Flaky Windows CI crash: UV_HANDLE_CLOSING assertion at ava worker exit on Node 24.19.0 #360

Symptom

A Windows CI job aborts after every test has already passed:

√ streaming-class › class compress output is byte-identical across all chunkings
Assertion failed: !(handle->flags & UV_HANDLE_CLOSING), file src\win\async.c, line 94
× __test__\streaming-class.spec.ts exited with a non-zero exit code: 3221226505
...
32 tests passed

Exit code 3221226505 is 0xC0000409, which MSVC uses for __fastfail / abort().
Zero tests fail. The ava worker process crashes at exit.

Occurrences

Both hits are in __test__/streaming-class.spec.ts, both on node@24, one per architecture.
The first is a lockfile-only commit, so this is not caused by any source change.
Rate is roughly 2%: 2 hits across 96 Windows test jobs.

Cause

This is a known Node.js bug, already fixed upstream. It is not a libuv bug and not a bug in
this repo's Rust code.

DelayedTaskScheduler in src/node_platform.cc owns a uv_async_t flush_tasks_. A V8
platform worker can post a delayed task after Stop() has begun closing that handle, so
uv_async_send() runs on a handle that already carries UV_HANDLE_CLOSING.

Node 24.19.0  ─►  DelayedTaskScheduler::PostDelayedTask() races Stop()
                  uv_async_send(&flush_tasks_) after uv_close
                          │
                          ▼
libuv 1.52.1  win/async.c:94   assert(!(flags & UV_HANDLE_CLOSING))  ─►  abort()  ─►  0xC0000409
                          ▲
                          └── Windows only. src/unix/async.c has no such assert.

The upstream fix is nodejs/node#61999, which
closed #56645 and
#64322:

   void PostDelayedTask(...) {
     auto locked = tasks_.Lock();
+    if (has_shut_down_) return;
     ...
   void Stop() {
     auto locked = tasks_.Lock();
+    has_shut_down_ = true;
     locked.Push(std::make_unique<StopTask>(this));
     uv_async_send(&flush_tasks_);
   }

I checked src/node_platform.cc at each release tag. DelayedTaskScheduler::Stop sets
has_shut_down_ only from 24.20.0 on:

Why CI ran 24.19.0

.github/workflows/CI.yaml pins node-version: 24 and does not set check-latest, which
defaults to false. The runner served a stale tool cache entry:

Found in cache @ C:\hostedtoolcache\windows\node\24.19.0\x64

What was ruled out

I traced the full control flow of the class API before blaming Node.

• DecompressorState::spawn() (src/stream.rs:250) starts a plain OS thread. It holds only
  std::sync::mpsc channels and a JoinHandle. It never calls into napi, V8 or libuv, so it
  cannot reach uv_async_send.
• That thread is joined in into_finish() (src/stream.rs:299 and :316). Every test in the
  spec awaits finish(), so every thread was joined.
• Decompressor has no Drop and no custom finalizer. No test leaves one unfinished.
• src/stream_web.rs runs no code in this spec. The spec calls only Compressor,
  Decompressor and uncompressSync.
• Local repro on macOS arm64, Node 24.21.0: 20 runs of the spec and 4 × 150 child-process
  teardown probes, all clean. Expected — the assert does not exist in src/unix/async.c, and
  24.21.0 already carries the fix.

Suggested fix

Set check-latest: true on actions/setup-node, or pin node-version: 24.20.0 or later.

Warning: Node 22 is still exposed. The fix is not in any 22.x release, so node@22 in the
matrix can still hit this.

Not proven

• Which uv_async_t asserted. The message does not name the handle. Three live in that
  process: Node's flush_tasks_, libuv's internal loop->wq_async, and the custom-GC
  threadsafe function napi-rs creates per environment. The version evidence points at
  flush_tasks_, but that is inference, not a stack trace.
• Why only streaming-class.spec.ts. That spec creates and joins the most threads and issues
  the most async work, so it plausibly holds the widest teardown window. n=2 is not proof.
• That upgrading Node removes it. That is a version-consistent prediction, not an observation.

Links

• src: fix libuv assertion on windows nodejs/node#61999
• libuv assertion on Windows with Node.js 23.x nodejs/node#56645
• Assertion failed: !(handle->flags & UV_HANDLE_CLOSING), file src\win\async.c, line 76 nodejs/node#64322
• uv_async_send() inconsistency libuv/libuv#3622
• win: fix race condition in uv_async_send libuv/libuv#5079 (removes the assert; not in libuv 1.52.1)

🤖 Generated with Claude Code

https://claude.ai/code/session_01Qa2uh946PBdYTn9Tjw19TZ