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mock.module: validate callback before running resolver #28946

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12 changes: 6 additions & 6 deletions src/bun.js/bindings/BunPlugin.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -526,6 +526,12 @@
return {};
}

JSC::JSValue callbackValue = callframe->argument(1);
if (!callbackValue.isCell() || !callbackValue.isCallable()) {
scope.throwException(lexicalGlobalObject, JSC::createTypeError(lexicalGlobalObject, "mock(module, fn) requires a function"_s));
return {};
}

auto resolveSpecifier = [&]() -> void {
JSC::SourceOrigin sourceOrigin = callframe->callerSourceOrigin(vm);
if (sourceOrigin.isNull())
Expand Down Expand Up @@ -578,18 +584,12 @@
}
};

resolveSpecifier();
RETURN_IF_EXCEPTION(scope, {});

JSC::JSValue callbackValue = callframe->argument(1);
if (!callbackValue.isCell() || !callbackValue.isCallable()) {
scope.throwException(lexicalGlobalObject, JSC::createTypeError(lexicalGlobalObject, "mock(module, fn) requires a function"_s));
return {};
}

JSC::JSObject* callback = callbackValue.getObject();

JSModuleMock* mock = JSModuleMock::create(vm, globalObject->mockModule.mockModuleStructure.getInitializedOnMainThread(globalObject), callback);

Check notice on line 592 in src/bun.js/bindings/BunPlugin.cpp

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Claude / Claude Code Review

Infinite spin loop in getJSValue when mock factory returns a pending Promise 

Pre-existing bug in the `getJSValue` lambda (same function this PR modifies): the `while` loop that unwraps Promise chains has no escape path for a Pending promise — `default: { break; }` exits only the `switch`, not the `while`, so when `executeOnce` returns a pending Promise the loop busy-spins forever, permanently hanging the JS event loop thread. This is triggered by calling `mock.module('already-imported-module', async () => ({ default: 42 }))` — the async factory returns a pending Promise
Comment on lines 587 to 592

@claude claude Bot Apr 7, 2026

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🟣 Pre-existing bug in the getJSValue lambda (same function this PR modifies): the while loop that unwraps Promise chains has no escape path for a Pending promise — default: { break; } exits only the switch, not the while, so when executeOnce returns a pending Promise the loop busy-spins forever, permanently hanging the JS event loop thread. This is triggered by calling mock.module('already-imported-module', async () => ({ default: 42 })) — the async factory returns a pending Promise synchronously, and the loop never exits.

Extended reasoning...

The bug: In the getJSValue lambda (lines 603–620 of BunPlugin.cpp), a while loop unwraps Promise chains returned by executeOnce. The loop condition is:

while (JSC::JSPromise* promise = jsDynamicCast<JSC::JSPromise*>(result))

The three cases are:

  • Rejected → throws and returns (correctly exits the loop)
  • Fulfilled → updates result with the resolved value (loop exits on next iteration once result is no longer a Promise)
  • Pending (default) → break; which in C++ exits only the enclosing switch statement, not the while loop

Why the loop spins forever: Because result is never updated in the Pending branch, jsDynamicCast<JSC::JSPromise*>(result) evaluates to the same non-null pointer on the very next iteration. The loop has no sleep, no yield, and no way to drain microtasks between iterations — so a Pending promise can never transition to Fulfilled during iteration. The thread spins at 100% CPU until the process is killed.

Concrete trigger sequence:

  1. import 'my-module' — module is now in the ESM registry
  2. mock.module('my-module', async () => ({ default: 42 }))
  3. Inside JSMock__jsModuleMock, esm->get returns the existing entry, so getJSValue() is called
  4. executeOnce calls the async factory; an async function always returns a Promise in Pending state synchronously (the microtask that resolves it hasn't run yet)
  5. The while loop receives a Pending JSPromise, enters default: { break; }, exits the switch, immediately re-evaluates the while condition with the same unchanged result — Pending again
  6. Infinite loop — the JS event loop thread is permanently hung

Why existing code doesn't prevent it: The TODO comment // TODO: blocking wait for promise explicitly acknowledges that blocking on the promise is unimplemented. But the implementation is actively incorrect: instead of returning an error or the pending promise to the caller (as runVirtualModule correctly does at line ~930), it re-spins endlessly.

Contrast with runVirtualModule: That function handles Pending by return promise; — returning the pending promise to the caller for proper async resolution. getJSValue has no equivalent escape.

Fix: In the default/Pending case, either goto / break out of the while loop (e.g., using a labeled break or a flag variable), throw a descriptive error, or return the pending promise to the caller and handle it asynchronously as runVirtualModule does. The minimal safe fix is to add goto done; or restructure to break out of the outer loop when the status is Pending.


auto* esm = globalObject->esmRegistryMap();
RETURN_IF_EXCEPTION(scope, {});
Expand Down
17 changes: 17 additions & 0 deletions test/js/bun/test/mock/mock-module-non-string.test.ts
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Original file line number Diff line number Diff line change
Expand Up @@ -16,3 +16,20 @@ test("mock.module still works with valid string argument", async () => {
const m = await import("mock-module-non-string-test-fixture");
expect(m.default).toBe(42);
});

test("mock.module throws TypeError without resolving when callback is missing", () => {
// Running the resolver on an arbitrary user-provided string can enter the
// package-manager auto-install code path and crash. When the caller didn't
// pass a callable second argument, we must fail fast before touching the
// resolver.
const specifiers = [
"function f3() {}",
"function foo(a, b) { return a + b; }",
"() => 1",
"some bogus package name {with braces}",
];
for (const specifier of specifiers) {
// @ts-expect-error missing callback on purpose
expect(() => mock.module(specifier)).toThrow("mock(module, fn) requires a function");
}
});
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