Fix hang when a macro awaits crypto.subtle.digest

[robobun]

Problem

• await crypto.subtle.digest(...) inside a macro makes Bun.build() and bun run hang forever. Regression from 1.3 to 1.4 (await crypto.subtle.digest("SHA-256", buffer) in a macro cause Bun.build() to hang indefinitely in Bun 1.4 #39900).
• It is not specific to digest. Anything a macro awaits whose completion reaches the VM through a post that does not know which loop the macro is waiting on hangs the same way on 1.4.0 and passes on 1.3.14: WebAssembly.compile/instantiate and Atomics.waitAsync (JSC's DeferredWorkTimer, JSCScheduler.rs), MessageChannel and BroadcastChannel deliveries and small (< 64 byte) digests (postTaskTo from the same thread), Worker messages/online/exit (postTaskTo from the worker thread), and the rest of SubtleCrypto (sign, encrypt, ECDH deriveBits, …). All of these were hardcoded to LoopKind::Regular (VmHandle::post_cpp_task, Bun__queueJSCDeferredWorkTaskConcurrently, Bun__VmHandle__refKeepAlive), while a macro's await ticks only the macro loop (wait_for_promise).
• The opposite direction is event loop: run completions left on the macro loop after the macro returned #38304: work a macro starts but does not await takes a Macro ticket, finishes after the macro returned, and sits on a loop nothing ticks, its keep-alive holding the process open. 1.3 posted to whichever loop was current at completion time, so both directions worked there (through a cross-thread read of the JS thread's loop pointer, which Worker / worker_threads: WebCore-shaped lifetimes, joined threads, one ordered VM teardown #37075 removed).

Fix

Every post carries the loop that was current on the JS thread when its work was initiated — the rule tickets already follow — and the regular loop adopts whatever is left on the macro loop once no macro is running. The macro's wait services only what the macro started; nothing can be stranded; no thread reads another thread's loop state.

• LoopKind crosses the FFI as BunLoopKind (BunLoopKind.h); Bun__VM__currentLoopKind exposes current_loop_kind() to the C++ initiators. Bun__VmHandle__postAndRelease / refKeepAlive and VmHandle::post_cpp_task take the kind instead of assuming Regular.
• WebCrypto: the work-queue closures start on the JS thread with the context in hand, so they capture context.currentLoopKind() next to context.identifier() and pass it to postTaskTo from the pool thread; the sub-64-byte same-thread paths pass it directly. ConcurrentCppTask is back to main's event_loop_shared().ref_keep_alive() + vm.ticket().
• JSCTaskScheduler: the two pending-ticket sets become maps from JSC ticket to the loop kind onAddPendingWork (JS thread) captured, and onScheduleWorkSoon posts the completion to it. The keep-alive onAddPendingWork takes is applied directly (Bun__eventLoop__refKeepAlive, it is on the JS thread) so its release does not depend on which loop ticks next.
• ScriptExecutionContext::postTaskTo(id, loopKind, task) — no default, every caller states it: from the target's own thread the task joins the loop that thread is running (still through the concurrent queue, so per-tick refill bounding is unchanged); from another thread it joins loopKind, which the cross-thread callers supply: WorkerMessagingProxy captures the parent's loop at new Worker() for everything it posts back (postTaskToWorkerObject); the worker heap-snapshot/statistics/CPU-usage request-replies capture it at request time; MessagePortPipe records it next to ctxId when a side attaches to a context and posts drains and the peer-close notification to it; the BroadcastChannel registry records it per subscriber. ScriptExecutionContext::currentLoopKind() is the single C++ accessor.
• Work no script initiated — the debugger, signal delivery, hot reload, napi finalizers from GC threads, memory-pressure, webview socket refs, threadsafe: true FFI callbacks invoked from a foreign thread — stays Regular. Those are now the only Regular constants left.
• EventLoop::macro_loop_if_not_running (from event loop: run completions left on the macro loop after the macro returned #38304): when this is the regular loop, the VM has run a macro, and none is running now, tick_concurrent_with_count first folds the macro loop's concurrent_ref delta and moves its concurrent batch into tasks (through take_concurrent_tasks, which now takes the batch), then drains its own queue as before; has_pending_tasks / has_pending_refs / is_event_loop_alive see the same through has_concurrent_tasks. The macro loop never adopts the regular loop's queue.
• Correct because at any moment exactly one of the two loops ticks. While a macro runs, only completions of work the macro (transitively) started carry Macro, and the wait drains exactly those; the program's completions carry Regular and wait for the macro to return, as its same-thread tasks always have. After the macro returns the regular loop owns both queues, and a late Macro post or unref wakes the shared platform loop, so it is picked up on the next tick. No poster reads the JS thread's macro state: same-thread initiators read their own thread's state, cross-thread ones use a value captured earlier on the JS thread.
• Supersedes event loop: run completions left on the macro loop after the macro returned #38304 (its drain and its four test cases are included here, on top of the routing change).

Verified on a Windows debug build: test/regression/issue/39900.test.ts (12 cases, including a port transferred to a Worker and a BroadcastChannel message sent from a Worker; all time out on 1.4.0, pass on 1.3.14 except Atomics.waitAsync, which hung there too), the new event loop routing around macros block in test/bundler/transpiler/macro-test.test.ts (7 cases), plus web-crypto, web-crypto-sha3, atomics, message-channel, message-port-pipe, worker-postmessage-transfer, message-port-closed-leak, broadcast-channel, broadcast-channel-worker-gc, worker_threads, worker-late-completion and the source lints.

Background

• A VM embeds two EventLoops, regular_event_loop and macro_event_loop, and vm.event_loop points at whichever is current. MacroModeGuard points it at the macro loop while a macro runs (and while the macro module loads) so that the program's queued work does not run underneath the transpiler; a macro that returns a promise is settled by ticking that loop. Both sit on one uSockets/libuv platform loop, whose active count keep-alives adjust.
• Each EventLoop has two things other threads write: concurrent_tasks (completions; each tick moves a batch into tasks and runs it) and concurrent_ref (a pending keep-alive delta folded into the platform loop at the top of each tick). A Ticket records which loop's pair it writes to when it is taken on the JS thread; a weak post (VmHandle::post, what postTaskTo(identifier) becomes) is told which by its caller.
• In bun run, the entry file's macros, and those of any module transpiled on the main thread (e.g. one that is require()d), execute in the main VM; that is where routing matters. Macros in files transpiled on the thread pool and in bun build run in per-thread VMs.

Notes

• Why not drain the regular queue from the macro wait (this PR's first revision): it fixes the hang, but it runs the program's pending completions mid-transpile, and any work those callbacks start takes a Macro ticket and is stranded once the macro returns unless something later drains the macro loop. The a program completion that arrives during a macro waits for the macro to return test pins this: with that revision the macro observes ["program"]; with this one it observes [].
• Why not pick the loop at post time (what 1.3 did): the posting thread would read the JS thread's macro state again, and a program completion that merely arrives during a macro would still run inside it. Every producer here has a JS-thread initiation point where the loop is known exactly, so it is captured there instead.
• postTaskTo from the target's own thread keeps going through the concurrent queue rather than postTask's direct enqueue on purpose: same-thread MessageChannel ping-pong relies on the per-tick concurrent refill bound to let timers and I/O run.
• Atomics.waitAsync with a timeout inside a macro hangs on 1.3.14 as well; it goes through the same DeferredWorkTimer path and is fixed by the same change, so it is in the producer list even though it is not strictly a 1.4 regression.
• A sync macro that calls setImmediate() still hangs (immediates are per-loop, as on 1.3.14); untouched here.
• On the Windows debug build, four worker-late-completion rows (Bun.file().text(), Bun.Image().metadata(), fs.read, thread-pool dns) do not observe a late completion because those paths run on the libuv loop thread there; unrelated to this change (that describe block is debug/ASAN-only and CI runs it on Linux). worker.test.ts's message-flood case needs the first worker message within ~30 ms, which a Windows debug build's worker startup does not meet.

[robobun]

^{Updated 8:42 PM PT - Aug 21st, 2026}

@dylan-conway, your commit 8f1658d is building: #103185

[coderabbitai]

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Walkthrough

Changes

The change adds regular and macro loop selection to VM task APIs, routes worker and WebCrypto callbacks through the originating loop, drains finished macro-loop tasks, and returns C++ task replies through tickets. Tests cover macro asynchronous work and process completion.

Changes

Macro event-loop scheduling

Layer / File(s)	Summary
Loop-aware task contracts and delivery src/jsc/CppTask.rs, src/jsc/VmHandle.rs, src/jsc/bindings/BunLoopKind.h, src/jsc/bindings/EventLoopTaskNoContext.*, src/jsc/bindings/BunClientData.h, src/jsc/virtual_machine_exports.rs, src/runtime/webview/*	C++ task replies now return event-loop tasks. VM APIs accept LoopKind, and callers select the regular or macro loop.
Deferred ticket lifecycle src/jsc/bindings/JSCTaskScheduler.*	Pending tickets now store loop kind and keep-alive state. Shared cleanup handles cancellation, shutdown, execution, and queued-job reclamation.
Macro-loop draining and liveness src/jsc/event_loop.rs, src/jsc/VirtualMachine.rs	Concurrent-task detection and draining include finished macro-loop work. VM liveness uses the event-loop concurrent-task query.
Worker, messaging, and WebCrypto callback routing src/jsc/bindings/ScriptExecutionContext.*, src/jsc/bindings/webcore/*, src/jsc/bindings/webcrypto/*	Worker and messaging callbacks preserve loop kind. WebCrypto work returns WorkQueue::Reply callbacks instead of posting nested context tasks.
Regression coverage test/bundler/transpiler/macro-test.test.ts, test/regression/issue/39900.test.ts, test/js/web/workers/worker-late-completion.test.ts	Tests cover macro asynchronous work, awaited and unawaited operations, worker completion, bun run, and Bun.build.

Suggested reviewers: dylan-conway

🚥 Pre-merge checks | ✅ 4

✅ Passed checks (4 passed)

Check name	Status	Explanation
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Description check	✅ Passed	The description clearly explains the problem, implementation, scope, background, and verification results, although it uses different headings than the template.
Title check	✅ Passed	The title clearly identifies the primary fix: preventing macro hangs when awaiting crypto.subtle.digest.

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Actionable comments posted: 1

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In `@test/regression/issue/39900.test.ts`:
- Around line 5-8: Condense the regression comment above the test to one line
containing the issue URL and a concise description of the failure; remove the
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In `@test/regression/issue/39900.test.ts`:
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[claude]

I reviewed this PR and didn't find any bugs. Because it changes cross-loop task routing in the core event loop (draining the regular loop's concurrent queue into the macro loop's task queue during a macro wait, plus new unsafe sibling-field projections in regular_loop_during_macro), a human look from someone familiar with the macro/regular loop split would still be worthwhile.

What was reviewed:

• tick_concurrent_with_count refactor: take_concurrent_batch is the old body extracted verbatim; return-count arithmetic unchanged.
• regular_loop_during_macro aliasing: addr_of! projects sibling VM fields without forming &VirtualMachine; pop_batch(&self) and apply_concurrent_ref_delta(&self) need only shared access, and the returned &EventLoop is dead before &mut self is reborrowed for take_concurrent_batch.
• Keep-alive pairing: ref_keep_alive on regular_event_loop is balanced by ticket.unref_keep_alive() on a LoopKind::Regular ticket, and that delta now folds during macro ticks.
• Tests follow harness conventions (tempDir, bunEnv, concurrent pipe drain, exit-code last).

Extended reasoning...

Overview

The PR fixes a hang when a macro awaits crypto.subtle.digest(). It touches three core event-loop files: src/jsc/event_loop.rs (tick_concurrent_with_count now also drains the regular loop's concurrent queue when ticking as the macro loop; adds take_concurrent_batch and regular_loop_during_macro), src/jsc/VmHandle.rs (adds VirtualMachine::regular_ticket), and src/jsc/CppTask.rs (ConcurrentCppTask__createAndRun pins its keep-alive and ticket to the regular loop). A regression test with two subprocess cases (bun run and Bun.build) is added.

Security risks

None identified. This is internal event-loop scheduling; no user input parsing, auth, or crypto verification paths are touched. The WebCrypto operation itself is unchanged — only where its completion is queued.

Level of scrutiny

High. This is core runtime concurrency code: cross-loop task routing, keep-alive ref-count folding across two embedded event loops, and an unsafe block that materializes &EventLoop for a sibling field via raw-pointer projection to avoid aliasing &mut self (a value field of the same VM). The semantic change — regular-loop concurrent tasks now run inside the macro loop's tick during a macro wait — is argued to restore 1.3 behavior, but it affects what can execute during macro evaluation and interacts with the complementary fix in #38304. A maintainer familiar with the macro/regular loop split should confirm this is the intended layering.

Other factors

The PR description is thorough (mechanism explained, verified against 1.4.0 hang, related suites run, pre-existing worker.test.ts failures noted). All bot review threads (comment-cop, coderabbit) are resolved. The bug-hunting system found nothing. The refactor of tick_concurrent_with_count into take_concurrent_batch preserves the original body byte-for-byte. Still, per the approval guidelines, changes to critical code paths with non-obvious concurrency semantics warrant human review rather than auto-approval.

[robobun]

CI status: the only red lane is test/js/bun/http/bun-server.test.ts on Windows 2019, which also fails on main (reported separately). The other failures passed on retry and are on suites this diff does not touch. The regression tests pass on all lanes.

[claude]

I reviewed this PR and the follow-up commits address all three of my earlier inline findings (the take_concurrent_tasks fold, the issue_ticket extraction, and the documented ticketed-Regular gap). No new issues found this pass. Because this reworks cross-thread task routing between the macro and regular event loops, carries an acknowledged gap that #38304 closes, and is step 1 of a two-PR stack, a maintainer look at the design shape and landing order is still worthwhile.

Checked: the regular_loop_during_macro addr_of! projection keeps &mut self (macro loop) and &regular_event_loop disjoint; into_task was already the batch-move path in the deleted take_concurrent_tasks, so the refactored drain is behavior-preserving; ref_keep_alive on regular_event_loop folds immediately on the JS thread so the CppTask.rs pinning does not depend on which loop next ticks.

Extended reasoning...

Overview

Four files: src/jsc/event_loop.rs (macro-loop tick now also drains the regular loop's concurrent queue and folds its keep-alive delta; take_concurrent_batch extracted and take_concurrent_tasks deleted), src/jsc/CppTask.rs (WebCrypto work-queue keep-alive and ticket pinned to the regular loop), src/jsc/VmHandle.rs (new regular_ticket() and shared issue_ticket(kind)), and a three-case regression test.

Security risks

None identified. No untrusted input parsing, no auth/crypto correctness surface — the change is task-routing plumbing between two per-VM event loops.

Level of scrutiny

High. event_loop.rs is the core JS-thread dispatch path; tick_concurrent_with_count runs on every tick. The change adds an unsafe sibling-field projection (regular_loop_during_macro), reworks the batch-drain to drop the deferred-destruction dance in favor of into_task, and changes which loop a WebCrypto keep-alive lands on. All of that is subtle enough — and explicitly coupled to a follow-up PR (#38304) that closes the remaining ticketed-Regular-follow-up strand — that the design shape and stack ordering are maintainer calls.

Other factors

My three prior inline findings are all addressed (e6933c1 folded the duplicate drain helper, 6a2a313 extracted issue_ticket, and the ticketed-Regular gap is now documented in the PR body with #38304 named as the closer). The tests are hermetic subprocess spawns that cover both entry points (bun run, Bun.build) plus the un-awaited keep-alive path, and the evidence block shows they hang on the unfixed debug build. CI is green apart from a pre-existing Windows failure the author called out. Nothing blocks from my side; deferring for a human sign-off on the two-loop routing design and the stack landing order rather than for any open defect.

[RiskyMH]

this fixes a hanging bug i got in macros with 1.4 too

[robobun]

Thanks for confirming. Any macro that awaits WebCrypto work hits the same path, so this fix should cover your case too.

[coderabbitai]

Actionable comments posted: 1

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⚠️ Outside diff range comments (1)

src/jsc/bindings/webcore/MessagePortPipe.h (1)

66-70: 🩺 Stability & Availability | 🔵 Trivial | ⚡ Quick win

Add a macro-loop regression test. MessagePortPipe.cpp records and forwards currentLoopKind() for attachment, transfer, drain, and peer-close paths. Existing tests cover transfer and re-attach, but not Macro loop delivery. Add coverage and run the relevant tests.

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In `@src/jsc/bindings/webcore/MessagePortPipe.h` around lines 66 - 70, Add a
regression test covering Macro loop delivery through MessagePortPipe, including
the peer-close path that forwards currentLoopKind(). Reuse the existing transfer
and re-attach test setup and assertions, configure the loop kind as Macro, and
verify the close event is delivered on the expected loop.

Source: Coding guidelines

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Inline comments:
In `@src/jsc/bindings/webcore/MessagePortPipe.cpp`:
- Around line 59-67: Use both context ID and loop kind as callback identity in
MessagePortPipe::scheduleDrain: capture the expected BunLoopKind and have
drainAndDispatch reject stale work when either stored value differs. Apply the
same expected-loop-kind capture and validation to the peer-close callback at
src/jsc/bindings/webcore/MessagePortPipe.cpp lines 336-357; both sites require
changes.

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Outside diff comments:
In `@src/jsc/bindings/webcore/MessagePortPipe.h`:
- Around line 66-70: Add a regression test covering Macro loop delivery through
MessagePortPipe, including the peer-close path that forwards currentLoopKind().
Reuse the existing transfer and re-attach test setup and assertions, configure
the loop kind as Macro, and verify the close event is delivered on the expected
loop.

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• src/jsc/bindings/BunDebugger.cpp
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• src/jsc/bindings/JSCTaskScheduler.cpp
• src/jsc/bindings/JSCTaskScheduler.h
• src/jsc/bindings/ScriptExecutionContext.cpp
• src/jsc/bindings/ScriptExecutionContext.h
• src/jsc/bindings/webcore/BunBroadcastChannelRegistry.cpp
• src/jsc/bindings/webcore/BunBroadcastChannelRegistry.h
• src/jsc/bindings/webcore/JSWorker.cpp
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• src/jsc/bindings/webcore/MessagePortPipe.h
• src/jsc/bindings/webcore/WorkerMessagingProxy.cpp
• src/jsc/bindings/webcore/WorkerMessagingProxy.h
• src/jsc/bindings/webcrypto/CryptoAlgorithmSHA1.cpp
• src/jsc/bindings/webcrypto/CryptoAlgorithmSHA224.cpp
• src/jsc/bindings/webcrypto/CryptoAlgorithmSHA256.cpp
• src/jsc/bindings/webcrypto/CryptoAlgorithmSHA3.cpp
• src/jsc/bindings/webcrypto/CryptoAlgorithmSHA384.cpp
• src/jsc/bindings/webcrypto/CryptoAlgorithmSHA512.cpp

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