src: avoid race condition in tracing code

[addaleax]

json_trace_writer_ is protected by stream_mutex_,
but one access to it was not guarded by a lock on said mutex.

Refs: #25512

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• commit message follows commit guidelines

[nodejs-github-bot]

@addaleax build started: https://ci.nodejs.org/blue/organizations/jenkins/node-test-pull-request-lite-pipeline/detail/node-test-pull-request-lite-pipeline/2331/pipeline

[addaleax]

Stress test CI for parallel/test-trace-events-fs-sync with this PR: https://ci.nodejs.org/job/node-stress-single-test/2142/
Regular CI: https://ci.nodejs.org/job/node-test-pull-request/20250/

[bnoordhuis]

Aside: is the call to WriteFile() on line 136 sound? FlushPrivate() reads num_write_requests_ while holding the lock, then passes that value to WriteFile() after dropping the lock. It looks like you could observe duplicate request ids.

[bnoordhuis]

Is it necessary to nest locks?¹ And if so, can you perhaps add a comment explaining why? It's currently kind of hard to eyeball whether this is correct or a deadlock waiting to happen.

It would be nice to have an abstraction that enforces that locks are only taken out in a specific order so you don't run into AB-BA deadlocks.

¹ I think the answer is 'yes' because otherwise there's a race window.

[addaleax]

Is it necessary to nest locks?¹

I think so, yes, for the reason you mentioned. But I am not a hundred percent sure either. I have a hard time figuring out this code myself as well.

And if so, can you perhaps add a comment explaining why? It's currently kind of hard to eyeball whether this is correct or a deadlock waiting to happen.

Added a comment; I did check that the locks are never required in another order, so this shouldn’t be making anything worse…

[addaleax]

is the call to WriteFile() on line 136 sound? FlushPrivate() reads num_write_requests_ while holding the lock, then passes that value to WriteFile() after dropping the lock. It looks like you could observe duplicate request ids.

I can’t say for sure. /cc @nodejs/trace-events

[addaleax]

CI: https://ci.nodejs.org/job/node-test-pull-request/20352/

[addaleax]

Landed in 06da364