[ruby-core:106708] [Ruby master Bug#18411] Introduce `Fiber.blocking` for disabling scheduler.

Issue #18411 has been updated by ioquatix (Samuel Williams).


@matz this isn't strictly necessary but makes pure Ruby implementation of IO read/write hooks more efficient. I'm not sure if there is valid case for general user code. Can we consider it for Ruby 3.1?

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Bug #18411: Introduce `Fiber.blocking` for disabling scheduler.
https://bugs.ruby-lang.org/issues/18411#change-95393

* Author: ioquatix (Samuel Williams)
* Status: Open
* Priority: Normal
* Backport: 2.6: UNKNOWN, 2.7: UNKNOWN, 3.0: UNKNOWN
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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`.



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