process: Change default --unhandled-rejections=warn-with-error-code

[dfabulich]

This is a semver-major change that resolves DEP0018.

In addition to changing the default value for --unhandled-rejections,
this PR changes the exit code from 1 to 66. The warning message has
been updated to mention the new exit code, which I selected in order
to make this problem easier to search for on the web.

Users can prevent this behavior by setting a non-default value for the
--unhandled-rejections flag (throw, strict, warn, or none) or by setting
a hook for 'unhandledRejection'. This change has no effect on users who
have already set the flag or installed an 'unhandledRejection' hook; it
will only impact users who currently see a DEP0018 warning today.

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• tests and/or benchmarks are included
• documentation is changed or added
• commit message follows commit guidelines

[dfabulich]

When I posted this PR, I hadn't yet seen @BridgeAR's comment #33021 (comment):

We just discussed these PRs in the TSC meeting and we try to move these forward fast. The issue about the deprecation notice was around for way to long that we did not act upon it so far.

The last agreement on our Collaborator Summit was to get a survey about this topic and I am going to write a blog post about it that I'll post in the promises debugging working group and the website working group so that it can be reviewed before being published.
As soon as that's done, we'll get a survey together to get a wider audience about the preferred future default value.

It is unlikely that any PR that tries to change the default before that is going to land before that point of time. I guess this PR might land as semver-minor in case it does not change the default anymore. The name could be changed to e.g., semi-strict in that case.

Well, the work is done, and I figure it's better to have the code out there while we wait for results.

[devsnek]

needs another 6

[dfabulich]

For the sake of discussion, I've resolved conflicts on this PR now that PR #33475 has merged. The TSC will need to decided whether to merge this PR or PR #33021. (I would prefer to merge 33021.)

[nylen]

issue a warning, and possibly set the exit code to non-zero when the process eventually happens to exit

That's what this PR does, right?

Why would anyone want this behavior, much less making it the default?

[dfabulich]

Most of the discussion around this PR is on the thread for PR #33021, which is my preferred solution.

tl;dr: Everybody would like to get rid of DEP0018, but there's disagreement about how to do that. Some Node TSC members think the default should be to throw an exception (and thereby crash, by default), and others think the default should be to just log a warning and delete the DEP0018 warning, saying it was a mistake to deprecate unhandled rejections.

(One TC39 member in the thread argued that the default shouldn't even log a warning on unhandled rejections, though he accepted logging a warning as a compromise.)

The option adopted here, to set an exit code while logging the warning, was raised as a compromise solution between the two main groups. It doesn't crash the process, which may satisfy those who feel that rejected promises are not exceptional, but it does indicate a problem via the exit code.

TSC plans to survey node users about this in a few weeks, and then vote on a solution.

[nylen]

but it does indicate a problem via the exit code

...at some point, when the process finally exits. This could be hours or even weeks later.

And that's if no other code sets process.exitCode to some other value in the meantime!

Frankly, this "compromise solution" is basically useless, and I will repeat what I said on the other PR here:

unhandled errors should abort the process. I think we (programmers in general) learned a long time ago that unhandled errors should abort, any other behavior is asking for trouble in complex systems.

Anyway thanks for making the PRs!

[mmarchini]

Closing as decision was to move forward with throw