Throw within queueMicrotask callbacks should not crash Node

[Huxpro]

• Version:v15.9.0
• Platform: macOS x86
• Subsystem: task_queues

What steps will reproduce the bug?

λ node --trace-uncaught
Welcome to Node.js v15.9.0.
Type ".help" for more information.
> queueMicrotask(_ => { throw 1 });
undefined
>
node:internal/process/task_queues:94
    runMicrotasks();
    ^
1
Thrown at:
    at processTicksAndRejections (node:internal/process/task_queues:94:5)

How often does it reproduce? Is there a required condition?

Always.

What is the expected behavior?

If I understood the spec correct:

The queueMicrotask(callback) method must queue a microtask to invoke callback, and if callback throws an exception, report the exception.

It should behave similarly as Timer tasks.

> setTimeout(_ => {throw 1});
Timeout {
  ...
}
> Uncaught 1

What do you see instead?

Node crashed

Additional information

I did some preliminary triages that hopefully helps a bit:

• task_queues.js invoke runMicrotask
• which is a JS binding from node_task_queue.cc which invokes V8's PerformCheckpoint under the hood.
• V8's PerformCheckpoint invoked V8's RunMicrotasks to actually evaluate those JS callbacks.

But since V8's PerformCheckpoint returns void. I'm not sure how can Node be aware of any exceptions threw during the evaluation of those JS callbacks. Chromium seems to handle this fine but I haven't got time looking at its source.

[devsnek]

The exception is reported via the uncaughtException event. If nothing handles that, the exception is fatal, like all other uncaught exceptions.

[Huxpro]

@devsnek ah that make sense.

Out of line question: there isn't an equivalent API on the Web that can do what process.on('uncaughtException', ..) on Node, right?

[legendecas]

@Huxpro: there isn't an equivalent API on the Web that can do what process.on('uncaughtException', ..) on Node, right?

If you mean to monitor those uncaught exceptions, try window.addEventListener('error', ...) (and unhandledrejection for unhandledRejection in Node.js respectively).

[Huxpro]

@devsnek although it's spec-conformant in that sense. Why wouldn't queueMicrotask behave the same as setTimeout?

[targos]

I'm reopening because it seems worth fixing (if possible) in the REPL.

[Linkgoron]

I'm reopening because it seems worth fixing (if possible) in the REPL.

This actually works in the REPL of Node 12 and 14.4 but stopped working in Node 14.5. I think that the issue is that errors thrown in queueMicroTask don't get captured by the domain, and don't reach the domain error handler (in the repl). The behaviour change was probably caused by this change #33859

[EladKeyshawn]

@Linkgoron Does it still need fixing?

[BridgeAR]

@EladKeyshawn yes, it does.

The question is how something like that might be solved. One way would be to use child processes to execute the code and to detect fatal exceptions leading to a new child process. However, that would only recover the REPL but all former input would be lost. Thus, we would have to print a clear warning about what happened. This is the best solution I can think of and it would also resolve multiple other reports.