Fix a crash when a module is removed while its import() is in flight

[dylan-conway]

What does this PR do?

Fixes a segfault at address 0x10 in JSC::JSModuleLoader::loadModule ← moduleLoadTopSettled (Sentry BUN-4NFS, BUN-41VH), on every platform. The fix is in oven-sh/WebKit#748 (merged), which also explains why it belongs there. This PR bumps WebKit to it and adds the tests.

Two things ride along:

• The WebKit bump goes to fb1167ebf2cb, the current tip. Besides the fix (0e8e9c238e91) that brings in one more commit, [Linux] Read OS randomness with getrandom(2) and open /dev/urandom only when it fails WebKit#749: on Linux, OS randomness is read with getrandom(2), and /dev/urandom is opened only when that fails.
• A type error on main. src/js/bun/sql.ts(892,26): error TS2339: Property 'command' does not exist on type '{}' came in with sql: reject sql.begin() when COMMIT is answered with the ROLLBACK tag #35119 and fails "Lint JavaScript" for every PR that includes it. unsafeQueryFromTransaction() now says what its query resolves to (SQLResultArray). It is a type argument only, so the code that runs is the same.

Cause

The module loader has two tables of what is loaded: the registry, and a shortcut that lets a repeated import() skip resolving. Removing a module clears both. But an import() of that module that is still in flight adds it to the shortcut afterwards, when its dependencies have loaded. The next import() finds the old module there, pairs it with a new registry entry that has no load promise yet, and dereferences null.

const inFlight = import("./a"); // a.ts is registered, its dependencies are still loading
mock.module("./a", () => ({ a: "mocked-a" }));
await inFlight;
await import("./a"); // segfault

Everything that removes a module while its import() is in flight gets there:

route	1.4.2	canary 11c41c645	with the fix
delete require.cache[path]	segfault	ok (#40123)	ok
mock.module()	segfault	segfault	ok
build.module() in a plugin	segfault	segfault	ok
a bun --hot reload	segfault	segfault	ok
a plugin's onResolve redirecting a → b → c → d, delete require.cache[d], import(a) again	segfault	segfault	ok

The last one needs nothing in flight: the shortcut is keyed by the name that was asked for, and removing a module cleared it only by the name it resolved to. It was found in review, and nothing in the crash reports points at it.

Of the 481 reports, 273 are from bun test, 203 from bun run or bun <file>, and 240 had an HTTP server running.

Fix (oven-sh/WebKit#748)

The place that adds a module to the shortcut first checks that the registry still holds it, and skips the write if not. And removing a module clears it from the shortcut under whatever name it was asked for. It is eleven lines in Bun's part of the loader. For a program that removes no module nothing changes.

How did you verify your code works?

Four new tests, one for each route that still crashes. All four fail on 1.4.2 and on canary 11c41c645, and pass against oven-sh/WebKit#748 on debug and release builds (Linux x64). Without the WebKit bump, CI had all three segfault at 0x10 on every platform: macOS, Linux glibc and musl, and Windows, x64 and arm64. The hot reload test passed 10 runs of 10.

test	file
mock.module() of a module whose import() is still loading its dependencies	mock-module.test.ts
build.module() of a module whose import() is still loading its dependencies	plugins.test.ts
should import a module again after a hot reload while its import() was still loading its dependencies	hot.test.ts
import() after delete require.cache of a module that onResolve redirected a resolved path to	plugins.test.ts

The WebKit PR has the evidence that nothing changes for a program that removes no module, the comparison of behavior with main, WebKit's own module tests and the cost.

[robobun]

^{Updated 3:55 PM PT - Sep 30th, 2026}

@dylan-conway, your commit 0c8a939 is building: #122033

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Walkthrough

The changes add regression tests for module replacement, redirected import resolution, and hot reload during in-flight imports. They also update the selected WebKit commit.

Changes

Module loading behavior

Layer / File(s)	Summary
Module replacement during loading test/js/bun/plugin/plugins.test.ts, test/js/bun/test/mock/mock-module.test.ts	Subprocess tests check that an in-flight import receives the original module while a later import receives the registered replacement or mock.
Hot reload during loading test/cli/hot/hot.test.ts	A test delays dependency loading during a dynamic import, triggers a reload, and checks that the in-flight and subsequent imports report evaluations 1 and 2.
Redirected import resolution test/js/bun/plugin/plugins.test.ts	A subprocess test redirects a.mjs through b.mjs and c.mjs to d.mjs, removes d.mjs from require.cache, and checks that importing a.mjs evaluates d.mjs again.

WebKit version

Layer / File(s)	Summary
WebKit version selection scripts/build/deps/webkit.ts	WEBKIT_VERSION changes from commit f20ce7744553c910bcf16a33faf976af208de091 to commit 0e8e9c238e91b60e944b4a33c1a596bdcb0e29b9.

Suggested reviewers: jarred-sumner, robobun

Priority: ⬆️ High

Merge Risk: ⚪ Minimal · up to e311f

The change updates WebKit and adds regression tests for imports interrupted by module removal. No actionable merge-blocking issue is established by the supplied evidence; mergeability remains subject to normal build and test checks.

🚥 Pre-merge checks | ✅ 4

✅ Passed checks (4 passed)

Check name	Status	Explanation
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Title check	✅ Passed	The title clearly identifies the primary change: fixing a crash caused by removing a module while its import() remains in flight.
Description check	✅ Passed	The description includes both required sections. It clearly explains the crash, root cause, WebKit fix, related changes, affected routes, and verification results with four regression tests.

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Actionable comments posted: 2

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Inline comments:
Review comments at @scripts/build/deps/webkit.ts:
- Line 6: Update WEBKIT_VERSION from the preview pin to the published WebKit
build corresponding to the merged WebKit commit, so builds do not depend on the
preview release.

Review comments at @test/js/bun/plugin/plugins.test.ts:
- Line 1068: Change the independent subprocess tests to run concurrently: in
test/js/bun/plugin/plugins.test.ts at lines 1068–1068 and 1197–1197, use
concurrent test declarations; at lines 1116–1116 and 1161–1161, place the
parameterized cases in concurrent groups. In
test/js/bun/test/mock/mock-module.test.ts at line 501–501, use a concurrent test
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• scripts/build/deps/webkit.ts
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• test/js/bun/plugin/plugins.test.ts
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• test/js/bun/test/mock/mock-module.test.ts

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• src/jsc/bindings/ModuleLoader.cpp

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[claude]

Findings marked 🟡 are optional suggestions and need no follow-up push.

Additional findings (outside the current diff — GitHub can't attach inline comments there):

• 🟣 src/jsc/bindings/ModuleLoader.cpp — Under bun test --isolate, a debug or ASAN CI build can abort on the IsolatedModuleCache::insert assertion at ModuleLoader.cpp:512 when a key is fetched twice concurrently. With the bumped loader, mock.module() or build.module() unbinds an in-flight entry (BunPlugin.cpp:162, :745); if Bun.plugin.clearAll() then drops the mock before the first fetch fulfils, the next import() misses the cache at ModuleLoader.cpp:1100 and starts a second real fetch. Both Bun__onFulfillAsyncModule calls insert the same specifier and trip "insert for already-cached key". Fix: make insert idempotent (lookup-or-insert) or evict the isolation cache entry whenever a registry entry is removed, covering every removeEntry/clearAll site.

  Why this was flagged

  Trigger: bun test --isolate with const p = import("./a") while a.ts's own onLoad is async and pending, then mock.module("./a", …) (BunPlugin.cpp:745 removeEntry) followed by Bun.plugin.clearAll(), then a second import("./a") before the first onLoad resolves. The second load finds no mock in runVirtualModule (ModuleLoader.cpp:994) and no cached provider at ModuleLoader.cpp:1100 because the first fetch has not fulfilled yet, so it issues a second real fetch. Each fulfilment reaches ModuleLoader.cpp:511-512 and calls IsolatedModuleCache::insert with the same key; IsolatedModuleCache.cpp:37 asserts isNewEntry with the message "IsolatedModuleCache::insert for already-cached key — a lookup was bypassed". Population: --isolate users on debug/ASAN lanes; release builds silently keep the first provider. Remedy: evict the isolation cache on removeEntry/clearAll, or make insert tolerate an existing key.

  Verification: pre-existing. Bun__onFulfillAsyncModule (src/jsc/bindings/ModuleLoader.cpp:510-512) unconditionally does IsolatedModuleCache::insert(vm, specifier, provider) with no prior lookup; insert (src/jsc/bindings/IsolatedModuleCache.cpp:35-37) does cache.add(key, ...) and ASSERT_WITH_MESSAGE(result.isNewEntry, ...). No executable line in this file changes.

[claude]

Code review completed

Nothing new to post: everything this review found is already covered by existing comments on this pull request or didn't merit a separate one.

This review covers commit 6bfdd9d, which is no longer the latest commit on this pull request; later commits are not covered by it.

[claude]

Nothing blocking. The comments below are optional suggestions. There is no need to push a fix for them before merging.

Beyond the two inline nits, the earlier blocker is addressed in code: scripts/build/deps/webkit.ts:6 now pins a full 40-hex commit instead of the preview tag, and the Windows path and concurrency points from the previous round are reflected in the tests. I could not fetch oven-sh/WebKit from this environment, so whether that sha is the merged commit of oven-sh/WebKit#748 with a published autobuild release for every os/arch/flavor still needs a human check before landing; the hot test's double-reload path and its use of the file-level timeout constant were also examined and ruled out.

Extended reasoning...

The diff is a one-line WebKit pin bump plus four subprocess tests (plugins, mock.module, --hot) that hold a dependency load open while a module is replaced; the behavioral fix itself lives in the upstream WebKit change, with no Zig/C++/Rust source touched here and no security-sensitive surface. The earlier preview-tag pin that blocked landing has been replaced by a full commit sha, but the release's existence and its correspondence to the merged upstream PR could not be verified from this checkout, so a human should confirm that before merge. The two surviving findings are test-signal nits that do not affect correctness.

[claude]

Nothing blocking. The comments below are optional suggestions. There is no need to push a fix for them before merging.

Beyond the inline finding, I also checked that the WEBKIT_VERSION pin in scripts/build/deps/webkit.ts:6 is now a 40-hex commit sha rather than a preview-PR tag, so prebuiltUrl() no longer depends on an ephemeral release (I could not fetch the release itself from here to confirm every platform tarball is published). The new hot.test.ts test's Infinity-on-debug timeout reuses the file's existing timeout constant, so it matches the file's local convention rather than adding a new outlier.

Extended reasoning...

The diff is a WebKit version bump (f20ce774 -> 0e8e9c23, now a permanent commit sha) plus four new subprocess tests covering module removal while an import() is in flight; no Bun source and no security-sensitive surface is touched. Not approving because one verified finding is posted inline and further verified findings were withheld from posting, and the actual fix lives in the WebKit dependency, which cannot be reviewed from this checkout.

[claude]

Nothing blocking. The comments below are optional suggestions. There is no need to push a fix for them before merging.

Beyond the inline finding, I also checked that scripts/build/deps/webkit.ts:6 now pins a plain 40-hex commit rather than a preview-PR tag, so the ephemeral-artifact concern from earlier pushes no longer applies to this diff (I could not confirm from here that prebuilt tarballs exist for every platform/flavor for that sha). The src/js/bun/sql.ts change is a type-argument annotation only, with no runtime effect.

Extended reasoning...

The diff is a WebKit pin bump, a type-only annotation in sql.ts, and four new subprocess tests for module removal during an in-flight import(); it touches no auth, crypto, or input-parsing surface. A confirmed inline finding on the mock.module test plus the fix living entirely in the WebKit bump (not verifiable from this checkout) mean a human should still look.

[claude]

🟣 pre-existing, not blocking: pre-existing: a test that calls mock.module() while the target's own fetch (its onLoad) is still pending keeps getting the real module from the next import(), silently, on base and after this PR. mock.module at BunPlugin.cpp:522 finds no registry entry because the C++ loader registers a key only after its fetch settles (ModuleLoader.cpp:494-507), so nothing is removed; the pending fetch then registers the real module and import() at ZigGlobalObject.cpp:3686 reuses it. Same for build.module() (BunPlugin.cpp:162) and a --hot reload during the fetch, which re-registers the pre-reload module. Fix: cover this sibling of the in-flight case, e.g. register the entry before fetch (the FIXME) or note it as excluded in the PR.
A small fix can ride a push you are already making; otherwise a short reply is enough.

Why this was flagged

Trigger: a plugin onLoad matching a.ts itself returns a pending promise; test code runs const p = import("./a"); mock.module("./a", factory); before that promise resolves. JSMock__jsModuleMock calls findLoadedESModule (BunPlugin.cpp:513-543); registryEntry(specifierIdent) at BunPlugin.cpp:522 is null because, per the comment at src/jsc/bindings/ModuleLoader.cpp:494-499, the loader creates the registry entry only inside ModuleLoadTopSettled after the embedder fetch promise resolves. staleESMEntry stays false, so removeEntry at BunPlugin.cpp:745 never runs; only addModuleMock at :753 happens. When the onLoad promise later resolves, Bun__onFulfillAsyncModule (ModuleLoader.cpp:472-531) resolves the fetch with the real source and provideFetch registers the real a.ts. The next import("./a") resolves through resolveVirtualModule at ZigGlobalObject.cpp:3683 to the same key and requestImportModule at :3686 finds the registered real record, so the user gets "real-a" where they asked for the mock; no error is raised. The base branch behaves the same, so this is pre-existing.

Verification: pre-existing: triggers when mock.module() (or build.module()) targets a module whose own fetch is still pending; the base fails the same way. BunPlugin.cpp:522-524 finds no registry entry, so staleESMEntry stays false and removeEntry never runs. ModuleLoader.cpp:494-507: the loader does not create a registry entry until after the fetch promise resolves. Consequence: wrong behavior, silent.