[ruby-core:109971] [Ruby master Feature#18411] Introduce `Fiber.blocking` for disabling scheduler.

["ko1 (Koichi Sasada)" <noreply@ruby-lang.org>]

Issue #18411 has been updated by ko1 (Koichi Sasada).


Let me confirm that why `write_nonblock` calls a fiber scheduler?
Or does it solve if `write_nonblock` doesn't call a fiber scheduler?

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Feature #18411: Introduce `Fiber.blocking` for disabling scheduler.
https://bugs.ruby-lang.org/issues/18411#change-99221

* Author: ioquatix (Samuel Williams)
* Status: Open
* Priority: Normal
----------------------------------------
When implementing pure-ruby IO scheduler, we may need to invoke some Ruby IO operations without entering the scheduler.

```ruby
def io_write(fiber, io, buffer, length)
  offset = 0
  
  while length > 0
    # From offset until the end:
    chunk = buffer.to_str(offset, length)
    case result = io.write_nonblock(chunk, exception: false)
    when :wait_readable
      self.io_wait(fiber, io, IO::READABLE)
    when :wait_writable
      self.io_wait(fiber, io, IO::WRITABLE)
    else
      offset += result
      length -= result
    end
  end
  
  return offset
end
```

There are some cases where even in this code `read_nonblock` can invoke fiber scheduler creating infinite recursion.

Therefore, I propose to introduce `Fiber.blocking{...}` which has almost identical implementation to `Fiber.new(blocking: true) {}.resume`.

In the above code, we change the line:

```
    case result = io.write_nonblock(chunk, exception: false)
```

to

```
    case result = Fiber.blocking{io.write_nonblock(chunk, exception: false)}
```

This ensures that `write_nonblock` can never enter the scheduler again.



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