[ruby-core:118456] [Ruby master Bug#20413] Enumerator can block fiber scheduler.

6 Jul 2024

      Issue #20413 has been updated by nagachika (Tomoyuki Chikanaga).

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

Sorry, I accidentally handled a different ticket (#20414).

ruby_3_2 commit:2f8f17e842666abb05ca522d6072c957fab0e12e merged revision(s) commit:5d1702e01a36e11b183fe29ce10780a9b1a41cf0.

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Bug #20413: Enumerator can block fiber scheduler.
https://bugs.ruby-lang.org/issues/20413#change-108970

* Author: ioquatix (Samuel Williams)
* Status: Closed
* Assignee: ioquatix (Samuel Williams)
* Backport: 3.0: UNKNOWN, 3.1: REQUIRED, 3.2: DONE, 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 &lt;https://github.com/ruby/ruby/pull/10481> for the fix.

-- 
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