[ruby-core:112667] [Ruby master Bug#19473] can't be called from trap context (ThreadError) is too limiting

Issue #19473 has been updated by Eregon (Benoit Daloze).


Another approach would be to do it like e.g. Java which runs signal handlers on a separate thread.
Then there is no potential problem of the signal handler being reentrant to the main thread/fiber.
But that is a much bigger semantic change of course, so I think just removing the `can't be called from trap context (ThreadError)` is much more actionable.

----------------------------------------
Bug #19473: can't be called from trap context (ThreadError) is too limiting
https://bugs.ruby-lang.org/issues/19473#change-102116

* Author: Eregon (Benoit Daloze)
* Status: Open
* Priority: Normal
* ruby -v: ruby 3.2.1 (2023-02-08 revision 31819e82c8) [x86_64-linux]
* Backport: 2.7: UNKNOWN, 3.0: UNKNOWN, 3.1: UNKNOWN, 3.2: UNKNOWN
----------------------------------------
Simple reproducer:
```
$ ruby -ve 'm=Mutex.new; trap(:HUP) { m.synchronize { p :OK } }; Process.kill :HUP, Process.pid; sleep 0.1'
ruby 3.2.1 (2023-02-08 revision 31819e82c8) [x86_64-linux]
-e:1:in `synchronize': can't be called from trap context (ThreadError)
	from -e:1:in `block in <main>'
	from -e:1:in `kill'
	from -e:1:in `<main>'
```

Expected behavior:
```
$ ruby -ve 'm=Mutex.new; trap(:HUP) { m.synchronize { p :OK } }; Process.kill :HUP, Process.pid; sleep 0.1'
truffleruby 22.3.1, like ruby 3.0.3, GraalVM CE Native [x86_64-linux]
:OK

$ ruby -ve 'm=Mutex.new; trap(:HUP) { m.synchronize { p :OK } }; Process.kill :HUP, Process.pid; sleep 0.1'
jruby 9.4.0.0 (3.1.0) 2022-11-23 95c0ec159f OpenJDK 64-Bit Server VM 17.0.6+10 on 17.0.6+10 +jit [x86_64-linux]
:OK
```

This exception is highly problematic, for instance it breaks `Timeout.timeout` in `trap`:
https://github.com/ruby/timeout/issues/17#issuecomment-1142035939

I suppose this behavior is because *sometimes* it's problematic to lock a Mutex in trap, e.g., if it's already locked by the main thread/fiber.
However, that's just one case, not all, so we should not raise an exception early like that.

There seems to be no valid reason to prevent *all* `Mutex#synchronize` in `trap`.
After all, if the Mutex for instance is only used in `trap`, it's well-defined AFAIK.
For instance a given trap handler does not seem executed concurrently:
```
$ ruby -ve 'trap(:HUP) { puts "in trap\n"+caller.join("\n")+"\n\n"; sleep 0.1 }; pid = Process.pid; Process.wait fork { 20.times { Process.kill :HUP, pid } }; sleep 1'
ruby 3.2.1 (2023-02-08 revision 31819e82c8) [x86_64-linux]
in trap
-e:1:in `wait'
-e:1:in `<main>'

in trap
-e:1:in `wait'
-e:1:in `<main>'

in trap
-e:1:in `wait'
-e:1:in `<main>'

in trap
-e:1:in `wait'
-e:1:in `<main>'

in trap
-e:1:in `wait'
-e:1:in `<main>'

in trap
-e:1:in `wait'
-e:1:in `<main>'

```

And if the trap handler using the Mutex is never called while the Mutex is held by the main thread/fiber, there is also no problem.



-- 
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