bun:test: implement proper construct semantics for mock functions - #37030
bun:test: implement proper construct semantics for mock functions#37030robobun wants to merge 4 commits into
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Included review availability: Your plan provides up to 5 included reviews per hour; 0 remain after this review. WalkthroughChangesJSMockFunction now separates call and construct handlers. Shared invocation logic records constructor instances. Construction supports prototypes, Mock function construction
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I didn't find any bugs. The refactor is mechanical and the new construct handler follows standard JSC patterns with correct RETURN_IF_EXCEPTION placement, but this is ~30 lines of new native [[Construct]] code in JSC bindings, so it's worth a human look.
Checked: newTarget() is guaranteed to be an object in a construct callback, so asObject is safe; every fallible call (get on prototype, getInstances, push, the impl call) is followed by an exception check; returning {} on exception is fine since JSC checks the exception before asObject(result).
Ruled out: GC hazard on thisObject (stack-rooted, and instances is a visited WriteBarrier); mock.instances being pushed before a throwing implementation matches Jest.
Extended reasoning...
Overview
This PR fixes a crash where new (jest.fn()) triggered an isCell() assertion (debug) or leaked a non-object from a new expression (release), because JSMockFunction reused its call handler as its construct handler and JSC requires native [[Construct]] to return an object. The fix extracts the shared recording logic into jsMockFunctionCallImpl and adds a dedicated jsMockFunctionConstruct that creates this from newTarget.prototype, records it in mock.instances, runs the mock body with it, and returns the body's result only when it is an object. Eight new tests in mock-fn.test.js cover the construct variants.
Security risks
None material. This is test-runner-only surface (bun:test mocks). The change removes UB (release builds previously treated a primitive's value bits as a JSObject* in C++ callers of JSC::construct), which is a strict improvement. No untrusted-input parsing, no privilege boundaries.
Level of scrutiny
Medium-high. Although the logical scope is small and the fix is clearly correct at the JS-semantics level (matches ordinary [[Construct]] and Jest), it is new native C++ in src/jsc/bindings/ — the repo's most-blocked review category. Exception-scope discipline, GC rooting, and the asObject contract on callframe->newTarget() all need to be right. I traced each: exception checks are placed after every JS-observable call (get for .prototype, getInstances, push, and the impl call), RELEASE_AND_RETURN is used on the tail call in the call path, thisObject is on the C stack until return so it is conservatively rooted, and JSC guarantees newTarget is an object in a construct callback.
Other factors
The refactor of jsMockFunctionCall into jsMockFunctionCallImpl is a pure extraction — the call path's behavior is unchanged (same callee check, same callframe->thisValue()). Test coverage is thorough (no-impl, this binding, object vs primitive return, custom prototype, Reflect.construct with separate newTarget, throwing impl, spy construct), and the PR reports a clean run under BUN_JSC_validateExceptionChecks=1. One minor semantic note: mock.instances is now populated only on construct, whereas Jest populates it on every invocation with this — but that is strictly better than the previous behavior (never populated) and out of scope for this crash fix. Deferring only because native JSC construct handlers warrant a maintainer's eyes.
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Updated 4:39 PM PT - Aug 18th, 2026
✅ @robobun, your commit 99db3a1143816146f896b37410591196ef8aa977 passed in 🧪 To try this PR locally: bunx bun-pr 37030That installs a local version of the PR into your bun-37030 --bun |
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LGTM — the earlier note about mock.instances alignment has been addressed in afda1c1 (push moved into jsMockFunctionCallImpl alongside contexts, with a covering test).
What was reviewed:
jsMockFunctionConstruct: exception check afternewTarget->get(prototype), object-or-instance return contract satisfied,RELEASE_AND_RETURNon the call-path wrapper.instancespush mirrors the existingcontextsblock byte-for-byte;visitChildrenImplalready visitsfn->instances.- Existing tests that read
mock.instancesaftermockClear/mockResetstill hold (arrays are cleared/re-created).
Extended reasoning...
Overview
Fixes an assertion failure / UB when mock functions are used with new: JSMockFunction previously registered the same native handler for both [[Call]] and [[Construct]], but JSC requires a construct handler to return an object. The change extracts the shared body into jsMockFunctionCallImpl, adds a dedicated jsMockFunctionConstruct implementing ordinary-function construct semantics (create instance from newTarget.prototype, run body with instance as this, return object result or the instance), and records thisValue in mock.instances on every invocation for Jest parity. Nine new tests cover the construct path.
Prior feedback
My earlier inline comment noted that mock.instances was only populated on construct, leaving it misaligned with calls/contexts when calls and constructs are mixed. Commit afda1c1 moved the push into jsMockFunctionCallImpl next to the contexts push and added the "instances records this for every invocation" test — exactly the suggested fix. The comment-cop note about the long comment was also addressed (e943cc1) and is marked resolved.
Security risks
None. This is test-runner mock machinery; no untrusted input parsing, no auth/crypto/permissions surface.
Level of scrutiny
Native C++ JSC bindings warrant careful memory-safety review, but the change is small and follows in-file patterns exactly: the instances block is a copy of the adjacent contexts block, and the construct handler uses the standard get + RETURN_IF_EXCEPTION + constructEmptyObject sequence. callframe->newTarget() is guaranteed to be an object in a construct callback, so asObject(...) is safe. Exception scopes are correct (RELEASE_AND_RETURN on the tail call in jsMockFunctionCall, RETURN_IF_EXCEPTION after the observable .prototype get and after the impl call). fn->instances was already visited in visitAdditionalChildrenInGCThread, so no GC-rooting change is needed.
Other factors
The PR description shows the new tests fail on main (assertion in debug, wrong values in release) and pass on the branch under debug+ASAN and BUN_JSC_validateExceptionChecks=1. I spot-checked existing tests that touch mock.instances (mockReset works, mockClear works, mockClear doesnt update existing object) — all remain correct now that instances is populated on plain calls, since they only assert emptiness after clear or array identity.
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LGTM — the earlier mock.instances alignment feedback was addressed (push moved into jsMockFunctionCallImpl with a covering test), and this pass found nothing further.
Checked: the call/construct split preserves the original call-path behavior (only thisValue is now a parameter); exception scopes in both wrappers are balanced (RELEASE_AND_RETURN on the call tail, RETURN_IF_EXCEPTION after get(prototype) and after the impl call in construct); asObject(newTarget) is safe since JSC guarantees an object newTarget in a construct frame; the new instances recording block is byte-for-byte the contexts shape and is cleared alongside it in clear().
Extended reasoning...
Overview
Splits jsMockFunctionCall into a shared jsMockFunctionCallImpl plus two thin wrappers, and registers a dedicated jsMockFunctionConstruct as the InternalFunction construct callback. The construct wrapper implements ordinary-function [[Construct]]: derive the instance from newTarget.prototype (Object.prototype fallback), run the impl with that instance as this, and return the impl result only if it is an object. Also wires up mock.instances recording in the shared path. ~60 lines of C++ in JSMockFunction.cpp plus 9 new tests in mock-fn.test.js.
Security risks
None. This is test-runner mock plumbing; no untrusted input parsing, I/O, or privilege boundaries involved.
Level of scrutiny
Medium — it is JSC bindings C++, so exception-scope and GC discipline matter, but the change introduces no new ownership: thisObject is a fresh GC object on the stack, fn is the callee held by the frame, and the instances block is a verbatim copy of the adjacent contexts block (already visited in visitChildrenImpl and cleared in clear()). Exception checks follow the file's existing pattern and the PR reports a clean BUN_JSC_validateExceptionChecks=1 run.
Other factors
My previous review flagged that instances was only pushed on construct, breaking index alignment with calls; the author moved the push into the shared impl and added a mixed-call/construct test that pins instances[i] to calls[i]. All inline threads are resolved, the comment-cop nag was addressed (the remaining comment is one line), and the test matrix covers no-impl, this binding, object vs non-object return, custom prototype, Reflect.construct with a distinct newTarget, throwing impls, spy construction, and instances alignment. The gate evidence shows the suite crashes/fails on main and passes on the PR under both debug+ASAN and release.
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Actionable comments posted: 1
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Inline comments:
In `@src/jsc/bindings/JSMockFunction.cpp`:
- Around line 995-1000: The fallback construction in the newTarget handling must
use Object.prototype from newTarget’s realm when newTarget.prototype is
primitive or null, rather than the current globalObject realm. Update the
relevant JSMockFunction construction path while preserving the explicit
prototype path, and add a cross-realm Reflect.construct regression test covering
a primitive or null newTarget.prototype.
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Fix all unresolved CodeRabbit comments on this PR:
- Push a commit to this branch (recommended)
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src/jsc/bindings/JSMockFunction.cpptest/js/bun/test/mock-fn.test.js
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LGTM — the earlier mock.instances alignment feedback and the comment-cop note are both addressed.
What was reviewed:
jsMockFunctionConstructimplements ordinary[[Construct]](newTarget.prototype → instance asthis→ object-return-wins), always returning an object soexecuteConstruct'sasObjectis safe.- Exception checks after
newTarget->get(prototype)and afterjsMockFunctionCallImpl;RELEASE_AND_RETURNon the call-path tail; PR reports clean underBUN_JSC_validateExceptionChecks=1. instancespush now sits in the shared impl next tocontexts(same lazy-create shape), andfn->instancesis already visited invisitAdditionalChildrenInGCThread.- New tests cover no-impl,
thisbinding, object vs non-object return, customprototype,Reflect.constructwith a separate newTarget, throwing impl, constructing a spy, andinstances/callsalignment across mixed calls.
Extended reasoning...
Overview
Fixes a fuzzer-found crash where JSMockFunction used jsMockFunctionCall as its construct callback. JSC's Interpreter::executeConstruct calls asObject() on a native construct's return value, so a mock returning a non-object (e.g. new (jest.fn())) tripped a debug assertion and produced UB in release. The change:
- Extracts the existing call body into
jsMockFunctionCallImpl(globalObject, fn, callframe, thisValue)— behavior-preserving refactor; only the callee validation andthisValueread move to the wrapper. - Adds
jsMockFunctionConstruct: readsnewTarget.prototype, allocates the instance from it (orObject.prototype), runs the shared impl with the instance asthis, and returns the impl's result if it's an object, else the instance. This is exactly ordinary-function[[Construct]]and matches Jest, whose mocks are plain JS functions. - Populates
mock.instanceson every invocation (previously always empty) by pushingthisValuein the shared impl next to thecontextspush, soinstances[i]lines up withcalls[i]— this addresses my prior review comment.
Nine new tests in mock-fn.test.js, seven of which fail on the current release build.
Security risks
None. This is test-runner mock machinery; no external input parsing, no auth/crypto, no filesystem or network. The change closes a type-confusion path (non-object bits treated as JSObject* by callers of JSC::construct), so it's a strict safety improvement.
Level of scrutiny
Medium-high — it's C++ JSC bindings, so memory safety and exception-scope discipline matter. On those axes:
- All new JSValues are stack locals (conservatively scanned) or stored via
WriteBarrier;fn->instanceswas already declared and visited invisitAdditionalChildrenInGCThread, so populating it introduces no new GC edge. asObject(callframe->newTarget())is safe: JSC guaranteesnewTargetis an object inside a native[[Construct]].- Exception checks:
RETURN_IF_EXCEPTIONafternewTarget->get(...), after eachpush, and afterjsMockFunctionCallImpl; the call wrapper usesRELEASE_AND_RETURNon its tail call. The construct path's tail is non-throwingJSValue::encode, so no release is needed. PR description confirms a clean run underBUN_JSC_validateExceptionChecks=1. - The
instancesblock is byte-for-byte the same lazy-create-or-push shape as the adjacentcontextsblock, so it inherits that pattern's review history.
Other factors
- My previous inline comment (instances only pushed on construct → misaligned with
calls) was addressed exactly as suggested, with a covering test that mixesfn(),obj.fn(), andnew fn()and asserts index-by-index alignment. - The comment-cop note about a paragraph-long comment was addressed; the remaining comment above
jsMockFunctionConstructis one line. - Evidence gate shows the suite fails on main (assertion crash on debug, 7 failures on release) and passes on the PR under both debug+ASAN and release.
- No CODEOWNERS entries match the touched paths.
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LGTM — the construct handler follows the established getFunctionRealm + createSubclassStructure pattern, and the earlier feedback on mock.instances alignment and realm-aware fallback has been addressed.
What was reviewed
- The refactor of
jsMockFunctionCallinto a sharedjsMockFunctionCallImplis behavior-preserving; the only addition is theinstancespush, which mirrors the adjacentcontextsblock exactly. - Exception handling in
jsMockFunctionConstruct:getFunctionRealm,createSubclassStructure, and the shared call each have aRETURN_IF_EXCEPTION, and the wrapper usesRELEASE_AND_RETURNfor the tail call. - The construct pattern (
getFunctionRealm(globalObject, newTarget)→createSubclassStructure(..., functionGlobalObject->objectStructureForObjectConstructor())) matchesNapiClass.cppand other in-tree constructors. - Test coverage spans no-impl,
thisbinding, object vs non-object returns, customprototype,Reflect.constructwith a separate newTarget, throwing implementations, spy construct, cross-realm fallback, andinstances/callsalignment.
Extended reasoning...
Overview
This PR fixes an assertion failure / UB when constructing a bun:test mock function. JSMockFunction previously registered jsMockFunctionCall for both [[Call]] and [[Construct]], but JSC requires a native construct callback to return an object (Interpreter::executeConstruct calls asObject on the result). The fix:
- Extracts the invocation body into
jsMockFunctionCallImpl(globalObject, fn, callframe, thisValue)and adds aninstancespush next to the existingcontextspush. - Adds
jsMockFunctionConstruct, which creates the instance viagetFunctionRealm+InternalFunction::createSubclassStructure(withobjectStructureForObjectConstructor()as the fallback), runs the shared body with the instance asthis, and returns the body's result if it is an object, otherwise the instance — standard ordinary-function[[Construct]]semantics. - Wires the new handler into the
InternalFunctionbase constructor. - Adds ten tests to
mock-fn.test.jsand anode:vmimport for the cross-realm case.
Security risks
None. This is bun:test mock machinery — test-runner-only code with no auth, crypto, filesystem, or network exposure. The fix removes a debug assertion crash and a release-build type-confusion where non-object bits were treated as JSObject*; strictly a safety improvement.
Level of scrutiny
Medium. Native JSC binding code implementing [[Construct]] warrants care around exception scopes and GC, but the change is small, follows a codebase-wide idiom verbatim (NapiClass.cpp:49-54, JSX509Certificate.cpp:191-194, NodeVMScript.cpp:126-129), and is confined to the test runner's mock function. The refactored call path is a mechanical extraction — the only new logic in the shared body is the instances push, byte-for-byte the same shape as the neighbouring contexts block.
Other factors
- Two prior review comments were addressed: my note that
instancesshould be recorded on every invocation (fixed in 4d768b3 by moving the push into the shared path, with a test asserting alignment across mixed calls), and CodeRabbit's note that the non-object-prototype fallback should use newTarget's realm (fixed in 3ee079c by switching togetFunctionRealm+createSubclassStructure, with arunInNewContexttest). - Exception handling is correct: each throwing call in
jsMockFunctionConstructis followed byRETURN_IF_EXCEPTION,jsMockFunctionCalltail-calls viaRELEASE_AND_RETURN, and the newinstances->pushhas its ownRETURN_IF_EXCEPTIONmatching the sibling arrays. - The gated evidence in the PR description shows 7 tests failing on release without the fix and a debug+ASAN build failure, all passing with the fix; the author reports
BUN_JSC_validateExceptionChecks=1is clean. - No open reviewer threads; the comment-cop and both inline threads are resolved.
JSMockFunction registered its call handler as the construct handler. JSC requires a native construct callback to return an object, so constructing a mock whose implementation returned a non-object (any bare jest.fn() or spy) tripped the isCell assertion in debug builds and let a non-object escape from new in release builds. Give mocks a real construct handler matching ordinary function and Jest semantics: create the instance from newTarget's prototype, run the mock with it as this, and return the implementation's result only when it is an object. The instance is also recorded in mock.instances.
jest-mock pushes this onto instances alongside contexts and calls on each invocation, so the three arrays always have the same length and instances[i] corresponds to calls[i]. Move the push into the shared call path so plain calls and constructs both record it.
Use getFunctionRealm plus InternalFunction::createSubclassStructure, the same shape JSC's Object constructor and the napi class constructor use, so a newTarget whose prototype is not an object falls back to the Object.prototype of its own realm rather than the mock's.
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Note on the merge risk summary above: the cross-realm fallback it describes is already handled. The construct handler derives the instance structure with getFunctionRealm and InternalFunction::createSubclassStructure (JSMockFunction.cpp lines 997 to 999 on the current head), so a newTarget whose prototype is not an object gets the Object.prototype of its own realm. The test "falls back to Object.prototype of newTarget's realm when its prototype is not an object" in mock-fn.test.js covers it with a node:vm context, and it fails on the earlier version of this branch. |
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LGTM — the follow-up commits address both earlier review points (instances now recorded on every invocation alongside contexts, and the fallback prototype comes from newTarget's realm via getFunctionRealm + createSubclassStructure).
What was reviewed:
- Construct handler follows the same pattern as
NapiClass.cpp/ JSC'sObjectConstructor; every fallible step (getFunctionRealm,createSubclassStructure, the shared impl call) has aRETURN_IF_EXCEPTION, and the object-vs-non-object return rule matches ordinary[[Construct]]. instancespush mirrors the neighboringcontextslazy-create-or-push shape, so the arrays stay aligned;clear()andvisitChildrenalready covered the field.- Tests cover no-impl,
thisbinding, object/primitive returns, custom prototype,Reflect.constructnewTarget, throwing impl, spy construct, mixed call/new alignment, and the cross-realm null-prototype fallback.
Extended reasoning...
Overview
Fixes a fuzzer-found crash where JSMockFunction reused its call callback as its construct callback. JSC's Interpreter::executeConstruct calls asObject() on a native construct's return value, so a mock returning undefined (the default) hit an isCell() assertion in debug and produced a bogus JSObject* in release. The fix adds a dedicated jsMockFunctionConstruct that creates the instance from newTarget.prototype (falling back to newTarget's realm's Object.prototype), invokes the shared recording/dispatch path with that instance as this, and returns the impl's result only if it is an object. While here, mock.instances is populated on every invocation next to contexts, matching jest-mock. Two files touched: ~60 lines in JSMockFunction.cpp and 10 new tests in mock-fn.test.js.
Security risks
None. This is test-runner mock machinery; no untrusted-input parsing, auth, crypto, or filesystem paths are involved. The only memory-safety-relevant surface is the JSC exception discipline, and every call that can throw or run user JS (getFunctionRealm for revoked proxies, createSubclassStructure for the prototype getter, the shared impl for the user callback) is followed by RETURN_IF_EXCEPTION. The PR description confirms the path is clean under BUN_JSC_validateExceptionChecks=1.
Level of scrutiny
Medium. This is C++ JSC bindings code, so mistakes can UAF or corrupt, but the change is a focused crash fix that copies the exact getFunctionRealm → objectStructureForObjectConstructor() → InternalFunction::createSubclassStructure sequence already used in NapiClass.cpp and mirrored across other in-tree constructors (NodeFSStatBinding, JSSign, JSDOMFile, generated classes). The refactor of jsMockFunctionCall into a thin wrapper plus jsMockFunctionCallImpl is mechanical — the body is unchanged except for the added instances push, which is a byte-for-byte copy of the adjacent contexts block.
Other factors
- Two prior review comments were addressed in follow-up commits (094ccef moved the
instancespush into the shared path; 99db3a1 switched to realm-aware structure derivation). All review threads are resolved. - Test coverage is thorough (10 new cases) and the evidence block shows the suite fails on main / crashes under ASAN and passes with the fix on both debug+ASAN and release.
- No API surface change beyond making
mock.instancesactually work, which was previously always empty.
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What does this PR do?
Fixes a crash found by fuzzing (assertion
isCell()inJSCJSValue.h:766, reachable from plainbun:testusage).JSMockFunctionpassedjsMockFunctionCallas both the call and the construct callback. JSC requires a native[[Construct]]callback to return an object (Interpreter::executeConstructdoesasObject(result)on the return value), but the call handler returns whatever the mock implementation returns. Sonew (jest.fn()), orReflect.constructon any mock or spy whose implementation yields a non-object, failed the assertion in debug builds. In release builds the non-object value escaped thenewexpression (new (jest.fn())evaluated toundefined), and C++ callers ofJSC::constructwould treat those value bits as aJSObject*.Repro on release or debug builds before this change:
The fix gives mock functions a real construct handler with ordinary-function semantics, which is also what Jest's mocks do (they are plain JS functions):
newTarget.prototype, falling back to theObject.prototypeof newTarget's own realm when that is not an object (viagetFunctionRealmandInternalFunction::createSubclassStructure, the same path JSC'sObjectconstructor and the napi class constructor use)this, sojest.fn(function () { this.x = 1; })works withnewWhile here,
mock.instancesis now populated. It was always empty before. It recordsthison every invocation, next tocallsandcontexts, which is what jest-mock does, soinstances[i]lines up withcalls[i]whether the mock was called or constructed. The rest of the shared recording logic (calls, contexts, results, invocation order, implementation dispatch) is unchanged.Rebase note: #39509 landed on main in the meantime. It converts a scope-object receiver to
undefinedwithtoThis(..., ECMAMode::strict())on the line injsMockFunctionCallthat readcallframe->thisValue(). This PR turns that receiver into a parameter of the shared body, so the conversion now lives in the call wrapper, right where the receiver is read. The construct wrapper passes the new instance and needs no conversion. A bare call through a captured scope recordsundefinedincontextsand ininstances, and the three tests from #39509 pass together with the ones added here.How did you verify your code works?
test/js/bun/test/mock-fn.test.jscovering construct with no implementation,thisbinding, object vs non-object return values, customprototype,Reflect.constructwith a separatenewTarget, throwing implementations, constructing a spy,instancesstaying aligned withcallsacross mixed calls and constructs, and a cross-realmnewTarget(vianode:vm) with a nullprototypegetting its own realm'sObject.prototype. 8 of them fail on the current release build.mock-fn.test.js,mock-disposable.test.ts,spyMatchers.test.ts, andtest/js/bun/test/mock/pass (269 tests after the rebase onto bun:test: convert scope receivers to undefined in mock host functions #39509), as does theERR_INVALID_THIStest inglobals.test.jsthat bun:test: convert scope receivers to undefined in mock host functions #39509 extended. The construct path is clean underBUN_JSC_validateExceptionChecks=1, including the throwing path for a revoked Proxy asnewTarget.[review] gate passed · iteration 0 · 2 files touched
fails on main (without fix)
passes on PR (with fix)
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gate history · 2 passed · 0 rejected · iteration 0
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