From: "Eregon (Benoit Daloze)" <noreply@...>
Date: 2022-03-18T16:20:56+00:00
Subject: [ruby-core:107980] [Ruby master Bug#18649] Enumerable#first breaks out of the incorect block when across threads

Issue #18649 has been reported by Eregon (Benoit Daloze).

----------------------------------------
Bug #18649: Enumerable#first breaks out of the incorect block when across threads
https://bugs.ruby-lang.org/issues/18649

* Author: Eregon (Benoit Daloze)
* Status: Open
* Priority: Normal
* ruby -v: ruby 3.1.1p18 (2022-02-18 revision 53f5fc4236) [x86_64-linux]
* Backport: 2.6: UNKNOWN, 2.7: UNKNOWN, 3.0: UNKNOWN, 3.1: UNKNOWN
----------------------------------------
```ruby
def synchronize
  yield
end

def execute(task)
  success = true
  value = reason = nil
  end_sync = false

  synchronize do
    begin
      p :before
      value = task.call
      p :never_reached
      success = true
    rescue StandardError => ex
      p [:rescue, ex]
      reason = ex
      success = false
    end

    end_sync = true
    p :end_sync
  end

  p :should_not_reach_here! unless end_sync
  [success, value, reason]
end

def foo
  Thread.new do
    result = execute(-> { yield 42 })
    p [:result, result]
  end.join
end

p [:first, to_enum(:foo).first]
```

This code should raise LocalJumpError (and that should get `rescue`'d) because Enumerable#first can't break/return across threads.
But instead, it seems to break out of the block given to `synchronize`, which is clearly wrong.
That case is shown as `:should_not_reach_here!`.

Results:
```
ruby 3.0.3p157 (2021-11-24 revision 3fb7d2cadc) [x86_64-linux]
:before
:should_not_reach_here!
[:result, [true, nil, nil]]
[:first, 42]

ruby 3.1.1p18 (2022-02-18 revision 53f5fc4236) [x86_64-linux]
:before
:should_not_reach_here!
[:result, [true, nil, nil]]
[:first, 42]
```

CRuby (3.0 and 3.1) print `:should_not_reach_here!`, which is a semantic bug, if we get to the end of `execute` we should have gotten to the end of the block given to synchronize.

This is related to #18474 and https://github.com/ruby-concurrency/concurrent-ruby/issues/931.



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