[ruby-core:118074] [Ruby master Bug#20413] Enumerator can block fiber scheduler.

Issue #20413 has been updated by k0kubun (Takashi Kokubun).

Backport changed from 3.0: UNKNOWN, 3.1: REQUIRED, 3.2: REQUIRED, 3.3: REQUIRED to 3.0: UNKNOWN, 3.1: REQUIRED, 3.2: REQUIRED, 3.3: DONE

ruby_3_3 commit:5688bcb54a640b353bed4ff49032ea00f947e1aa merged revision(s) commit:5d1702e01a36e11b183fe29ce10780a9b1a41cf0.

----------------------------------------
Bug #20413: Enumerator can block fiber scheduler.
https://bugs.ruby-lang.org/issues/20413#change-108501

* Author: ioquatix (Samuel Williams)
* Status: Closed
* Assignee: ioquatix (Samuel Williams)
* Backport: 3.0: UNKNOWN, 3.1: REQUIRED, 3.2: REQUIRED, 3.3: DONE
----------------------------------------
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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