branch (debug build) vs node v26.3.0; worker source iseval: true. Node'sAggregateErrorresults carryname: "AggregateError"as an own property on anError-prototyped object; bun's clone has always produced a plainErrorfor anyAggregateError.AggregateError,e.errors = 42nullError "boom", stack kept"boom", stack keptAggregateError,errors = { length: 2**32-1 }nullError "boom""boom"AggregateError,errors= revoked ProxynullError "boom""boom"AggregateError,errorsgetternull(debug build hits an unrelated existing assertion in the formatter, so not in the test)Error "boom""boom"inspect.customon aTypeErrorwithcodenullTypeError "boom",codeand stack keptstackgetter on aRangeErrorwithcodeErrorwith the rendered source as messageRangeError "boom",codekept, nostacknamegetter on aTypeErrorTypeError "boom", stack keptTypeError "boom", no stacke.name = Symbol()on aTypeErrorTypeError "boom"e.name = "TypeError"on anError, throwingstackgetterTypeError "boom"(name mapping, as for any clone)Error-prototyped,name: "TypeError","boom"Error.prepareStackTraceError "boom", default stack textError "boom", no stackAggregateErrorwith throwingstackgetterError "boom", nostack"boom", no stacke.message = Symbol()Error ""Error ""TypeErrorthrow 42/throw "s"/throw {a: 1}throw function f() {}Error "[Function: f]"(rendered text)"[Function: f]"Two shapes are intentionally not in the test file: the
errorsgetter shape trips an existing debug assertion in the formatter's AggregateError printing, and the Symbolmessageshape trips an existing unchecked-exception report in the error printer underBUN_JSC_validateExceptionChecks=1, which the ASAN lanes enable for this file. Both are independent of this change and reported separately. The rows do not pin the first line oferr.stackeither: whether it readsAggregateError: boomor justErrordepends on whether a GC ran before the stack was first read, also a pre-existing issue.Earlier revision of this PR
The first revision left the serializer alone and added a second encoder in
WorkerMessagingProxy.cpp(postErrorSnapshotToWorkerObject): when the clone failed it re-read message / stack / position itself with sanitized accessors, unwrapped Proxies, posted the strings to the parent and rebuilt the error there. Review pointed out that this duplicated the serializer'sErrorInstancearm with slightly different rules (type fromerrorType()rather than fromname, soe.name = "TypeError"plus a hostilestackcame out asErrorwhile a plain clone of the same error givesTypeError), ran user getters twice, and overlapped #34424 in four places, while the serializer already had a per-call-site context to hang the tolerant policy on. The current revision is that restructuring; the Proxy unwrapping was dropped with it since node sends text for that shape as well.[review] gate passed · iteration 3 · 5 files touched
fails on main (without fix)
passes on PR (with fix)
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