fix(worker): don't keep the main thread alive for a global message listener

[0xfandom]

What does this PR do?

Fixes #24256. Setting a global message listener on the main thread kept the process running forever instead of exiting:

console.log("hello world");
globalThis.onmessage = () => {};
// Node: prints "hello world" and exits.
// Bun:  prints "hello world" and hangs indefinitely.

WorkerGlobalScope::onDidChangeListenerImpl refs the event loop whenever a message listener is added, so a Worker stays alive to receive messages from its parent. That is correct for worker threads, but on the main thread there is no parent message channel, so the ref is never balanced and the process never exits.

The fix skips the event-loop ref/unref when the global scope is on the main thread (ScriptExecutionContext::isMainThread()). Worker threads still get the keepalive, so they continue to receive messages. The same path backs addEventListener("message", ...), so both spellings are covered.

How did you verify your code works?

Added a parameterized test in test/js/web/workers/worker.test.ts that spawns a main-thread process setting a global message listener via both globalThis.onmessage = and addEventListener("message", ...), and asserts it exits on its own (exitCode === 0, signalCode === null). The test times out (hangs) on the released bun and passes on the debug build.

Also confirmed a Worker that sets globalThis.onmessage still receives messages from its parent (existing worker suite passes: 25 pass / 1 todo).

[claude]

Claude Code Review

This pull request is from a fork — automated review is disabled. A repository maintainer can comment @claude review to run a one-time review.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Path: .coderabbit.yaml

Review profile: ASSERTIVE

Plan: Pro

Run ID: 1024e99c-e1d8-4363-b33d-e05138df2304

📥 Commits

Reviewing files that changed from the base of the PR and between f9530ae and f78efeb.

📒 Files selected for processing (2)

• src/jsc/bindings/BunWorkerGlobalScope.cpp
• test/js/web/workers/worker.test.ts

---

Walkthrough

In WorkerGlobalScope::onDidChangeListenerImpl, a guard is added to retrieve scriptExecutionContext and return early when the context is null or belongs to the main thread. A parameterized regression test verifies that setting onmessage or calling addEventListener("message", ...) on the main thread no longer prevents process exit.

Changes

Main-thread message listener event loop fix

Layer / File(s)	Summary
Skip event-loop keepalive on main thread + regression test src/jsc/bindings/BunWorkerGlobalScope.cpp, test/js/web/workers/worker.test.ts	onDidChangeListenerImpl now returns early when scriptExecutionContext is null or is the main thread, preventing m_messageEventCount ref/unref from running outside worker threads. A test.each case spawns a Bun process with each of the two main-thread message listener forms and asserts exit code 0, stdout "ready", and no stderr errors.

🚥 Pre-merge checks | ✅ 4

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title accurately captures the main fix: preventing a global message listener on the main thread from keeping the process alive.
Description check	✅ Passed	The description completely covers both required sections with clear explanations of the issue, the fix, and the verification approach.
Linked Issues check	✅ Passed	The PR directly addresses issue #24256 by fixing the root cause: skipping event-loop ref/unref on the main thread when handling message listeners.
Out of Scope Changes check	✅ Passed	All changes are directly related to fixing the issue described in #24256; no unrelated modifications are present.

_{✏️ Tip: You can configure your own custom pre-merge checks in the settings.}

---

Thanks for using CodeRabbit! It's free for OSS, and your support helps us grow. If you like it, consider giving us a shout-out.

❤️ Share

• X
• Mastodon
• Reddit
• LinkedIn

_{Comment @coderabbitai help to get the list of available commands and usage tips.}