Fix multithreading to work in Deno and Bun #25947

Macil · 2025-12-13T04:37:02Z

Projects using Emscripten with pthreads don't work in Deno or Bun because those runtimes implement Web Worker APIs that Node doesn't, and Emscripten attempts to re-implement those APIs when running in Node/Deno/Bun in a way that conflicts with Deno and Bun's own implementations. This PR feature-detects Deno and Bun's postMessage implementation and avoids setting up its own conflicting implementation in that case.

Fixes: denoland/deno#17171

sbc100

Nice!

We also now have some testing under bun as of #25903. Can you add 1 or more pthread-based tests to the lists of bun tests in .circleci/config.yml?

src/runtime_common.js

Macil · 2025-12-14T04:01:55Z

src/wasm_worker.js

    return f;
  }

+  const parentPort = worker_threads['parentPort'];


(The lines below were relying on this variable being set in runtime_common.js, which I had moved into an if-block and made not global. I discovered this after tests failed. I figured the simplest solution was to make this code use its own local variable instead of re-introducing the global variable.)

To save on code size and keep this patch as small as possible can you revert this and hoist const parentPort = worker_threads['parentPort']; outside the if?

Also, we still use var in most places in our codebase sadly (there are a couple of reasons, that are probably not worth explaining here... i really should create an FAQ entry or some such for it).

test/pthread/test_pthread_unhandledrejection.c

Macil · 2025-12-14T04:09:12Z

I've added some pthread tests to the Bun tests and added some Deno tests too.

(I originally added a few tests for Deno that happened to use Emscripten's PROXY_TO_PTHREAD option, but those tests failed. I spent a while debugging it so I could maybe fix that issue in this PR too, but it turns out to be because Deno doesn't implement Atomics.waitAsync.)

Macil · 2025-12-15T00:08:27Z

test/common.py

+      return config.DENO_ENGINE
+    return None
+
+  def get_node_bun_or_deno(self):


(I'm not sure if some increasingly specific helper functions like this were the best route to go. One alternative would have been to make the get/require_node helpers to allow Deno/Bun too because they're mostly interchangeable, but it's probably better to be explicit. Another alternative would've been to make some generic helpers that take named parameters declaring which of Node/Node-Canary/Deno/Bun are allowed.)

I'm hoping we can just avoid this completely, but allowing NODE_JS_TEST to be set to /path/to/bun or /path/to/deno. No need for new config vars I think/hope.

I embraced that idea and it's much simpler now.

sbc100

To keep this change small and focused can you remove all the Deno stuff and we can consider that in a separate PR.

test/common.py

src/pthread_esm_startup.mjs

sbc100 · 2025-12-17T00:33:43Z

tools/shared.py

+  if override:
+    actual = override
+  else:
+    actual = utils.run_process(nodejs + ['--version'], stdout=PIPE).stdout.strip()


Ideally this change would live only in test/ code and only effect NODE_JS_TEST, not the main version of node js that we use.

i.e. can you make local function in test/common.py that does this override? Then use the local version instead of shared.get_node_version?

sbc100

Nice!

lgtm % one last comment

Fixes denoland/deno#17171 Also adds several pthread tests with Bun

Copilot

Pull request overview

This PR fixes multithreading support for Deno and Bun runtimes by detecting their native Web Worker API implementations and avoiding conflicts with Emscripten's own reimplementation. The fix uses feature detection to check if postMessage already exists on the global scope, and only sets up Emscripten's Node.js-specific message handling when needed.

Key changes:

Added feature detection for globalThis.postMessage in pthread worker initialization code
Introduced test infrastructure to override Node.js version detection for non-Node runtimes
Expanded CI testing to validate pthread functionality with Bun

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
`src/runtime_common.js`	Conditionally sets up message handling only when `postMessage` is not already defined, allowing Deno/Bun's native implementation to be used
`src/pthread_esm_startup.mjs`	Applies the same `postMessage` detection logic for ESM integration mode
`test/common.py`	Adds `get_node_test_version()` helper to allow overriding version detection via `OVERRIDE_NODE_JS_TEST_VERSION` environment variable
`test/test_other.py`	Updates all version check call sites to use the new `get_node_test_version()` helper
`.circleci/config.yml`	Configures Bun testing with version override and adds pthread test coverage

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Copilot · 2025-12-21T22:55:56Z

test/common.py

+  def get_node_test_version(self, nodejs):
+    override = os.environ.get('OVERRIDE_NODE_JS_TEST_VERSION')
+    if override:
+      override = override.removeprefix('v')
+      override = override.split('-')[0].split('.')
+      override = tuple(int(v) for v in override)
+      return override
+    return shared.get_node_version(nodejs)


Consider adding a docstring to document this helper function, especially the OVERRIDE_NODE_JS_TEST_VERSION environment variable behavior. This would help other developers understand when and why this override mechanism should be used. For example:
"""Get the Node.js version for testing purposes.

Returns the version tuple from OVERRIDE_NODE_JS_TEST_VERSION environment
variable if set, otherwise returns the actual version of the nodejs binary.
This allows testing with runtimes like Bun/Deno that masquerade as Node.js."""

sbc100 reviewed Dec 13, 2025

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Macil marked this pull request as draft December 16, 2025 02:58

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Macil marked this pull request as draft December 21, 2025 22:05

Macil force-pushed the fixThreadingForDenoAndBun branch 2 times, most recently from 4d052f0 to 071a233 Compare December 21, 2025 22:16

Fix multithreading support for Deno and Bun

0b0e5ed

Fixes denoland/deno#17171 Also adds several pthread tests with Bun

Macil force-pushed the fixThreadingForDenoAndBun branch from 071a233 to 0b0e5ed Compare December 21, 2025 22:36

Macil marked this pull request as ready for review December 21, 2025 22:52

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sbc100 enabled auto-merge (squash) December 22, 2025 00:50

Fix multithreading to work in Deno and Bun #25947

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Fix multithreading to work in Deno and Bun #25947

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Macil commented Dec 13, 2025 •

edited by sbc100

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