fix(worker): route worker_threads errors like the web surface, and fire once listeners once

[adrian-niculescu]

Errors thrown by parentPort message listeners skip the worker error handlers, and errors consumed by worker.on("error") listeners are also reported as unhandled. A once listener can run twice when another listener emits the same event recursively.

The relay uses rethrowing event dispatch, handled errors cancel the worker event, and once registrations record whether they fired. Node-style error listeners receive an Error rebuilt from the thrown value’s name, message and stack; names and messages preserve UTF-16, including embedded NULs and unpaired surrogates. A throwing stack getter cannot replace the reported error.

The corresponding Android fix is NativeScript/android#2065.

Validation: full TestRunner on an iPhone 17 simulator with Xcode 26.6: 1,782 tests, zero failures or errors (11 skipped). The AddressSanitizer run also had zero failures or errors (16 skipped). Both verdicts were checked in the exported Jasmine JUnit reports. Runtime JavaScript lint passed.

Concurrency

• Nested emits: a once registration is marked fired before its callback; an outer snapshot skips it if a nested emit already invoked it.
• Error delivery and worker teardown: the queued task owns copies of the payload and the parent persistent handle, without capturing the wrapper; an empty handle ends delivery.

Summary by CodeRabbit

• Bug Fixes

  • Worker errors now preserve their original name and message, including errors with unusual values or throwing stack accessors.
  • Errors handled by a worker error listener are no longer also reported globally.
  • Exceptions thrown by parent-port message listeners are now forwarded to the parent’s error handlers.
  • One-time listeners run only once during nested emissions, and event dispatch correctly reports whether listeners were called.

• Documentation

  • Clarified how worker errors are reconstructed and reported.

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📥 Commits

Reviewing files that changed from the base of the PR and between 73ec10a and 77808d2.

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📒 Files selected for processing (10)

• NativeScript/runtime/DataWrapper.h
• NativeScript/runtime/NativeScriptException.mm
• NativeScript/runtime/NsBuiltinModules.cpp
• NativeScript/runtime/Worker.h
• NativeScript/runtime/Worker.mm
• NativeScript/runtime/WorkerWrapper.mm
• NativeScript/runtime/js/node-worker-threads.js
• TestRunner/app/tests/MessagingTests.js
• docs/ns-builtin-modules.md
• docs/worker-threads.md

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🚧 Files skipped from review as they are similar to previous changes (2)

• docs/ns-builtin-modules.md
• docs/worker-threads.md

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📝 Walkthrough

Walkthrough

Worker error forwarding now preserves error names, messages, and stacks across isolates. The parent rebuilds errors for worker error events and suppresses global reporting when an error listener handles the event. Emitter behavior and parent-port exception relaying are also updated.

Changes

Worker error and event handling

Layer / File(s)	Summary
Forward thrown values across isolates NativeScript/runtime/DataWrapper.h, NativeScript/runtime/NativeScriptException.mm, NativeScript/runtime/WorkerWrapper.mm, NativeScript/runtime/Worker.h, NativeScript/runtime/Worker.mm	Worker error and rejection forwarding now carries the thrown value. The worker isolate derives its name and message, and the parent uses the result from Worker::EmitError to determine whether to dispatch a global error.
Rebuild and deliver worker errors NativeScript/runtime/js/primordials.js, NativeScript/runtime/js/worker-events.js, NativeScript/runtime/node-worker-threads.js, NativeScript/runtime/NsBuiltinModules.cpp, eslint.config.mjs, TestRunner/app/tests/messaging/*, docs/ns-builtin-modules.md, docs/worker-threads.md	worker-events rebuilds an error from its name, message, and stack. Worker error listeners receive that error, and tests and documentation cover error details, cancellation, and global reporting.
Update emitter and parent-port relay NativeScript/runtime/js/node-worker-threads.js, TestRunner/app/tests/MessagingTests.js, TestRunner/app/tests/messaging/parentPortThrowingWorker.js	WorkerEmitter.emit reports whether listeners exist and prevents a one-shot listener from firing again during reentrant emission. Parent-port message dispatch rethrows listener exceptions into the worker error chain.

Priority: ➖ Normal

Estimated code review effort: 4 (Complex) | ~45 minutes

Change: Bug fix

Sequence Diagram(s)

sequenceDiagram
  participant WorkerWrapper
  participant ParentLoop
  participant WorkerEmitError
  participant worker-events
  participant WorkerEmitter
  participant ParentGlobalScope
  WorkerWrapper->>ParentLoop: Post prepared error details
  ParentLoop->>WorkerEmitError: Call with name and message
  WorkerEmitError->>worker-events: Invoke emitError with error details
  worker-events->>WorkerEmitter: Dispatch event with rebuilt error
  WorkerEmitter->>ParentGlobalScope: Propagate when error is not handled

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Suggested reviewers: edusperoni

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Merge Risk: ⚪ Minimal · up to 77808

No new merge-blocking issue is established for worker error delivery or listener handling. The previously reported context-scope concern should be tracked separately.

Pre-merge checks | 4 | 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage		Docstring coverage is 42.86% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 7 functions across 13 files. (5 skipped: …	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Description Check		Check skipped - CodeRabbit’s high-level summary is enabled.
Title check		The title clearly identifies both main changes: worker error routing and preventing duplicate invocation of once listeners.
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.

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Full details: Docstring Coverage

Explanation

Docstring coverage is 42.86% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 7 functions across 13 files. (5 skipped: 5 unsupported.)

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Errors cross the threads with names just right.
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[edusperoni]

Thanks, the once-wrapper and relay changes look right. I checked the once semantics against Node 24.18 and lib/events.js on main: the fired flag matches _onceWrap, and the nested-emit case produces the same call sequence.

Two things before merging:

1. What worker.on("error") receives. worker.onerror forwards the runtime's ErrorEvent, so a Node-style listener gets an event object, not an Error. In Node the listener gets the deserialized error (instanceof Error is true, name and message preserved). The new spec only asserts error.message, which both shapes satisfy, so it can't tell them apart. Could you build an Error from the event in worker.onerror (message, with stack from event.stackTrace) and have the spec assert error instanceof Error? The error name doesn't survive the forwarding payload today; that can stay a follow-up.

2. The cancel. return self.emit("error", error) works because this runtime treats a truthy return from Worker.prototype.onerror as handled. Per HTML that's inverted for a Worker object: special error event handling only applies to global scopes, and on a Worker object return false is what cancels. We pin our behavior in the shared tests, so it isn't wrong here, but if (self.emit("error", error)) error.preventDefault(); says what it means and doesn't lean on that contract.

Not for this PR, just recording the remaining gap: Node terminates the worker after an uncaught throw and emits exit with code 1. We keep the worker alive and exit is always 0. No spec covers either side of that.

[adrian-niculescu]

Both done, here and in NativeScript/android#2065.

1. worker.on("error") now receives an Error. Building it from the event alone would have kept the wrong message: event.message is V8's Uncaught Error: boom text, not the error's own message. So the worker reads the thrown value's name and message and sends them with the rest of the error payload, and the parent rebuilds the error from those plus stackTrace, with the matching built-in constructor when there is one. That carries the name as well, so nothing is left for a follow-up, and a DOMException keeps its name too. The specs assert instanceof Error and the exact message, the parentPort one throws a TypeError and asserts instanceof TypeError, and a new one covers a DOMException. When nothing handles the error, the parent's global error event gets the same rebuilt error instead of Error("Uncaught Error: ..."). A thrown value with no string message, such as a string or a number, arrives as an Error whose message is its string form; worker-threads.md documents that, since Node hands over the raw value. The shim reads the rebuilt error off the event through a symbol from internal/worker-events, so event.error stays null, as the shared WorkerEvents spec expects.

2. Switched to if (self.emit("error", error)) event.preventDefault();.

On the exit code: agreed. I also corrected the #reportExit comment, which claimed the shared suite pins exit 0 for every way a worker ends; it only covers terminate(). I can make a worker that dies from an uncaught error terminate and emit exit with 1 in this PR, or leave it out. Which do you prefer?

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Actionable comments posted: 1

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Inline comments:
Review comments at @NativeScript/runtime/WorkerWrapper.mm:
- Around line 649-657: Update DescribeThrownValue to enter the supplied context
before converting the thrown value or accessing its name and message, so these
operations run with the worker context active for every caller.

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📒 Files selected for processing (15)

• NativeScript/runtime/DataWrapper.h
• NativeScript/runtime/NativeScriptException.mm
• NativeScript/runtime/NsBuiltinModules.cpp
• NativeScript/runtime/Worker.h
• NativeScript/runtime/Worker.mm
• NativeScript/runtime/WorkerWrapper.mm
• NativeScript/runtime/js/node-worker-threads.js
• NativeScript/runtime/js/primordials.js
• NativeScript/runtime/js/worker-events.js
• TestRunner/app/tests/MessagingTests.js
• TestRunner/app/tests/messaging/domExceptionThrowingWorker.js
• TestRunner/app/tests/messaging/parentPortThrowingWorker.js
• docs/ns-builtin-modules.md
• docs/worker-threads.md
• eslint.config.mjs

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[edusperoni]

@adrian-niculescu could you rebase on top of latest main? I merged a few changed on the workers (so we could have our own ns:worker_threads with our own typings which created some slight conflicts.

Also, please revert that testing change on the network side, if needed we'll address that on another PR

[adrian-niculescu]

@edusperoni Rebased on main and removed the network-test change in 77808d2.