weird behaviour from uncaughtExceptionMonitor for unhandled rejections

[mlarcher]

• Version: v14.2.0
• Platform: Linux f8ae88c27512 4.19.76-linuxkit deps: update openssl to 1.0.1j #1 SMP Tue May 26 11:42:35 UTC 2020 x86_64 GNU/Linux

What steps will reproduce the bug?

process.on('uncaughtExceptionMonitor', (err, origin) => {
  console.log('uncaughtExceptionMonitor handler', origin);
});

async function uncaughtPromise() {
  throw new Error('yo');
}

uncaughtPromise();

What is the expected behavior?

I should see the uncaughtExceptionMonitor handler

What do you see instead?

(node:39) UnhandledPromiseRejectionWarning: Error: yo
    at uncaughtPromise (/data/tmp.js:6:9)
    at Object.<anonymous> (/data/tmp.js:9:1)
    at Module._compile (internal/modules/cjs/loader.js:1176:30)
    at Object.Module._extensions..js (internal/modules/cjs/loader.js:1196:10)
    at Module.load (internal/modules/cjs/loader.js:1040:32)
    at Function.Module._load (internal/modules/cjs/loader.js:929:14)
    at Function.executeUserEntryPoint [as runMain] (internal/modules/run_main.js:71:12)
    at internal/main/run_main_module.js:17:47
(Use `node --trace-warnings ...` to show where the warning was created)
(node:39) UnhandledPromiseRejectionWarning: Unhandled promise rejection. This error originated either by throwing inside of an async function without a catch block, or by rejecting a promise which was not handled with .catch(). To terminate the node process on unhandled promise rejection, use the CLI flag `--unhandled-rejections=strict` (see https://nodejs.org/api/cli.html#cli_unhandled_rejections_mode). (rejection id: 1)
(node:39) [DEP0018] DeprecationWarning: Unhandled promise rejections are deprecated. In the future, promise rejections that are not handled will terminate the Node.js process with a non-zero exit code.

Additional information

If I add a handler for unhandledRejection that just re-throws the error like so:

process.on('unhandledRejection', (err) => {
  console.log('unhandledRejection handler‘)
  throw err;
});

process.on('uncaughtExceptionMonitor', (err, origin) => {
  console.log('uncaughtExceptionMonitor handler', origin);
});

async function uncaughtPromise() {
  throw new Error('yo');
}

uncaughtPromise();

then the uncaughtExceptionMonitor is reached :

unhandledRejection handler
uncaughtExceptionMonitor handler uncaughtException
/data/tmp.js:3
  throw err;
  ^

Error: yo
    at uncaughtPromise (/data/tmp.js:11:9)
    at Object.<anonymous> (/data/tmp.js:14:1)
    at Module._compile (internal/modules/cjs/loader.js:1176:30)
    at Object.Module._extensions..js (internal/modules/cjs/loader.js:1196:10)
    at Module.load (internal/modules/cjs/loader.js:1040:32)
    at Function.Module._load (internal/modules/cjs/loader.js:929:14)
    at Function.executeUserEntryPoint [as runMain] (internal/modules/run_main.js:71:12)
    at internal/main/run_main_module.js:17:47

(but that is now an uncaughtException and not an unhandledRejection anymore, when I come to think about it !)

Also, an exception not related to unhandled promise works as expected, i.e.:

process.on('uncaughtExceptionMonitor', (err, origin) => {
  console.log('uncaughtExceptionMonitor handler', origin);
});

unknownFunction();

gives me the expected

uncaughtExceptionMonitor handler uncaughtException
/data/tmp.js:5
unknownFunction();
^

ReferenceError: unknownFunction is not defined
    at Object.<anonymous> (/data/tmp.js:5:1)
    at Module._compile (internal/modules/cjs/loader.js:1176:30)
    at Object.Module._extensions..js (internal/modules/cjs/loader.js:1196:10)
    at Module.load (internal/modules/cjs/loader.js:1040:32)
    at Function.Module._load (internal/modules/cjs/loader.js:929:14)
    at Function.executeUserEntryPoint [as runMain] (internal/modules/run_main.js:71:12)
    at internal/main/run_main_module.js:17:47

So basically it seems to me that uncaughtExceptionMonitor doesn't work for unhandledRejection 🤷

[BridgeAR]

This seems to work as intended. The reason is that there's a difference between a promise rejection and an uncaught exception. The monitor only works for exceptions but not for rejections.

[mlarcher]

What is the point of the origin value then ? Docs states that "origin Indicates if the exception originates from an unhandled rejection or from synchronous errors. Can either be 'uncaughtException' or 'unhandledRejection'."
cf https://nodejs.org/api/process.html#process_event_uncaughtexceptionmonitor

[BridgeAR]

Seems like the documentation should be updated for the exception monitor. It should be identical to the documentation here https://nodejs.org/api/process.html#process_event_uncaughtexception. A PR is welcome :-)

[mlarcher]

So basically the handler will work as I expect only if I run node with a --unhandled-rejections flag set to strict... That makes sense and I did confirm it locally.
Thanks for your quick reply, I'll try to make a PR for the doc update.

[mlarcher]

Just found out that the issue is basically the same as #32907