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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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📒 Files selected for processing (58)
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
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- 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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📒 Files selected for processing (7)
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).
Conflicts: packages/bun-usockets/src/quic.c (kept the header include over the stale prototype, plus main's new verify-error extern) and src/jsc/web_worker.rs, where main's VmHandle teardown (#38299) replaced the vm_lock the parent-side ELU read relied on and moved parent reads into WorkerVmInit on the parent thread. The worker's execArgv, CA and cpu-prof inputs now travel in WorkerVmInit, and the parent reads ELU from a worker-published EluSource (loop plus copied stamps) that is taken back before teardown destroys the loop.
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.