[ruby-core:84214] [Ruby trunk Feature#14143] Thread.report_on_exception should be true by default

Issue #14143 has been updated by Eregon (Benoit Daloze).

Assignee changed from matz (Yukihiro Matsumoto) to Eregon (Benoit Daloze)

We discussed with @ko1 and @usa to add a hint to the warning produced by Thread.report_on_exception.
I am thinking to go with @usa's suggestion. The hint is between "()":

```
#<Thread:[e-mail]:3 run> terminated with exception (report_on_exception is true):
Traceback (most recent call last):
	4: from report.rb:4:in `block in <main>'
	3: from report.rb:4:in `times'
	2: from report.rb:5:in `block (2 levels) in <main>'
	1: from report.rb:5:in `times'
report.rb:6:in `block (3 levels) in <main>': unhandled exception
```

Then I think we can discuss possible ways to fix this warning (fix a bug, add rescue, set report to false) in more details
in the documentation of Thread.report_on_exception.

----------------------------------------
Feature #14143: Thread.report_on_exception should be true by default
https://bugs.ruby-lang.org/issues/14143#change-68337

* Author: Eregon (Benoit Daloze)
* Status: Open
* Priority: Normal
* Assignee: Eregon (Benoit Daloze)
* Target version: 2.5
----------------------------------------
Extracted from #6647 to focus on the default value now that the feature is implemented.

I strongly believe we should have Thread.report_on_exception = true by default.

It only adds some extra stderr output for apps which let threads die, which is very rarely intended.
If it is intended, then one can use Thread.current.report_on_exception = false
to clarify it's OK for that thread to die and the failure is handled by the app on Thread#join.

I enabled Thread.report_on_exception=true by default in ruby/spec, see https://github.com/ruby/spec/pull/517,
the only cases needing Thread.current.report_on_exception=false
are the specs testing report_on_exception itself and Thread#join/value/status/raise.

Enabling it for test-all shows a fair amount of extra output and failures, which I would bet some of them are bugs in the tests (I already found one, r60854 & r60870),
and other tests should simply more carefully test what they expect
(for instance assert_raise() inside the Thread just around the code raising an exception and join the Thread).

I am willing to help to reduce the extra output and failures in test-all,
but I would like a OK from Matz to try enabling Thread.report_on_exception by default.

Dear Matz, do you think it is reasonable to show exceptions killing threads on stderr by default,
instead of silently swallowing them until Thread#join ?
(if there is ever a Thread#join ..., often not or too late, when the rest of the application has crashed)



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