One door out of a VM's thread: tickets + a teardown that waits - #38299
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… them A worker VM is now destroyed only after everything it handed to another thread has come back. Work that runs on (or is referenced from) another thread on a VM's behalf holds a `bun_jsc::Ticket`; teardown forbids script, cancels what it can, then waits for the ticket count to reach zero while releasing whatever arrives on its own thread with the heap alive, and only then destroys the JSC VM, loops and VirtualMachine. Replaces the two-count (active + embedded) scheme, `VmHandle::borrow`, `Postable::release_refused`, `JobList::release_all_js` and the JsSide teardown partition with that single count. `VirtualMachine` is `!Send + !Sync`; the worker thread no longer dereferences its parent VM (options/env are copied at construction) and other threads reach a VM only through its handle. A source lint freezes the set of `unsafe impl Send/Sync` and direct thread-crossing calls in the VM crates.
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WalkthroughChangesThe PR replaces VM loop-handle and borrow tracking with counted VM ticket-based teardown
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…d69' into claude/vm-thread-safety-door-993d69
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Updated 10:37 PM PT - Aug 13th, 2026
⏳ @dylan-conway, your commit eccf810 is still building in
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…ket FFI, name the worker thread's Send payload
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Inline comments:
In `@src/jsc/bindings/EventLoopTaskNoContext.h`:
- Around line 8-30: Define WTF_MAKE_TZONE_ALLOCATED_IMPL(EventLoopTaskNoContext)
in the EventLoopTaskNoContext.cpp implementation file to match the class’s
WTF_MAKE_TZONE_ALLOCATED declaration, without adding unrelated changes.
In `@src/jsc/Debugger.rs`:
- Around line 507-513: Update the comment immediately before
event_loop_mut().tick() to remove the obsolete reference to the cached loop
variable while retaining that the active event-loop slot may switch between
regular_event_loop and macro_event_loop, requiring event_loop_mut() to re-read
it each time.
In `@src/jsc/event_loop.rs`:
- Around line 818-828: Update enqueue_task_concurrent_same_thread to check
closed_for_tasks before pushing; when closed, release the wrapped task with
__bun_release_task_unrun and free the carrier only if auto_delete() is true,
rather than invoking ConcurrentTask::release_refused alone. Preserve the
existing queue-and-wakeup behavior while teardown has not started.
In `@src/jsc/job.rs`:
- Around line 188-207: Update the JobContext::run documentation to state that
after calling Completion::finish, the completion may be queued and its backing
Job<C> may be freed immediately; therefore implementers must not access off or
vm, or otherwise rely on either borrow, after finish returns.
In `@src/jsc/web_worker.rs`:
- Around line 93-96: Update the documentation for terminated_by_parent and the
thread_main startup comment to replace references to the removed vm_lock and
vm_ptr with the current vm_handle mechanism, including its guarded lock and
published handle terminology. Do not alter behavior or unrelated documentation.
In `@src/runtime/api/JSTranspiler.rs`:
- Around line 755-757: Update the SAFETY comment above the tsconfig conversion
in the JSTranspiler path to state that the configuration remains alive under the
job’s Ticket, matching the `under_ticket` call and removing the obsolete borrow
wording.
In `@test/js/web/workers/worker-late-completion.test.ts`:
- Around line 22-40: Update the Row type to require exactly one producer marker:
ticket for ticketed work or weak for weak-post work, rather than leaving both
optional. Preserve the other row properties and ensure producer rows cannot omit
both fields or provide an ambiguous combination, so the interpolation around the
late-post search construction is type-safe.
- Line 235: Replace the test-body require("node:path") call used to create fifo
with a module-scope node:path import, while leaving require calls inside spawned
-e fixture strings unchanged.
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src/bun_core/env_var.rssrc/bundler/bundle_v2.rssrc/event_loop/AnyEventLoop.rssrc/event_loop/ConcurrentTask.rssrc/event_loop/lib.rssrc/jsc/AsyncModule.rssrc/jsc/CppTask.rssrc/jsc/Debugger.rssrc/jsc/JSSecrets.rssrc/jsc/RuntimeTranspilerStore.rssrc/jsc/VirtualMachine.rssrc/jsc/VmHandle.rssrc/jsc/bindings/EventLoopTaskNoContext.cppsrc/jsc/bindings/EventLoopTaskNoContext.hsrc/jsc/bindings/JSSecrets.cppsrc/jsc/bindings/webcrypto/PhonyWorkQueue.cppsrc/jsc/event_loop.rssrc/jsc/job.rssrc/jsc/lib.rssrc/jsc/node_path.rssrc/jsc/web_worker.rssrc/runtime/api/Archive.rssrc/runtime/api/BunObject.rssrc/runtime/api/JSTranspiler.rssrc/runtime/api/glob.rssrc/runtime/api/js_bundle_completion_task.rssrc/runtime/crypto/PBKDF2.rssrc/runtime/crypto/PasswordObject.rssrc/runtime/dns_jsc/dns.rssrc/runtime/image/Image.rssrc/runtime/jsc_hooks.rssrc/runtime/napi/napi_body.rssrc/runtime/node/node_crypto_binding.rssrc/runtime/node/node_fs.rssrc/runtime/node/node_fs_stat_watcher.rssrc/runtime/node/node_fs_watcher.rssrc/runtime/node/node_zlib_binding.rssrc/runtime/node/zlib/NativeBrotli.rssrc/runtime/node/zlib/NativeZlib.rssrc/runtime/node/zlib/NativeZstd.rssrc/runtime/shell/builtin/cp.rssrc/runtime/shell/builtin/rm.rssrc/runtime/shell/builtin/yes.rssrc/runtime/shell/interpreter.rssrc/runtime/shell/states/Async.rssrc/runtime/webcore/CompressionStreamCoder.rssrc/runtime/webcore/blob/copy_file.rssrc/runtime/webcore/blob/read_file.rssrc/runtime/webcore/blob/write_file.rssrc/runtime/webcore/fetch/FetchTasklet.rssrc/runtime/webcore/s3/client.rssrc/runtime/webcore/s3/download_stream.rssrc/runtime/webcore/s3/simple_request.rssrc/threading/Condition.rstest/internal/source-lints/vm-thread-door.inventory.jsontest/internal/source-lints/vm-thread-door.test.tstest/js/web/workers/worker-late-completion.test.tstest/js/web/workers/worker-refused-completion.test.ts
💤 Files with no reviewable changes (2)
- src/jsc/bindings/EventLoopTaskNoContext.cpp
- test/js/web/workers/worker-refused-completion.test.ts
…ent touch-ups, typed test rows
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Inline comments:
In `@test/js/web/workers/worker-late-completion.test.ts`:
- Around line 215-218: The diagnostic matching logic around the seen calculation
must validate ticket and weak markers as non-empty before searching lines. Keep
producer-variant selection separate from marker validation: use the ticketed
late-completion prefix only when a non-empty ticket exists, and use the weak
late-post prefix only when a non-empty weak marker exists; otherwise treat the
row as unmatched.
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src/jsc/Debugger.rssrc/jsc/event_loop.rssrc/jsc/job.rssrc/jsc/web_worker.rssrc/runtime/api/JSTranspiler.rssrc/runtime/node/node_crypto_binding.rstest/js/web/workers/worker-late-completion.test.ts
…t out only on the final callback
… first-level workers only, serialize gate output, more producer rows, comment fixes
…g, tidy the door (guards, folded test gate, keep-alive), pass the worker snapshot by value, simplify the fetch hand-back
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Additional findings (outside current diff — PR may have been updated during review):
-
🟡
src/runtime/webcore/fetch/FetchTasklet.rs:2417-2423— In the non-final (!is_done) branch,ticketborrows(*task).http_ticketin place (line 2420) and thenSelf::from_raw_mut(task)immediately forms&mut *taskover the whole tasklet, which under Stacked Borrows popsticket's SharedReadOnly tag; the laterticket.post(ct)at line 2559 reads through the invalidated tag whiletask_refis still live. Same shape inS3HttpDownloadStreamingTask::http_callback(download_stream.rs:299 → 303 → 311), where(*this).process_http_callback(...)autorefs&mut *thisfor&mut self. Theis_donearm borrows the localdone_ticketand is fine. No practical miscompile (Tree Borrows accepts it;task_refnever writeshttp_ticket), but it regresses the SB discipline this file and src/CLAUDE.md §provenance document — the pre-e65d10c6 code cloned an owned handle throughtask_ref. Fix: clone the ticket in theNonearm (oneArcclone + onefetch_add, same cost as the oldloop_handle.clone()), or derive it throughtask_refafter forming the&mut.Extended reasoning...
What the bug is
Commit e65d10c changed the non-final HTTP progress callbacks in
FetchTasklet::callbackandS3HttpDownloadStreamingTask::http_callbackto borrow the tasklet'shttp_ticketin place —(*task).http_ticket.as_ref()— rather than cloning an owned handle out. Immediately after that borrow, both callbacks form a&mutover the whole struct from the same raw pointer:- FetchTasklet.rs:2420 → 2423:
let ticket = (*task).http_ticket.as_ref()...thenlet task_ref = Self::from_raw_mut(task), wherefrom_raw_mut(line 322-324) is literallyunsafe { &mut *this }. - download_stream.rs:299 → 303:
let ticket = (*this).http_ticket.as_ref()...then(*this).process_http_callback(...), which takes&mut selfand so autorefs to&mut *this.
Under Stacked Borrows, deriving a
Uniquereborrow of the whole allocation fromtaskpops every tag abovetask's SRW on every byte of*task— including theSharedReadOnlytagticketholds on thehttp_ticketfield's bytes. The subsequentticket.post(...)(FetchTasklet.rs:2559, download_stream.rs:311) then readsticket.sharedthrough a tag SB has invalidated, while the&mutis still live (task_refused at line 2561; the S3 temporary&mut selfgoing out of scope after line 303 does not restore a popped tag). Miri under Stacked Borrows would flag this as UB.The
is_donebranch is unaffected: thereticketborrows the localdone_ticket(theSome(t) => tarm), not the tasklet's field, so no second reference into the allocation coexists with the&mut.Step-by-step proof (FetchTasklet, non-final callback)
- HTTP thread invokes
FetchTasklet::callback(task, ...)withresult.has_more == true, sois_done == falseanddone_ticket == None. - Line 2420:
Option::as_refon(*task).http_ticketautorefs&(*task).http_ticket, pushing an SRO tag on those bytes as a child oftask's tag; the returned&Ticketis bound toticket. - Line 2423:
Self::from_raw_mut(task)executes&mut *task— aUniqueretag of the wholeFetchTasklet. SB performs a write-like access on all of*task's bytes, popping everything abovetask's tag — including the SRO from step 2. - Lines 2425-2556: ~130 lines use
task_ref(mutex, result staging, buffer append, CAS onhas_schedule_callback). - Line 2559:
ticket.post(ct)readsself.shared(theArc<Shared>insideTicket) through the tag popped in step 3 → SB "attempting a read access using ... but that tag does not exist in the borrow stack". - Line 2561:
task_ref.mutex.unlock()—task_refis still live across the read in step 5, so this is not a case where the&mut's protector had already ended.
The S3 case follows the same steps with
process_http_callback(&mut self)at line 303 in place offrom_raw_mut.Why existing code doesn't prevent it
The codebase is explicitly SB-aware: src/CLAUDE.md's "Pointer provenance at FFI boundaries" section names exactly this hazard, and
rm.rs,interpreter.rs,node_fs.rs,jsc_hooks.rsetc. carry "Stacked Borrows" comments enforcing the discipline ("stay on raw pointers … no live&mutinto it may span that call"). The pre-PR code respected it: it cloned an ownedLoopHandlethroughtask_ref(task_ref.loop_handle.clone()) after forming the&mut, so the handle was a fresh value on the stack, not a borrow into the aliased allocation. e65d10c introduced the in-place borrow specifically to avoid a per-progress-update clone, and in doing so put an SRO borrow of the tasklet on both sides of the&mut *taskreborrow.Impact
None operationally today: Tree Borrows accepts this pattern (
task_refnever writeshttp_ticketin the non-final path, so the shared read is not conflicting), LLVM does not currently exploit it, and there is no evidence Miri runs in CI over the fetch/S3 paths. It is, however, a real Stacked Borrows violation Miri (SB) would report on any streaming fetch or S3 download in a worker, and it regresses a documented convention this PR otherwise upholds carefully (every other producer moves its ticket to a local before touching&mut *self). Hence nit: worth fixing, does not block merge.How to fix
Two equally cheap options, both matching idioms already in this file:
- Clone the ticket in the
Nonearm.Ticket::cloneis oneArc::clone+ onefetch_add— exactly what the oldloop_handle.clone()cost — so the "avoid a per-progress-update clone" motivation is not actually saved by the in-place borrow. Bind an ownedTicket(orCow-style local) instead of&Ticket, and drop it after the post. - Derive the borrow through
task_ref. Formtask_ref = Self::from_raw_mut(task)first, thenlet ticket = done_ticket.as_ref().unwrap_or_else(|| task_ref.http_ticket.as_ref().expect(...))— the SRO is now a child oftask_ref's Unique tag, and no later reborrow pops it (the intervening uses oftask_refwould need to be shared reborrows, which they mostly already are field-wise; if borrowck rejects the overlap, fall back to option 1).
For
download_stream.rs, either clone in theNonearm, or reorder so the&Ticketis taken afterprocess_http_callbackreturns (the ticket is only used at line 311). - FetchTasklet.rs:2420 → 2423:
…its VM stops A read of a pipe or tty with no data (or a write to a full one) parks on the io loop and holds its job's ticket, so a worker doing await Bun.stdin.text() could no longer be terminated: the wait never ended. The stop phase now walks the VM's live jobs and cancels the ones waiting on something external; for these two that is a compare-exchange handshake (IoParking) between the pool thread that parks, the io thread that arms/fires the poll, and the JS thread that cancels, ending in the existing close path with ECANCELED.
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Inline comments:
In `@src/jsc/VirtualMachine.rs`:
- Around line 693-703: Update the four stale SAFETY comments in VirtualMachine
to describe the single-JS-thread invariant and reference the negative Send/Sync
assertion using AmbiguousIfImpl, rather than referring to the removed unsafe
impl Sync. Leave the assertion and surrounding implementation unchanged.
In `@src/jsc/web_worker.rs`:
- Around line 445-462: Update the safety documentation for ThreadStart and its
unsafe Send implementation to explicitly state that WorkerVmInit contains only
owned byte buffers, scalar values, and thread-safe Arc references from
ProxyEnvSlots, bun_dotenv::Map, and TransformOptions; confirm that no JSC String
or AtomString enters ThreadStart or is dropped on the worker thread.
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📒 Files selected for processing (18)
src/event_loop/ConcurrentTask.rssrc/jsc/CppTask.rssrc/jsc/VirtualMachine.rssrc/jsc/VmHandle.rssrc/jsc/event_loop.rssrc/jsc/job.rssrc/jsc/web_worker.rssrc/runtime/api/js_bundle_completion_task.rssrc/runtime/dispatch.rssrc/runtime/napi/napi_body.rssrc/runtime/node/node_fs.rssrc/runtime/node/node_fs_stat_watcher.rssrc/runtime/node/node_zlib_binding.rssrc/runtime/shell/builtin/cp.rssrc/runtime/webcore/fetch/FetchTasklet.rssrc/runtime/webcore/s3/download_stream.rstest/internal/source-lints/vm-thread-door.test.tstest/js/web/workers/worker-late-completion.test.ts
💤 Files with no reviewable changes (2)
- src/runtime/node/node_zlib_binding.rs
- src/runtime/node/node_fs.rs
…y what WorkerVmInit carries
…ncellation; shell bounces via the yield queue; kqueue deletes are not addressed to their owner - Completion<C> holds the job's Ticket (JobContext::run loses its vm param; done.ticket() cannot outlive done.finish()); only CANCELLABLE jobs are linked into the VM's list; unstarted jobs are handed back once the VM is draining rather than merely stopping. - IoParking: cancellation is sticky (DOOMED) and park() is fallible, so a read that parks after the sweep is still cancelled; WriteFile returns straight after parking instead of re-reading errno; a failed EV_DELETE (one-shot already fired) can no longer be dispatched into a freed owner. - The shell's Async state and yes builtin re-arm through enqueue_task_after_yield; enqueue_task_concurrent_same_thread is gone. - Assorted tidying from review (names, docs, Debugger init on the stack, test host escaping, signal-driven FIFO test).
…ty-door-993d69 # Conflicts: # test/js/web/workers/worker-refused-completion.test.ts
…d69' into claude/vm-thread-safety-door-993d69
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I reviewed this revision and the bug-hunting system found no new issues; all seven findings from earlier passes (release-build Location import, ThreadSafe::drop mini-loop assert, dead Bun__VmTicket__* / JsTicket layers, SendPtr→ThreadStart, test_gate::closed, the offset_of!(Job, header) const-assert) were addressed in follow-up commits. Given the scope — a cross-cutting rearchitecture of VM-thread lifetime across ~60 files, new synchronization primitives (Ticket, close_and_wait, IoParking), and VirtualMachine becoming !Send + !Sync — a human review is still warranted.
What was reviewed:
- The
Ticket/VmHandlestate machine and Dekker-pair orderings inclose_and_wait; ticket-drop ↔Drainingandactive↔Closedsequencing. - Every producer's "move ticket out before post" discipline (fetch, S3, transpiler slot, napi, zlib, shell, fs.cp, bundle) — no site posts through a ticket that lives inside the freed payload.
- The
IoParkingCAS handshake for parkedReadFile/WriteFileand itscancel→re-schedule path. WorkerVmInitcopying parent state on the parent thread; no worker-thread deref ofparent.
Extended reasoning...
Overview
This PR replaces the previous VmHandle::borrow / embedded_work_{scheduled,finished} / Postable::release_refused / JsSide machinery with a single counted Ticket that off-thread work holds for its in-flight span, plus an unbounded close_and_wait in VirtualMachine::teardown that drains the concurrent queues until tickets == 0 before destroying the JSC VM. It touches 61 files: the core (VmHandle.rs, job.rs, VirtualMachine.rs, web_worker.rs), every pool/HTTP/bundle/uv producer in src/runtime/, C++ EventLoopTaskNoContext/PhonyWorkQueue, a new IoParking cancellation handshake for Bun.file reads/writes parked on the io loop, and two new test files (a 29-producer late-completion matrix under BUN_DEBUG_TEST_WORKER_TEARDOWN_GATE, and a source-lint that freezes unsafe impl Send/Sync and thread-crossing call sites in the VM crates). VirtualMachine is made !Send + !Sync with a compile-time negative assertion, and the worker thread no longer dereferences its parent VM (options/env are copied into WorkerVmInit on the parent's thread).
Security risks
None specific to this PR. It is a memory-safety hardening change; the risk profile is UAF / deadlock / leaked-ticket regressions, not injection or auth. The test_gate machinery is debug-assertions-only and gated on an env var.
Level of scrutiny
High. This is core runtime concurrency: SeqCst orderings justified against two Dekker pairs, an unbounded teardown wait whose liveness depends on every producer eventually dropping its ticket, a new three-thread IoParking state machine, and ~30 producers each rewritten to move a ticket out before posting. A missed "ticket lives inside the posted payload" site is a UAF; a ticket stored in a JS-owned/VM-owned object is a teardown deadlock (the module doc calls this out). The !Send + !Sync change on VirtualMachine and the removal of unsafe impl Sync shift how every existing &VirtualMachine use must be justified. These are exactly the design decisions a maintainer should sign off on.
Other factors
Seven prior inline findings across four review passes were all fixed by the author (commits eeae4b6/62a11e0/7e8bc9f/c2aba49/a386387/ab02eaa per the thread). CodeRabbit's remaining threads are resolved. The PR ships substantial deterministic test coverage (the gate forces every producer's completion to land during the wait, on debug+ASAN), and a source-lint inventory that will flag future bypasses. CI status on the latest commit (59148fa) is still building. The change is well-documented and internally consistent, but its size and the criticality of the invariants it introduces put it outside what automated review should approve.
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I reviewed this PR across several rounds and the automated bug-hunt found nothing further on the current revision — all earlier findings (the release-build Location import, the dead JsTicket layer, the test_gate::closed no-op, the removed offset_of! const-assert, stale SAFETY/doc comments) have been addressed. Given this is a 62-file redesign of cross-thread VM lifetime (new Ticket type, close_and_wait Dekker pair, IoParking state machine, VirtualMachine made !Send + !Sync, worker/parent VM access rewired), a maintainer look is still warranted.
What was reviewed:
close_and_wait's SeqCst store→load pairing againstTicket::dropand weakenter; the condvar notify-under-lock ordering.- Ticket move-out-before-post at every producer (fetch, S3, napi, zlib, transpiler, shell, bundler) so the JS thread can't free the struct under a live ticket field.
- The
IoParkingCAS lattice against double-completion and thecancel → schedulerearm on the JS thread. WorkerVmInit/ThreadStartfor JSC/atom-string leakage across the thread boundary — none found.
Extended reasoning...
Overview
This PR replaces Bun's previous mixture of cross-thread VM-lifetime mechanisms (active+embedded counters, VmHandle::borrow, Postable::release_refused, the JsSide teardown partition) with a single invariant enforced in one place: a VM is destroyed only after every Ticket held on another thread has been dropped. It touches 62 files across src/jsc/ (VmHandle, VirtualMachine, job, web_worker, event_loop, Debugger, CppTask), src/runtime/ (~30 producers rewired to carry tickets), src/event_loop/, C++ bindings, and adds a new IoParking cancellation state machine plus a source-lint inventory test freezing the unsafe impl Send/Sync set.
Security risks
No direct auth/crypto/permissions surface. The risk profile is memory safety: use-after-free of VM/heap/JS-buffer state from another thread if the ticket accounting is unbalanced anywhere, deadlock of terminate() if a ticket is held by something the VM itself owns, and data races on the new atomics (the tickets/active/state Dekker pairs, IoParking's five-state CAS). The design is sound in principle — RAII counting removes the forget-to-decrement class — and the ASAN-gated test forces 29 producers through the late-completion path deterministically.
Level of scrutiny
High. This is core runtime memory-safety infrastructure with pervasive unsafe, hand-rolled atomics with SeqCst ordering arguments, a new three-thread cancellation handshake, and removal of unsafe impl Send/Sync for VirtualMachine. REVIEW.md's most-blocked category (native memory safety, thread affinity, refcount balance on every terminal path) applies to essentially every hunk. This is well beyond the "simple, mechanical, or obvious" bar for auto-approval and is exactly the kind of architectural change a maintainer should sign off on.
Other factors
Over five prior review rounds every finding I raised was promptly addressed and all threads are resolved. Test coverage is unusually thorough for this class of change (per-producer deterministic late-completion under BUN_DEBUG_TEST_WORKER_TEARDOWN_GATE, terminate-cancels-parked-read, terminate-waits-for-uncancellable, plus the source-lint inventory). CI on the latest commit is still building. Nothing outstanding blocks merge from my side; deferring solely on scope and criticality.
…odule graph is a process global, not something a Worker carries
The merge commit 1984c81 recorded origin/main (a0921e1, #38299) as a parent but its tree kept this branch's pre-merge versions of the files that merge touched. This commit's tree is the actual three-way merge of 41103f4 with a0921e1 (conflicts in dispatch.rs / RuntimeTranspilerStore.rs resolved as before: the release_unrun_erased signature change applied to the generated release switch, both doc comments kept).
| unsafe fn cancel(this: *mut Self) { | ||
| // SAFETY: fn contract; `io_parking` is atomic, and a `true` means no | ||
| // other thread touches `io_request` until it is queued again here. | ||
| unsafe { | ||
| if (*this).io_parking.cancel() { | ||
| io::IoRequestLoop::schedule(&mut (*this).io_request); | ||
| } |
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🔴 When io_parking.cancel() wins ARMED→CANCELLED and calls IoRequestLoop::schedule(&mut (*this).io_request) (writes non-atomic request.scheduled), the io thread may concurrently be at dispatch.rs:792/797 where __bun_io_pollable_on_ready forms &mut ReadFile/&mut WriteFile over the whole struct before on_ready checks self.io_parking.fire() — two live &mut covering io_request's bytes on different threads, both lowered as noalias fn parameters. This is the exact shape io/lib.rs:922-928 already documents as UB and rewrote tick() around; the SAFETY comment ("a true means no other thread touches io_request") is accordingly inaccurate. Fix: check io_parking.fire() via raw-pointer field access in __bun_io_pollable_on_ready/_on_io_error before forming &mut ReadFile/&mut WriteFile, so the CAS establishes ownership before the retag (same for WriteFile::cancel).
Extended reasoning...
What the bug is
The new JobContext::cancel for ReadFile/WriteFile runs on the JS thread "concurrently with wherever the job is" (per its own doc). When (*this).io_parking.cancel() returns true (state was ARMED), the JS thread calls io::IoRequestLoop::schedule(&mut (*this).io_request) (read_file.rs:249, write_file.rs analogous). schedule (io/lib.rs:937-940) takes request: &mut Request and unconditionally writes the plain, non-atomic, non-UnsafeCell field request.scheduled = true.
At the same wall-clock moment the io thread's registered poll may fire. __bun_io_pollable_on_ready (dispatch.rs:788-798) does:
let this = unsafe { &mut *from_field_ptr!(ReadFile, io_poll, poll) };
this.on_ready();That forms &mut ReadFile over the whole struct — including io_request — and passes it as the &mut self (a noalias fn parameter) to on_ready, which only then calls self.io_parking.fire() and returns early on false. The IoParking CAS decides who continues (only one of fire()/cancel() wins ARMED), but nothing orders the io thread's &mut ReadFile retag (which precedes its CAS) against the JS thread's &mut io_request write (which follows its CAS). Two live &mut on different threads covering overlapping bytes is aliased-&mut UB under Stacked Borrows, regardless of whether the losing side actually reads those bytes.
__bun_io_pollable_on_io_error (dispatch.rs:813-828) has the identical shape for the error path.
Step-by-step proof
- Pool thread:
wait_for_readablecallsio_parking.park()(IDLE→PARKED) and queues the io request. io thread processes it, callsio_parking.arm()(PARKED→ARMED), and registers the poll. - Concurrently: (a) JS thread enters teardown →
JobList::cancel_all→ReadFile::cancel(this)→(*this).io_parking.cancel()CASes ARMED→CANCELLED, returnstrue. (b) io thread's poll fires →__bun_io_pollable_on_ready→let this = &mut *from_field_ptr!(ReadFile, io_poll, poll)— a&mut ReadFileis now live on the io thread, coveringio_request. - JS thread proceeds to
IoRequestLoop::schedule(&mut (*this).io_request)and writesrequest.scheduled = true— a non-atomic write to bytes the io thread's live&mut ReadFilecovers. - io thread calls
this.on_ready()(anothernoalias&mut self), which callsself.io_parking.fire(), sees CANCELLED (not ARMED), and returns early — but only after both&mutwere simultaneously live.
The JS thread's SeqCst CAS (ARMED→CANCELLED) happens-before the io thread's failed fire() CAS, but the io thread's retag at dispatch.rs:792 precedes its CAS, so no ordering separates the retag from the JS thread's write.
Why existing code doesn't prevent it
The IoParking handshake is correct at the byte-access level: the io thread that loses fire() never reads or writes io_request's bytes before bailing, so there is no data race on the same location. But &mut T asserts noalias over all of T's bytes for its lifetime, and both schedule(request: &mut Request) and on_ready(&mut self) are function parameters where LLVM applies noalias. This is exactly the class the codebase already treats as a bug worth restructuring around: io/lib.rs:922-928 says of tick(),
"A
&muthere would assertnoaliasover those bytes for the process lifetime … which is UB under Stacked Borrows regardless of the queue's internal atomics"
and rewrote that path to raw pointers. The same discipline appears in this PR's own web_worker.rs header ("materialising &mut WebWorker here would be aliased-&mut UB") and in spin()'s note about not binding a long-lived &mut VirtualMachine. REVIEW.md is explicit: "benign same-value races are still UB", and "SAFETY comments … must be accurate" — the comment at read_file.rs:245-246 ("a true means no other thread touches io_request") is byte-level correct but aliasing-model wrong: the io thread's live &mut ReadFile retag covers io_request before it can bail.
A secondary instance of the same shape: Job::run_on_pool binds off: &mut (*this).off (= &mut ReadFile) across the whole C::run call, while cancel_all() on the JS thread may concurrently reach (*this).io_parking via raw pointer. That path is weaker (in the IDLE state cancel() only touches the atomic and never forms &mut io_request), but it is the atomic-under-&mut gray area the JobContext::cancel doc's own caveat ("touch only what that tolerates") cannot resolve while run's parameter is &mut Self::OffThread.
Impact and severity
This PR's thesis is closing every off-thread path to a VM's memory; introducing new aliased-&mut UB in the very cancellation mechanism it adds is worth fixing before merge. There is no known LLVM miscompilation vector today (the losing path reads only the atomic), so the practical risk is low — but the repo's own io/lib.rs precedent, REVIEW.md's rules, and the inaccurate SAFETY comment all put this in the class the codebase fixes rather than argues about. Marking normal.
How to fix
Move the ownership CAS before the retag: in __bun_io_pollable_on_ready and __bun_io_pollable_on_io_error, recover this: *mut ReadFile/*mut WriteFile via from_field_ptr! as a raw pointer, check (*this).io_parking.fire() through raw-pointer field access, and only on true form &mut *this and dispatch. Then the CAS establishes exclusive ownership before any &mut exists, and ReadFile::cancel's schedule(&mut io_request) runs against a struct no other thread has &mut to. The fire() checks inside on_ready/on_io_error become dead and can be removed. For the secondary instance, either change JobContext::run's off parameter to *mut Self::OffThread for CANCELLABLE contexts, or wrap io_parking in an UnsafeCell-backed field that the &mut ReadFile does not assert noalias over.
| // The payload embeds only the JS-arm `ConcurrentTask`, so the | ||
| // mini arm heap-allocates an auto-deinit wrapper per bounce. |
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🟡 Three more stale-comment sites of the cleanup already applied in eccf810/a386387: (1) Async.rs:18-21 — the task field doc's "the intrusive concurrent-task node must live in a stable heap allocation" describes the ShellAsyncTask.concurrent_task field this PR removed from dispatch_tasks.rs; (2) Async.rs:162-163 — "The payload embeds only the JS-arm ConcurrentTask" — it embeds none now; (3) run_command.rs:1091-1093 — "(a u8 until the b2-cycle widens it to cli::HotReload)" describes the widening this PR just did (the as u8 cast is gone; the sibling comment in jsc_hooks.rs was deleted). Drop or reword to match the new state.
Extended reasoning...
What the finding is
Three doc comments still describe machinery this PR removes — the same class of stale-reference cleanup the author has already applied several times in this PR (eccf810 for read_file.rs, a386387 for four SAFETY comments in VirtualMachine.rs, plus node_path.rs, CompressionStreamCoder.rs, and the deleted jsc_hooks.rs "raw u8" comment). These three were missed.
Site 1: src/runtime/shell/states/Async.rs:18-21
/// Heap payload for the main-thread bounce. The node lives in the
/// reallocatable `Interpreter::nodes` arena, so the intrusive
/// concurrent-task node must live in a stable heap allocation instead.
/// Allocated in `init`, freed in `actually_deinit`.
task: *mut crate::shell::dispatch_tasks::ShellAsyncTask,The dispatch_tasks.rs diff removes pub concurrent_task: ConcurrentTask from ShellAsyncTask, leaving only { interp, node }. There is no intrusive concurrent-task node any more. The heap allocation is still needed — the Task queued via enqueue_task_after_yield (JS arm) and the AnyTaskWithExtraContext wrapper (mini arm) both hold *mut ShellAsyncTask, and the Async node itself lives in the reallocatable Interpreter::nodes arena, so the payload cannot be inlined there. But the stated reason ("the intrusive concurrent-task node") is the removed mechanism.
Site 2: src/runtime/shell/states/Async.rs:162-163
EventLoopHandle::Mini(mut mini) => {
// The payload embeds only the JS-arm `ConcurrentTask`, so the
// mini arm heap-allocates an auto-deinit wrapper per bounce.The payload embeds no ConcurrentTask at all now. The JS arm was rewritten to go through owner.enqueue_task_after_yield(bun_jsc::Task::init(task)) with a plain Task — no intrusive node, no ConcurrentTask. The mini arm still heap-allocates a wrapper per bounce (that part is accurate), but the premise the comment gives for it is now false.
Site 3: src/runtime/cli/run_command.rs:1091-1093
// `ctx.debug.hot_reload` → `vm.hot_reload` (a `u8` until the
// b2-cycle widens it to `cli::HotReload`); `Run::start` re-reads it
// from `self.ctx` to drive the hot-reloader enable.
vm.hot_reload = ctx.debug.hot_reload;This PR is the b2-cycle widening the parenthetical anticipates: VirtualMachine::hot_reload changed from pub hot_reload: u8 to pub hot_reload: HotReload (with pub use bun_options_types::context::HotReload), the HOT_RELOAD_HOT/HOT_RELOAD_WATCH u8 constants were deleted, and the as u8 cast on this exact line was removed. The PR deleted the analogous stale comment in jsc_hooks.rs ("The low-tier VirtualMachine.hot_reload slot is a raw u8; compare against the real HotReload enum discriminant"), so this is the last remaining reference to the old u8 representation.
Step-by-step proof
-
dispatch_tasks.rs diff removes the field:
pub(crate) struct ShellAsyncTask { pub interp: *mut Interpreter, pub node: NodeId, - pub concurrent_task: ConcurrentTask, }and drops the
use bun_jsc::ConcurrentTask::ConcurrentTaskimport.ShellAsyncTasknow has no intrusive node. -
Async.rs
enqueue_selfdiff rewrites the JS arm from(*task).concurrent_task.from(task, AutoDeinit::ManualDeinit)+poster.post_js(...)toowner.enqueue_task_after_yield(bun_jsc::Task::init(task)). So the JS arm neither reads nor needs any embeddedConcurrentTask. -
VirtualMachine.rs diff:
- pub hot_reload: u8, + pub hot_reload: HotReload, ... -pub const HOT_RELOAD_HOT: u8 = 1; -pub const HOT_RELOAD_WATCH: u8 = 2; +pub use bun_options_types::context::HotReload;
and run_command.rs diff on the very line the comment describes:
- vm.hot_reload = ctx.debug.hot_reload as u8; + vm.hot_reload = ctx.debug.hot_reload;
The comment at 1091-1093 appears as unchanged context lines around this hunk.
-
jsc_hooks.rs diff removes the sibling comment:
- // The low-tier - // `VirtualMachine.hot_reload` slot is a raw `u8`; compare against - // the real `HotReload` enum discriminant (`!= 0` would also match - // `.watch`, which is wrong).
confirming the author intended to sweep these; run_command.rs was missed.
Why existing code doesn't prevent it
Doc comments are not compiled; nothing checks that a comment's referents exist. All three sit in unchanged context lines of hunks whose surrounding code was modified — exactly how these get missed. Per REVIEW.md "Only comment what the code cannot say" — these now say something the code contradicts.
Impact
None functionally — doc comments only. But they now actively mislead the next reader: the Async.rs comments send them looking for a ConcurrentTask field that doesn't exist, and the run_command.rs parenthetical describes a future refactor that has already happened.
Fix
- Async.rs:18-21: reword to state the actual reason — e.g. "Heap payload for the main-thread bounce: the enqueued
Task/ mini-loop wrapper holds a raw pointer to it, and this node lives in the reallocatableInterpreter::nodesarena, so the payload needs a stable address. Allocated ininit, freed inactually_deinit." - Async.rs:162-163: drop the premise, or reword to something like "The mini loop's queue takes
AnyTaskWithExtraContext, so heap-allocate an auto-deinit wrapper per bounce." - run_command.rs:1091-1093: drop the parenthetical "(a
u8until the b2-cycle widens it tocli::HotReload)", or the whole first clause since the assignment is now self-explanatory; keep the "Run::startre-reads it fromself.ctx" half if it is still accurate.
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Went through this after it landed, on 8827f8f. The two things I had lined up from an earlier revision (queued jobs running to completion during the wait, and a Bun.file() read parked on the io loop holding its ticket forever) were both fixed in the later commits, checked both against a build of the merged head. What's left is in the threads below, all small follow-ups.
Two other things that don't attach to a line:
A few SAFETY comments still describe the model this PR removed ("read only under the pool borrow"): src/runtime/image/Image.rs:1401, src/runtime/api/JSTranspiler.rs:667, src/runtime/webcore/CompressionStreamCoder.rs:582.
Open PRs this decides or overlaps with, none of them mentioned here: #34154 is the same design (pin at creation, drop after the post, close_and_wait) and should close as done by this. #37170 and #38312 go the other way (don't wait for pool work that owns its memory); with this merged that needs a yes/no rather than a rebase, fs.readFile on a FIFO with no writer still hangs terminate() on the merged head, same as before this PR. #38304 still reproduces on the merged head (a macro that starts un-awaited fs work makes bun index.ts never exit; 1.3.14 exits), it just needs a rebase. #37518 and #37278 need rebases too, #37278 more so, see the job.rs thread.
| h.assert_js_thread(); | ||
| debug_assert!( | ||
| h.0.state() != State::Closed, | ||
| "off-thread work started after the VM finished draining" |
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In release this is the only thing between a ticket taken after the wait and a freed VM: ticket() and Ticket::post are debug_asserts, deliver() derefs hot.vm unconditionally, and the base's *hot.vm = null at close is gone, so a ticket issued during step C gets counted, posts onto a loop that step D frees, and nothing waits for it. Step C does issue tickets: an addon built as NAPI_VERSION_EXPERIMENTAL runs its finalizers inline during the final collect (napi.h mustDeferFinalizers), and napi_queue_async_work from one of them lands in napi_async_work::schedule, which takes a ticket with no state check. Either make this a real assert (track_caller is already there so it names the finalizer's caller), or have teardown re-check tickets_outstanding() after step C and go back into the wait. A row for it in worker-late-completion would pin whichever you pick.
| self.loop_handle.embedded_work_scheduled(); | ||
| // env, so the VM waits for it (Node likewise settles its threadpool | ||
| // requests before an environment is freed). | ||
| self.ticket = Some(self.global.bun_vm().ticket()); |
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Independently of the VmHandle thread: napi_queue_async_work has no state check, so once the VM is stopping this still takes a ticket and runs execute on the pool for a result nothing will read. Refusing (or queueing and immediately completing) when !script_allowed() would keep finalizer-time queueing, and anything else that queues late, out of the wait entirely.
| const seen = | ||
| "ticket" in row && row.ticket | ||
| ? lines.some(l => l.startsWith("[vm] late completion from ") && l.includes(row.ticket)) | ||
| : lines.some(l => l.startsWith(`[vm] late post: ${row.weak} (`)); |
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This matches the prefix only, and the gate line is late post: X (released by the wait) or late post: X (refused), so the three weak rows pass either way. The gate parks until Draining, so in practice they always take the released branch, and with worker-refused-completion.test.ts deleted the Closed -> Refused paths (post_cpp_task deleting the EventLoopTask on the posting thread, the waiter thread's refused path) have no test that reaches them on purpose any more. A second gate mode that parks until Closed (still with the don't-park-while-tickets-are-outstanding bypass so it can't deadlock the wait) plus asserting the outcome word would get that back for a few lines.
| // SAFETY: as above; the borrow keeps the VM (and any JsPtr target) alive. | ||
| if let Some(done) = C::run(unsafe { &mut (*this).off }, &vm, done) { | ||
| drop(vm); | ||
| if done.ticket().cancelled() { |
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A cancelled job is dropped without running, so a queued fs.close(fd) never closes its fd and a finished fs.open whose completion is released unrun drops the fd it opened. Worker doing open/close in a loop, parent terminates it 10 times: the merged head leaks 194 fds on macOS (main before this PR leaked 179 for the same run, so not new, but this path makes it the common case now). That's what #37278 fixes (release hook for Close, owning newtype for the open result); it wants a rebase onto this rather than closing.
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What does this PR do?
Makes it impossible for another thread to touch a worker's
VirtualMachine(heap, loops, JS-owned buffers, VM state) after it is destroyed, by enforcing one invariant in one place: a VM is destroyed only after everything that left its thread has come back. Anything running on, or referenced from, another thread on a VM's behalf holds abun_jsc::Ticket(src/jsc/VmHandle.rs); the VM's teardown forbids script, runs the stop phase, then waits for the ticket count to reach zero — releasing arriving completions on its own thread with the heap alive — before destroying the JSC VM, loops and struct. Holding the ticket is the count, and posting through it cannot fail, so no producer has a "VM already gone" path.This replaces the previous mixture (
active+embeddedcounts with hand-placedembedded_work_scheduled/finished,VmHandle::borrow,Postable::release_refused,JobList::release_all_js, theJsSideteardown partition). Every pool/HTTP/bundle/uv/thread producer injsc/runtimenow carries a ticket for its in-flight span; things that merely refer to a VM (other contexts' ports, JSC helper threads, the child waiter, napi tsfn, watcher threads) keep the uncountedVmHandle(deliver-or-refuse).VirtualMachineis!Send + !Sync; the worker thread no longer dereferences its parent VM (options/env copied atnew Worker(), as Node does) and a parent reaches a child VM only through its handle.test/internal/source-lints/vm-thread-door.test.tsfreezes theunsafe impl Send/Syncset and direct thread-crossing call sites in the VM crates.The stop phase keeps the wait short: pool jobs the VM has not started yet are handed back unrun once it is stopping (Node
uv_cancels them), and aBun.file/Bun.stdinread or write parked on a pipe with no data is cancelled through the VM's live-job list (JobContext::cancel, anIoParkinghandshake between the pool, io and JS threads) instead of holdingterminate()forever. What remains unbounded is genuinely uncancellable work (anode:fsread blocking a pool thread), as in Node; debug builds name the outstanding tickets' creation sites. Main-thread process exit is unchanged.How did you verify your code works?
test/js/web/workers/worker-late-completion.test.ts(debug+ASAN): 29 producers (fs incl. Buffer read/write, recursive readdir/fs.cp,Bun.$ls/cp/rm, crypto, zlib stream, transpiler slot,import(), fetch parked on a dead peer,Bun.build, WebCrypto, weak posters) each forced to complete during the worker's wait viaBUN_DEBUG_TEST_WORKER_TEARDOWN_GATE;terminate()cancellingBun.stdin.text()/Bun.file(fifo)reads parked on the io loop; and a FIFO-blockedfs.readFileshowingterminate()waits and the debug dump namesnode_fs.rs— 33/33 ×3. worker_threads 129/129, bun-write/bun-file/stdin suites green locally;cargo checkrelease + clippy. Relying on CI for the rest of the matrix.