From: "nobu (Nobuyoshi Nakada)" <noreply@...>
Date: 2022-04-19T09:46:11+00:00
Subject: [ruby-core:108292] [Ruby master Bug#18649] Enumerable#first breaks out of the incorect block when across threads

Issue #18649 has been updated by nobu (Nobuyoshi Nakada).


jeremyevans0 (Jeremy Evans) wrote in #note-2:
> With the block uncommented, the yielding thread raises LocalJumpError, but the calling thread gets the yielded value without an exception.

Isn't it natural because the test code rescues but doesn't re-raise?

----------------------------------------
Bug #18649: Enumerable#first breaks out of the incorect block when across threads
https://bugs.ruby-lang.org/issues/18649#change-97313

* 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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