[ruby-core:117456] [Ruby master Bug#20413] Enumerator can block fiber scheduler.

["ioquatix (Samuel Williams) via ruby-core" <ruby-core@ml.ruby-lang.org>]

Issue #20413 has been updated by ioquatix (Samuel Williams).


After discussing it with Eregon on Slack, he withdrew his objection to this change.

In general, this change:

- Aligns `Fiber.new` with `rb_fiber_new` so that they both produce non-blocking fibers.
- Doesn't affect existing code, as there is no obvious usage of `rb_fiber_new` by GitHub code search (small/zero blast radius).
- Only impacts usage within the fiber scheduler, i.e. no effect outside of fiber scheduler beside the predicate value itself.
- Even within the fiber scheduler, it is transparent to user code.

----------------------------------------
Bug #20413: Enumerator can block fiber scheduler.
https://bugs.ruby-lang.org/issues/20413#change-107841

* Author: ioquatix (Samuel Williams)
* Status: Closed
* Assignee: ioquatix (Samuel Williams)
* Backport: 3.0: UNKNOWN, 3.1: REQUIRED, 3.2: REQUIRED, 3.3: REQUIRED
----------------------------------------
Using `Enumerator` in the event loop can cause problems as the fiber created by `rb_fiber_new` is blocking by default. It should be non-blocking.

```ruby
#!/usr/bin/env ruby

require 'async'

Async do
  Async do
    while true
      puts "Hello"
      sleep 1
    end
  end
  
  enumerator = Enumerator.new do |yielder|
    while true
      yielder << "World"
      sleep 1
    end
  end
  
  while true
    puts enumerator.next
  end
end
```

Before this PR, the output is:

```
> ./test.rb
Hello
World
World
World
World
World
World
...
```

After this PR, the output is:

```
> ./test.rb
Hello
World
Hello
World
Hello
World
Hello
World
Hello
World
...
```

The reason why this happens, is because the enumerator `sleep` never yields to the event loop.

See <https://github.com/ruby/ruby/pull/10481> for the fix.



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