[ruby-core:122591] [Ruby Bug#19473] can't be called from trap context (ThreadError) is too limiting

Issue #19473 has been updated by ioquatix (Samuel Williams).


I agree, it would be nice to relax this restriction. But I also understand that it's unpredictable since the trap handler can run at any point the Ruby interpreter checks for interrupts (which for user code is fairly frequent)

As an alternative (to relaxing the restriction), we could expose the existing logic/flag, e.g. `Mutex#safe_in_trap_context = true/false`. It's a bit ugly, but it's one idea that might allow us to move forward while retaining the default existing behaviour.

> It's extremely hard to reason about a signal handler running on top of any line of code of the main thread.

In Async, we handle this using `Thread.handle_interrupt` to defer interrupts until a specific known point in the event loop execution: <https://github.com/socketry/async/blob/4206e32da4e04b65a4074080f9a59d66b827a347/lib/async/scheduler.rb#L520-L540>. This helps to make interrupt handling more predictable. This makes asynchronous interrupts a little bit safer, IMHO.

> There seems to be no valid reason to prevent all `Mutex#synchronize` in trap.

`pthread_mutex_` functions are not async-signal-safe. However, Ruby does not run code directly in the signal handler, so it's not the same risk - but I feel like in principle it's still the same semantic issue. So, it's not entirely clear to me that it's safe to use any kind of blocking operation in principle, in trap handlers.

----------------------------------------
Bug #19473: can't be called from trap context (ThreadError) is too limiting
https://bugs.ruby-lang.org/issues/19473#change-113826

* Author: Eregon (Benoit Daloze)
* Status: Open
* 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.
But that would otherwise already raise `deadlock; recursive locking (ThreadError)`, so there is no point to fail early.
And that's just one case, not all, so we should not always raise an exception.

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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