DeferredWorkTimer: keep the embedder's tickets in m_pendingTickets so the end of a collection cancels them - #487
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WalkthroughChangesDeferred work ticket handling now synchronizes pending-ticket access, supports embedder ticket data, adds ticket acquisition helpers, changes callback ownership, and updates cancellation ordering. Async waiter removal now permits tickets canceled before realm sweep cleanup. Deferred work ticket lifecycle
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…/WebKit#487 preview The scheduler kept every ticket in sets of its own, so the end of a collection, which cancels the tickets of dead realms by walking the timer's m_pendingTickets, found nothing, and a job queued for a realm that died in between still ran: on a destructed FinalizationRegistry, or it resolved a dead promise when an Atomics.waitAsync waiter was notified. With oven-sh/WebKit#487 the timer keeps the tickets itself. The add and cancel hooks only count the event loop ref of ImminentlyScheduled tickets, and a job runs its task only if takePendingWork() still finds the ticket pending. A ticket leaves the pending set once, by cancellation or by take, so the ref is released once. The job stores its client data because the ticket's owner can be dead when it runs. This replaces the HeapObserver that walked the scheduler's sets.
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Inline comments:
In `@Source/JavaScriptCore/runtime/DeferredWorkTimer.cpp`:
- Around line 339-354: Update the documentation for onCancelPendingWork to state
that, in addition to being callable from any thread, it may run during
CollectorPhase::End with the world stopped and therefore must not allocate,
execute JavaScript, or acquire the API lock.
In `@Source/JavaScriptCore/runtime/DeferredWorkTimer.h`:
- Around line 144-157: Add WTF_GUARDED_BY_LOCK(m_taskLock) to m_pendingTickets
in Source/JavaScriptCore/runtime/DeferredWorkTimer.h lines 144-157. In
Source/JavaScriptCore/runtime/DeferredWorkTimer.cpp lines 282-300, remove or
relocate the m_pendingTickets.contains access in the assertion because
cancelAndClear can call this path without m_taskLock; also lock m_taskLock
around the m_pendingTickets.isEmpty() read in runRunLoop.
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Fix all unresolved CodeRabbit comments on this PR:
- Push a commit to this branch (recommended)
- Create a new PR with the fixes
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Source/JavaScriptCore/runtime/DeferredWorkTimer.cppSource/JavaScriptCore/runtime/DeferredWorkTimer.hSource/JavaScriptCore/runtime/WaiterListManager.h
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Thanks for e70d2e8e — the four readers now take m_taskLock and the header comment names the cross-VM WaiterListManager path, which addresses the race I flagged. I re-reviewed the updated diff and found no further issues. Given this reworks the ticket ownership/lifecycle across GC-end, sweep, and cross-VM cancellation paths and changes the embedder hook contract, a human look is still worthwhile.
What was reviewed:
- Verified all
m_pendingTicketsaccesses (add/take/remove/queries,doWork, bothcancelPendingWorkoverloads,cancelPendingWorkSafe) are now underm_taskLock;unlockEarly()incancelPendingWork(VM&)releases before the hook so no re-entrancy deadlock. - Checked
addPendingWorkcallers for holdingm_taskLock— none do;doWorkdrops it around each task. - Confirmed the relaxed
WaiterList::takeFirstassertion matchesnotifyWaiterImpl's existing dead-ticket handling viascheduleWorkSoonIfActive. m_embedderDatasits in existing padding afterm_isCancelled, so noTicketsize change.
Extended reasoning...
Overview
This PR reworks DeferredWorkTimer so that when the embedder hooks (onAddPendingWork / onScheduleWorkSoon / onCancelPendingWork) are installed, the timer still owns tickets in m_pendingTickets rather than handing ownership to the embedder. This lets cancelPendingWork(VM&) — which the heap runs at the end of every collection — actually cancel tickets whose realm has died, fixing Bun crashes where FinalizationRegistry cleanup and Atomics.waitAsync resolution ran against destructed objects. It adds takePendingWork(Ticket&) for the embedder to claim a ticket before running its task, changes onAddPendingWork's signature to a notification (Ticket& instead of Ref<Ticket>&&, WorkType), adds a one-byte embedderData slot on Ticket, and relaxes an assertion in WaiterList::takeFirst. Three files touched: DeferredWorkTimer.{h,cpp} and WaiterListManager.h.
The follow-up commit e70d2e8e addressed my earlier review by taking m_taskLock in the four query functions and making the lock mutable, closing the cross-VM read/rehash race I described.
Security risks
No new attack surface in the classic sense (no parsing, no auth, no untrusted input). The risk profile here is memory-safety under concurrency: use-after-free of tickets/dependencies if the locking or lifecycle is wrong, particularly along the cross-VM ~SharedArrayBufferContents → WaiterListManager::unregister → cancelPendingWork path. After the follow-up commit, every structural access to m_pendingTickets I could find is under m_taskLock, and cancelPendingWork(VM&) clears dependencies under the lock and calls the embedder hook only after unlockEarly(). I did not find a remaining unguarded access, but concurrency correctness in this area is subtle enough that I'm not comfortable signing off without a human.
Level of scrutiny
High. This is core JSC runtime code at the intersection of GC end-phase, sweep, cross-thread cancellation, and an embedder API contract that a companion Bun PR (oven-sh/bun#39994) depends on. It changes a public hook signature and adds exported API. The default (non-hook) mode is meant to be behavior-preserving — I checked that doWork still purges cancelled tickets and the only default-mode change is that addPendingWork now briefly takes m_taskLock for the insert — but the hook-mode paths are new logic with multi-thread callers.
Other factors
- My earlier review's only finding was addressed exactly as suggested; the PR description was updated to reflect it.
- No in-repo tests are added; verification is via the Bun-side PR's tests, which is typical for this fork but means correctness depends on the paired change.
- The
onAddPendingWorksignature change is a breaking API change for the embedder; it is intentional and coordinated, but is another reason a human should confirm the pairing. - The relaxed
takeFirstassertion is justified by the new lifecycle (tickets can be released at GC-end before the realm's sweep unregisters the waiter), and the comment explains it — but dropping an assertion always deserves a second pair of eyes.
Given all of the above, this is well outside the "simple/mechanical" bar for auto-approval. Deferring to a human reviewer.
…/WebKit#487 preview The scheduler kept every ticket in sets of its own, so the end of a collection, which cancels the tickets of dead realms by walking the timer's m_pendingTickets, found nothing, and a job queued for a realm that died in between still ran: on a destructed FinalizationRegistry, or it resolved a dead promise when an Atomics.waitAsync waiter was notified. With oven-sh/WebKit#487 the timer keeps the tickets itself. The add and cancel hooks only count the event loop ref of ImminentlyScheduled tickets, and a job runs its task only if takePendingWork() still finds the ticket pending. A ticket leaves the pending set once, by cancellation or by take, so the ref is released once. The loop the work belongs to, which the scheduler used to keep in a map of its own, is stored in the ticket's embedder byte. The job stores its client data because the ticket's owner can be dead when it runs.
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Thanks — e70d2e8e addresses the race I flagged: the four m_pendingTickets readers now take m_taskLock, and the header comment names the cross-VM WaiterListManager writer. This run found no further bugs. Given the scope (ticket lifecycle across GC end phase, cross-thread cancellation, embedder hook contract change), a human look is still worthwhile.
On the two open CodeRabbit points, I checked and believe they don't bite:
runRunLoop's unlockedisEmpty()and theASSERT(onCancelPendingWork || m_pendingTickets.contains(...))are both reached only in default (no-hook) mode, wherecancelPendingWork(Ticket&)only flipsm_isCancelledand never structurally mutates the set — so no rehash race. In hook mode the||short-circuits beforecontains().- The
onCancelPendingWorkGC-end-phase doc note is a nice-to-have, not a correctness gap.
Extended reasoning...
Overview
The PR reworks DeferredWorkTimer so that when the embedder hooks are installed (Bun), tickets are still held in m_pendingTickets rather than handed off entirely to the embedder. This lets cancelPendingWork(VM&) — run by Heap at the end of every collection — actually cancel tickets whose realm died, fixing crashes where a queued FinalizationRegistry cleanup or Atomics.waitAsync resolution ran against a destructed object. It adds takePendingWork() for the embedder to claim a ticket before running its task, changes onAddPendingWork to a notification taking Ticket&, adds a one-byte embedderData slot on Ticket, puts every m_pendingTickets access in the hook-mode paths under m_taskLock, and relaxes a WaiterList::takeFirst assertion that no longer holds once tickets can be released before the realm's sweep.
Security risks
None identified. This is internal runtime lifecycle/concurrency plumbing with no attacker-reachable input surface beyond what already exists (Atomics.waitAsync, FinalizationRegistry). The change tightens lifetime handling rather than loosening it.
Level of scrutiny
High. This is concurrency-sensitive code at the intersection of the GC end phase, cross-VM WaiterListManager teardown, and the embedder's event loop. It changes the public hook contract (onAddPendingWork signature, new takePendingWork protocol) that Bun's scheduler must match (oven-sh/bun#39994). Lock ordering, the removeIf + unlockEarly + callback sequence in cancelPendingWork(VM&), and the mode-split in cancelPendingWorkSafe all deserve human eyes.
Other factors
- My earlier review flagged that the four query functions read
m_pendingTicketswithoutm_taskLockwhile a foreign VM's thread could be structurally mutating it via~SharedArrayBufferContents → WaiterListManager::unregister → cancelPendingWork. The author fixed this in e70d2e8e (queries now lock;m_taskLockmademutable; header comment updated). That thread is resolved. - CodeRabbit's two open comments were re-examined this run and ruled out: the two remaining unlocked reads (
runRunLoopand thecontainsin the ASSERT) are gated to default mode where no concurrent structural mutation of the set occurs, and the||short-circuit skipscontains()in hook mode. AddingWTF_GUARDED_BY_LOCKwould flag exactly those two benign sites, which is presumably why it was left off. The GC-end-phase doc suggestion is a nit. - The Bun-side PR carries the regression tests; nothing here is unit-testable in isolation without the embedder.
Not a candidate for auto-approval given the concurrency surface and the coupled embedder-side change.
…/WebKit#487 preview The scheduler kept every ticket in sets of its own, so the end of a collection, which cancels the tickets of dead realms by walking the timer's m_pendingTickets, found nothing, and a job queued for a realm that died in between still ran: on a destructed FinalizationRegistry, or it resolved a dead promise when an Atomics.waitAsync waiter was notified. With oven-sh/WebKit#487 the timer keeps the tickets itself. The add and cancel hooks only count the event loop ref of ImminentlyScheduled tickets, and a job runs its task only if takePendingWork() still finds the ticket pending. A ticket leaves the pending set once, by cancellation or by take, so the ref is released once. The loop the work belongs to, which the scheduler used to keep in a map of its own, is stored in the ticket's embedder byte. The job stores its client data because the ticket's owner can be dead when it runs.
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Thanks for e70d2e8e — the four readers now take m_taskLock and the header comment names the cross-VM path, which addresses my earlier concern. This pass found no further issues. Given the scope — reworked ticket-ownership contract across the embedder hooks, cross-thread cancellation from the collector and from a foreign VM's WaiterListManager, and a coordinated Bun-side scheduler change — a human look is still worthwhile.
What was reviewed:
- Lock discipline on
m_pendingTicketsafter the follow-up:addPendingWork,takePendingTicket, bothcancelPendingWorkoverloads,cancelPendingWorkSafe, and the four queries all acquirem_taskLock. - The two remaining unlocked reads CodeRabbit flagged: the
ASSERTatcancelPendingWork(Ticket&)short-circuits ononCancelPendingWorkin hook mode socontainsis only reached in the unchanged default path, andrunRunLoopis default-mode-only (jsc shell) — neither races with the new hook-mode writers. cancelPendingWork(VM&)hook branch:removeIfunder the lock, tickets retained indeadTickets,unlockEarlybefore invokingonCancelPendingWork; the!isCancelled() && isValid()short-circuit avoidstarget()on cleared tickets.- The relaxed
WaiterList::takeFirstassertion matches the new window where a ticket is released at collection end before the realm's sweep unregisters the waiter;notifyWaiterImplalready handles a deadWeakTicket.
Extended reasoning...
Overview
The PR changes DeferredWorkTimer so that when embedder hooks (onAddPendingWork / onScheduleWorkSoon / onCancelPendingWork) are installed, tickets are still owned by m_pendingTickets rather than handed off to the embedder. This lets cancelPendingWork(VM&) — run at the end of every collection — actually cancel tickets whose realm died, fixing crashes where FinalizationRegistry cleanup jobs and Atomics.waitAsync resolutions ran against destructed objects. It adds takePendingWork(Ticket&) for the embedder to claim a ticket before running its task, changes onAddPendingWork's signature to (Ticket&), adds a one-byte embedderData slot on Ticket, and puts every hook-mode access to m_pendingTickets under m_taskLock. WaiterList::takeFirst's assertion is relaxed to permit an async waiter whose ticket was already released.
Security risks
None identified. This is internal runtime lifecycle/concurrency plumbing with no untrusted-input parsing, auth, or crypto surface. The risk class here is memory safety (use-after-free, data races), which is what the change is fixing, not introducing an attack surface.
Level of scrutiny
High. This is concurrency-sensitive code at the intersection of GC end-phase, sweeping, cross-VM WaiterListManager callbacks, and an embedder event loop. The hook contract is being redefined (breaking API change coordinated with oven-sh/bun#39994), and correctness depends on subtle ordering (remove-before-flag, unlock-before-callback, which thread holds which VM's API lock). One round of review already surfaced a real race in the readers, which was fixed. This is well beyond a mechanical change and warrants human sign-off.
Other factors
- My earlier concern (unlocked reads of
m_pendingTicketsracing with cross-VMremove()) was addressed in e70d2e8e; the current diff reflects that. - The bug-hunting pass this run examined CodeRabbit's remaining "two unguarded accesses / missing
WTF_GUARDED_BY_LOCK" finding and concluded it is not a real bug: the debugASSERTshort-circuits in hook mode, andrunRunLoopis not reached with hooks installed. Whether to add the annotation anyway (and lockdoWork's andrunRunLoop's default-mode accesses to satisfy it) is a style call for the maintainers. - CodeRabbit's trivial doc nit (state that
onCancelPendingWorkmay run duringCollectorPhase::Endand must not allocate/run JS/take the API lock) is still open; it's a comment-only suggestion. - Tests live on the Bun side (oven-sh/bun#39994) and can't be exercised here.
…/WebKit#487 preview The scheduler kept every ticket in sets of its own, so the end of a collection, which cancels the tickets of dead realms by walking the timer's m_pendingTickets, found nothing, and a job queued for a realm that died in between still ran: on a destructed FinalizationRegistry, or it resolved a dead promise when an Atomics.waitAsync waiter was notified. With oven-sh/WebKit#487 the timer keeps the tickets itself. The add and cancel hooks only count the event loop ref of ImminentlyScheduled tickets, and a job runs its task only if takePendingWork() still finds the ticket pending. A ticket leaves the pending set once, by cancellation or by take, so the ref is released once. The loop the work belongs to, which the scheduler used to keep in a map of its own, is stored in the ticket's embedder byte. The job stores its client data because the ticket's owner can be dead when it runs.
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I re-reviewed after e70d2e8e/53ed6cc8 and didn't find further issues — the four query methods now take m_taskLock, closing the cross-VM rehash race I flagged earlier, and the onCancelPendingWork collector-phase constraint is documented. Because this reworks the ticket-ownership contract across GC end-phase, cross-VM WaiterListManager cancellation, and a coordinated Bun-side scheduler change, a human look is still worthwhile.
What was reviewed:
- Lock discipline on
m_pendingTicketsin hook mode:addPendingWork,takePendingTicket,cancelPendingWork(VM&), and the four queries all takem_taskLock;cancelPendingWorkSafein hook mode avoids recursive locking by lettingtakePendingTickettake it per-ticket. onCancelPendingWorkonce-per-ticket guarantee: concurrentcancelPendingWork(Ticket&)callers race ontakePendingTicket, and only the winner calls the hook;cancelPendingWork(VM&)only sees tickets still in the set.- Default (no-hook) mode:
addPendingWorknow takes the lock for insertion — checked that no caller holds it (doWorkdrops it around tasks;cancelPendingWork(VM&)returns before reconciliation). WaiterList::takeFirstassertion relaxation matches the new window where a dead realm's ticket is released before its waiter is unregistered.
Extended reasoning...
Overview
This PR changes DeferredWorkTimer so that when embedder hooks are installed (Bun's mode), tickets are still owned by the timer's m_pendingTickets set rather than being handed off to the embedder. The embedder is notified via onAddPendingWork(Ticket&) and must call the new takePendingWork(Ticket&) before running a task. This lets cancelPendingWork(VM&) — which the heap runs at the end of every collection — actually see and cancel tickets whose realm died, fixing use-after-free crashes in FinalizationRegistry cleanup and Atomics.waitAsync (BUN-4NCK, BUN-4M46). The change also adds a per-ticket embedderData() byte, makes m_taskLock mutable so const query methods can take it, and relaxes a WaiterList::takeFirst assertion to allow the new window where a ticket is released before the realm's sweep unregisters the waiter. Three files touched: DeferredWorkTimer.{h,cpp} and WaiterListManager.h.
Security risks
No injection, auth, or data-exposure surface. The risk here is memory safety: races on m_pendingTickets (a rehashing HashSet) between the collector thread, cross-VM WaiterListManager::unregister, and the embedder's event-loop thread. My previous review identified that the four query methods read the set without m_taskLock while hook-mode cancellation could rehash it from a foreign VM's thread; this was fixed in e70d2e8e. I re-checked for recursive-lock deadlocks (none: cancelPendingWorkSafe in hook mode does not take the lock itself, cancelPendingWork(VM&) does unlockEarly() before invoking the hook, and doWork drops the lock around tasks so re-entrant addPendingWork is safe) and for double-notification of onCancelPendingWork (concurrent cancellers serialize on takePendingTicket and only the one that removes the entry calls the hook).
Level of scrutiny
High. This is concurrency-sensitive runtime code sitting at the intersection of GC end-phase (world-stopped collector thread), cross-VM SharedArrayBufferContents teardown, and the embedder's event loop. It also changes a public hook signature (onAddPendingWork went from (Ref<Ticket>&&, WorkType) to (Ticket&)) that requires a coordinated Bun-side change (oven-sh/bun#39994). None of this is mechanical; it is a redesign of an ownership contract with subtle lock-ordering and lifetime reasoning. That is well outside the auto-approval bar of "simple, mechanical, or obvious".
Other factors
All prior review threads are resolved: my locking concern (e70d2e8e), CodeRabbit's collector-phase documentation nit (53ed6cc8), and CodeRabbit's WTF_GUARDED_BY_LOCK request (declined with a reasoned explanation that the annotation would force suppressions on upstream no-hook paths this PR does not otherwise touch — CodeRabbit withdrew). The Bun side has two regression tests that segfault on 1.4.0 and pass with this change. The bug-hunting pass on the current revision found nothing. Given the scope and the cross-repo coordination, deferring to a human reviewer is the right call.
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Thanks — e70d2e8e addressed the concern I raised earlier: the four query functions now take m_taskLock, and the header comment names the cross-VM WaiterListManager writer. I re-reviewed the current revision and didn't find further issues.
Given that this reworks the ticket-ownership contract and lock discipline across the VM thread, collector thread (world-stopped), and foreign-VM threads, and pairs with a coordinated scheduler change on the Bun side, a human look would still be worthwhile.
What was reviewed:
- Lock discipline for
m_pendingTicketsin hook mode — every access (add/take/cancel/4 queries) now underm_taskLock;cancelPendingWork(VM&)unlocks before invoking the hook. addPendingWorknewly takingm_taskLockin the no-hook path —doWorkdrops the lock around each task, so no re-entry deadlock; thehasPendingWork/hasDependencyInPendingWorkcallers inWasmStreamingCompilerare assertions outside any lock.- Remove-before-cancel ordering vs.
takePendingWork's "still in the set ⇒ live" invariant — consistent acrosscancelPendingWork(Ticket&)andcancelPendingWork(VM&). WaiterList::takeFirstassertion relaxation — matches the new window where the collector releases a dead realm's ticket before sweep unregisters the waiter.
Extended reasoning...
Overview
This PR changes DeferredWorkTimer so that when embedder hooks are installed (Bun's mode), tickets are still owned by the timer in m_pendingTickets rather than being handed off to the embedder. This lets cancelPendingWork(VM&) — run by the heap at the end of every collection — cancel tickets whose realm died, fixing crashes where FinalizationRegistry cleanup jobs and Atomics.waitAsync resolutions ran against destructed objects. The change adds takePendingWork() for the embedder to acquire a ticket before running its task, redefines the onAddPendingWork/onCancelPendingWork hook contract (signature change: Ref<Ticket>&&, WorkType → Ticket&), adds a byte of embedderData to Ticket, and puts every hook-mode access to m_pendingTickets under m_taskLock. It also relaxes an assertion in WaiterList::takeFirst to allow a waiter whose ticket was already released by the collector.
Security risks
No injection, auth, or data-exposure surface. The risk profile is memory safety and concurrency: cross-thread mutation of a HashSet (rehash during iteration), use-after-free of ticket dependencies during GC end phase, and lock-ordering. The earlier revision had one such gap (unlocked reads of m_pendingTickets racing with a foreign-VM remove()), which was fixed in e70d2e8e by taking m_taskLock in the four query functions and making the lock mutable.
Level of scrutiny
High. This is core JSC runtime code on the GC/mutator boundary, with three concurrent actors (VM thread holding the API lock, collector thread with world stopped, and a foreign VM's thread via the process-global WaiterListManager). The hook contract is a design change requiring a coordinated Bun-side update (oven-sh/bun#39994), so both sides need to agree on the invariants (e.g., "remove from set before flagging cancelled", "onCancelPendingWork may run with world stopped"). The no-hook upstream path is also touched (addPendingWork now takes m_taskLock; the four queries now take it), which is a behavior change for the jsc shell / WebKit path even though it appears benign.
Other factors
- My earlier inline finding (unlocked query reads vs. cross-VM
remove()) was addressed in e70d2e8e and confirmed by the author. - Two coderabbit findings were resolved: the collector-phase constraint on
onCancelPendingWorkis now documented in the header (53ed6cc842), and theWTF_GUARDED_BY_LOCKannotation was intentionally omitted with a reasoned explanation (upstream no-hook callers reach the assertion both with and without the lock; annotating would require suppressions on unchanged upstream code). - All inline threads on the PR are resolved.
- Tests exist on the Bun side for the two crash scenarios; there are no new tests in this repo, which is typical for embedder-hook changes here.
- I checked that the new
m_taskLockacquisitions inaddPendingWorkand the query functions don't introduce re-entry deadlocks in the no-hook path:doWorkdrops the lock around task execution, and the only external callers ofhasPendingWork/hasDependencyInPendingWorkare debug assertions inWasmStreamingCompileroutside any lock.
Given the concurrency subtlety and the coordinated cross-repo API change, this warrants a human reviewer's sign-off rather than auto-approval.
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Re-reviewed after e70d2e8e/53ed6cc8 — the four m_pendingTickets readers now take m_taskLock, and the onCancelPendingWork collector-phase constraint is documented. This run found no further issues. Given how much of this change is about lock discipline across the collector thread, cross-VM WaiterListManager cancellation, and the reworked embedder ticket-ownership contract, a human pass is still worthwhile before merge.
What was reviewed:
- Hook-mode ticket lifecycle:
addPendingWorkinserts under the lock before calling the hook, and no caller holdsm_taskLockat that point (doWorkdrops it around each task;cancelPendingWork(VM&)returns before reconciliation), so no self-deadlock. cancelPendingWork(VM&)hook branch:removeIfshort-circuits onisCancelled()beforeisValidreadstarget(), dead tickets are retained in aVector, and the hook runs afterunlockEarly().cancelPendingWorkSafehook branch iteratesm_weakTickets(notm_pendingTickets), sotakePendingTicket'sremove()under the lock does not invalidate the outer iteration.- The relaxed
takeFirstassertion matches the new window where a dead-realm ticket is released before sweep unregisters the waiter;notifyWaiterImplalready tolerates a deadWeakTicket.
Extended reasoning...
Overview
This PR reworks the embedder-hook contract in DeferredWorkTimer so that when Bun's hooks are installed, tickets remain in m_pendingTickets (previously they were handed off to the embedder and the set stayed empty). This lets cancelPendingWork(VM&) — run by the heap at the end of every collection — actually cancel tickets whose realm died, fixing use-after-free crashes where FinalizationRegistry cleanup jobs and Atomics.waitAsync resolutions ran against destructed objects. It adds takePendingWork() for the embedder to atomically claim a ticket before running its task, an embedderData byte on Ticket, and puts every hook-mode access to m_pendingTickets under m_taskLock. It also relaxes an assertion in WaiterList::takeFirst that no longer holds in the new lifecycle.
Security risks
No new attack surface. The change is internal lifecycle/locking for deferred-work tickets. The risk class is memory safety under concurrency (use-after-free, iterator invalidation, races on HashSet rehash) rather than injection/auth/data exposure. The PR is fixing two UAF crash reports; the review focus was on whether the new locking introduces different races or deadlocks.
Level of scrutiny
High. This is concurrency-sensitive code at the intersection of GC end-phase (world stopped, collector thread), cross-VM cancellation via the process-wide WaiterListManager singleton, and the embedder's event-loop thread. My earlier review found one real race (the four query functions reading the set unlocked while a foreign VM's thread could remove() and rehash); that was fixed in e70d2e8e. CodeRabbit's follow-ups on the collector-phase callback constraints and the WTF_GUARDED_BY_LOCK annotation were addressed/explained in 53ed6cc8 and the thread. The current diff looks internally consistent, but the correctness argument leans on several non-local invariants (hook mode never runs doWork/runRunLoop, the no-hook assertion path short-circuits on onCancelPendingWork, addPendingWork is never called with m_taskLock held) that a human familiar with Bun's scheduler side (oven-sh/bun#39994) should confirm.
Other factors
- The PR has a matching Bun-side change with two regression tests (FinalizationRegistry and
Atomics.waitAsyncin a collectednode:vmcontext) that segfault on 1.4.0 and pass with this change. - The
onAddPendingWorksignature changed from(Ref<Ticket>&&, WorkType)to(Ticket&)— the embedder side must be updated in lockstep, which the linked Bun PR handles. - All prior review threads (mine and CodeRabbit's) are resolved; no outstanding comments.
- The default (no-hook) upstream path is left behaviorally unchanged apart from
addPendingWorknow takingm_taskLockfor the insertion, which is safe given no caller holds it.
Given the above, I'm not auto-approving: this is exactly the kind of subtle multi-threaded lifecycle change where a second human reader adds real value, even though the automated passes are now clean.
…/WebKit#487 preview The scheduler kept every ticket in sets of its own, so the end of a collection, which cancels the tickets of dead realms by walking the timer's m_pendingTickets, found nothing, and a job queued for a realm that died in between still ran: on a destructed FinalizationRegistry, or it resolved a dead promise when an Atomics.waitAsync waiter was notified. With oven-sh/WebKit#487 the timer keeps the tickets itself. The add and cancel hooks only count the event loop ref of ImminentlyScheduled tickets, and a job runs its task only if takePendingWork() still finds the ticket pending. A ticket leaves the pending set once, by cancellation or by take, so the ref is released once. The loop the work belongs to, which the scheduler used to keep in a map of its own, is stored in the ticket's embedder byte. The job stores its client data because the ticket's owner can be dead when it runs.
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I re-reviewed after e70d2e8e added m_taskLock to the four query functions (addressing my earlier race finding) and a080c97 documented the collector-phase constraint on onCancelPendingWork. No further issues found. Given this reworks the embedder ticket-ownership contract and the lock discipline across the collector thread, the VM's thread, and the cross-VM WaiterListManager path, a human look would still be worthwhile.
What was reviewed:
- Lock ordering in hook mode:
addPendingWork/takePendingTicket/the four queries all takem_taskLock;cancelPendingWork(VM&)releases it before invoking the hook, andcancelPendingWorkSafe's hook branch does not hold it when callingcancelPendingWork(Ticket&)— no recursive acquire. onCancelPendingWorkonce-per-ticket: dead tickets are removed and flagged under the lock beforeunlockEarly, so a concurrent foreign-VMcancelPendingWork(Ticket&)seesisCancelled()and returns without a second callback.doWorkdropsm_taskLockaround each task, soaddPendingWork's new lock acquire does not deadlock in the no-hook path.- The relaxed
WaiterList::takeFirstassertion matchesnotifyWaiterImpl's existing handling of a deadWeakTicket.
Extended reasoning...
Overview
This PR changes DeferredWorkTimer so that when the Bun embedder hooks are installed, tickets remain owned by the timer's m_pendingTickets set rather than being handed off entirely to the embedder. This lets cancelPendingWork(VM&) — run by the heap at the end of every collection — actually cancel tickets whose realm died, fixing use-after-free crashes (BUN-4NCK, BUN-4M46) where FinalizationRegistry cleanup jobs and Atomics.waitAsync resolutions ran against destructed objects. The change touches DeferredWorkTimer.{h,cpp} (new takePendingWork/takePendingTicket API, reworked addPendingWork/cancelPendingWork*, embedderData byte on Ticket, m_taskLock now taken on all set accesses in hook mode) and relaxes one assertion in WaiterListManager.h.
Security risks
No injection, auth, or data-exposure surface. The risk profile is memory safety and concurrency: the change introduces structural HashSet mutation reachable from a foreign VM's thread (via ~SharedArrayBufferContents → WaiterListManager::unregister) and from the collector thread with the world stopped, concurrent with the VM's own thread taking tickets. My earlier review flagged an unlocked-read/locked-write race on m_pendingTickets; the author fixed it by taking m_taskLock in the four query functions and making the lock mutable. I re-checked the remaining unguarded reads (runRunLoop, the ASSERT in cancelPendingWork(Ticket&)) and confirmed they are on the no-hook upstream path only, per the author's explanation.
Level of scrutiny
High. This is core JSC runtime code on the GC ↔ mutator boundary, with a three-way threading model (VM thread, collector thread, foreign-VM thread) and a redesigned embedder contract that a paired Bun-side PR depends on. The fix targets real production crashes. Mistakes here manifest as rare data races or use-after-free, not test failures. That warrants a human reviewer familiar with JSC's heap phases and Bun's scheduler.
Other factors
All prior review threads (mine and CodeRabbit's) are resolved. The author's responses were detailed and correct on the points I could verify. Tests exist on the Bun side (oven-sh/bun#39994). The onAddPendingWork signature change (Ref<Ticket>&&, WorkType → Ticket&) is a breaking API change for the embedder, which is expected given the paired Bun PR but is another reason for a human to sign off on the contract.
…/WebKit#487 preview The scheduler kept every ticket in sets of its own, so the end of a collection, which cancels the tickets of dead realms by walking the timer's m_pendingTickets, found nothing, and a job queued for a realm that died in between still ran: on a destructed FinalizationRegistry, or it resolved a dead promise when an Atomics.waitAsync waiter was notified. With oven-sh/WebKit#487 the timer keeps the tickets itself. The add and cancel hooks only count the event loop ref of ImminentlyScheduled tickets, and a job runs its task only if takePendingWork() still finds the ticket pending. A ticket leaves the pending set once, by cancellation or by take, so the ref is released once. The loop the work belongs to, which the scheduler used to keep in a map of its own, is stored in the ticket's embedder byte. The job stores its client data because the ticket's owner can be dead when it runs.
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…/WebKit#487 preview The scheduler kept every ticket in sets of its own, so the end of a collection, which cancels the tickets of dead realms by walking the timer's m_pendingTickets, found nothing, and a job queued for a realm that died in between still ran: on a destructed FinalizationRegistry, or it resolved a dead promise when an Atomics.waitAsync waiter was notified. With oven-sh/WebKit#487 the timer keeps the tickets itself. The add and cancel hooks only count the event loop ref of ImminentlyScheduled tickets, and a job runs its task only if takePendingWork() still finds the ticket pending. A ticket leaves the pending set once, by cancellation or by take, so the ref is released once. The loop the work belongs to, which the scheduler used to keep in a map of its own, is stored in the ticket's embedder byte. The job stores its client data because the ticket's owner can be dead when it runs.
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…/WebKit#487 preview The scheduler kept every ticket in sets of its own, so the end of a collection, which cancels the tickets of dead realms by walking the timer's m_pendingTickets, found nothing, and a job queued for a realm that died in between still ran: on a destructed FinalizationRegistry, or it resolved a dead promise when an Atomics.waitAsync waiter was notified. With oven-sh/WebKit#487 the timer keeps the tickets itself. The add and cancel hooks only count the event loop ref of ImminentlyScheduled tickets, and a job runs its task only if takePendingWork() still finds the ticket pending. A ticket leaves the pending set once, by cancellation or by take, so the ref is released once. The loop the work belongs to, which the scheduler used to keep in a map of its own, is stored in the ticket's embedder byte. The job stores its client data because the ticket's owner can be dead when it runs.
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…/WebKit#487 preview The scheduler kept every ticket in sets of its own, so the end of a collection, which cancels the tickets of dead realms by walking the timer's m_pendingTickets, found nothing, and a job queued for a realm that died in between still ran: on a destructed FinalizationRegistry, or it resolved a dead promise when an Atomics.waitAsync waiter was notified. With oven-sh/WebKit#487 the timer keeps the tickets itself. The add and cancel hooks only count the event loop ref of ImminentlyScheduled tickets, and a job runs its task only if takePendingWork() still finds the ticket pending. A ticket leaves the pending set once, by cancellation or by take, so the ref is released once. The loop the work belongs to, which the scheduler used to keep in a map of its own, is stored in the ticket's embedder byte. The job stores its client data because the ticket's owner can be dead when it runs.
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…/WebKit#487 preview The scheduler kept every ticket in sets of its own, so the end of a collection, which cancels the tickets of dead realms by walking the timer's m_pendingTickets, found nothing, and a job queued for a realm that died in between still ran: on a destructed FinalizationRegistry, or it resolved a dead promise when an Atomics.waitAsync waiter was notified. With oven-sh/WebKit#487 the timer keeps the tickets itself. The add and cancel hooks only count the event loop ref of ImminentlyScheduled tickets, and a job runs its task only if takePendingWork() still finds the ticket pending. A ticket leaves the pending set once, by cancellation or by take, so the ref is released once. The loop the work belongs to, which the scheduler used to keep in a map of its own, is stored in the ticket's embedder byte. The job stores its client data because the ticket's owner can be dead when it runs.
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…/WebKit#487 preview The scheduler kept every ticket in sets of its own, so the end of a collection, which cancels the tickets of dead realms by walking the timer's m_pendingTickets, found nothing, and a job queued for a realm that died in between still ran: on a destructed FinalizationRegistry, or it resolved a dead promise when an Atomics.waitAsync waiter was notified. With oven-sh/WebKit#487 the timer keeps the tickets itself. The add and cancel hooks only count the event loop ref of ImminentlyScheduled tickets, and a job runs its task only if takePendingWork() still finds the ticket pending. A ticket leaves the pending set once, by cancellation or by take, so the ref is released once. The loop the work belongs to, which the scheduler used to keep in a map of its own, is stored in the ticket's embedder byte. The job stores its client data because the ticket's owner can be dead when it runs.
With the embedder hooks installed, addPendingWork handed the ticket to the embedder instead of storing it. cancelPendingWork(VM&), which the end of every collection runs to cancel the tickets whose realm died, therefore walked an empty set, and a task the embedder had already queued for such a ticket still ran: on a destructed FinalizationRegistry, or it resolved a dead promise when an Atomics.waitAsync waiter was notified. The timer now owns the tickets in both modes. The hooks become notifications: onAddPendingWork(Ticket&) when a ticket becomes pending, onCancelPendingWork(Ticket&) exactly once when a pending ticket is cancelled. The embedder calls takePendingWork() before it runs a task, which removes the ticket from the set and says whether the task may run. Since doWork does not run in this mode, the cancellation paths remove the tickets from the set themselves. Every access to the set is under m_taskLock, including the four queries: a cancellation can come from a collector thread, or from another VM's thread through WaiterListManager (~SharedArrayBufferContents), while the embedder takes tickets on the VM's thread. A ticket gets one byte of embedder data, written from onAddPendingWork and read from onScheduleWorkSoon on whichever thread schedules the work. Bun records which of its event loops the work belongs to. WaiterList::takeFirst no longer asserts that an async waiter's ticket is alive. The end of a collection can now release the ticket before the realm's sweep unregisters the waiter; notifyWaiterImpl already handles that and schedules nothing.
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…/WebKit#487 preview The scheduler kept every ticket in sets of its own, so the end of a collection, which cancels the tickets of dead realms by walking the timer's m_pendingTickets, found nothing, and a job queued for a realm that died in between still ran: on a destructed FinalizationRegistry, or it resolved a dead promise when an Atomics.waitAsync waiter was notified. With oven-sh/WebKit#487 the timer keeps the tickets itself. The add and cancel hooks only count the event loop ref of ImminentlyScheduled tickets, and a job runs its task only if takePendingWork() still finds the ticket pending. A ticket leaves the pending set once, by cancellation or by take, so the ref is released once. The loop the work belongs to, which the scheduler used to keep in a map of its own, is stored in the ticket's embedder byte. The job stores its client data because the ticket's owner can be dead when it runs.
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Problem
With the embedder hooks installed (Bun),
DeferredWorkTimer::addPendingWorkhanded the ticket to the embedder instead of adding it tom_pendingTickets.cancelPendingWork(VM&), whichHeapruns at the end of every collection to cancel the tickets whose realm died, therefore walked an empty set. A task the embedder had already queued for such a ticket still ran after the sweep: a FinalizationRegistry cleanup job read the destructed registry (Bun crash reports BUN-4NCK and BUN-4M46), and a notifiedAtomics.waitAsyncwaiter of a dead context resolved a dead promise. Bun fix: oven-sh/bun#39994.Fix
The timer owns the tickets in both modes. With the hooks installed:
onAddPendingWork(Ticket&)is a notification. The ticket is inm_pendingTicketslike in the default mode.takePendingWork(Ticket&)is what the embedder calls before it runs a task. It removes the ticket from the set and returns whether the task may run. This is the checkdoWorkmakes before it runs a task.onCancelPendingWork(Ticket&)is called exactly once for a pending ticket that is cancelled, bycancelPendingWork(Ticket&),cancelPendingWorkSafeorcancelPendingWork(VM&). It is not called for a ticket the embedder has taken. SincedoWorkdoes not run in this mode, these paths remove the tickets from the set themselves.m_taskLock, the four queries included.cancelPendingWork(VM&)runs on the collecting thread,~JSGlobalObjectruns on whichever thread sweeps, and~SharedArrayBufferContentscancels another VM's waiter from its own thread (WaiterListManager::unregister(uint8_t*, size_t)), while the embedder takes tickets on the VM's thread.cancelPendingWork(VM&)still clears the dependencies under the lock and calls the hook after it releases it.embedderData()/setEmbedderData()), written fromonAddPendingWorkbefore the ticket is handed out and read fromonScheduleWorkSoonon whichever thread schedules the work. Bun records which of its two event loops the work belongs to, which it used to keep in a map of its own.The default mode keeps its behavior:
doWorkstill purges cancelled tickets, andaddPendingWorknow takes the lock for the insertion. Nothing calls it with the lock held (doWorkdrops the lock around each task, and reconciliation runs aftercancelPendingWork(VM&)returns).WaiterList::takeFirstasserted that an async waiter's ticket is still alive. The end of a collection can now release the ticket before the realm's sweep unregisters the waiter.notifyWaiterImplhandles that already:scheduleWorkSoonIfActivefails on the deadWeakTicketand nothing is scheduled. The default mode has the same window betweendoWorkpurging a cancelled ticket and the sweep.hasPendingWork,hasDependencyInPendingWork,hasAnyPendingWorkandhasImminentlyScheduledWorknow answer correctly with the hooks installed. TheStreamingCompilerassertions thatBUN_SKIP_FAILING_ASSERTIONSdisables are the ones this made fail before. They are left as they are in this change.Verification
The Bun side (oven-sh/bun#39994) has two tests, a FinalizationRegistry and an
Atomics.waitAsyncin anode:vmcontext that is collected while the work is queued. Both segfault on Bun 1.4.0 and pass with this change and the matching scheduler. The modified files compile cleanly with-Wall -Wextra -Wthread-safetyunder Bun's debug flags.