[ruby-core:121213] [Ruby master Bug#21158] Ractor using 'receive_if' sits in busy wait when there are skipped messages in the incoming queue

["luke-gru (Luke Gruber) via ruby-core" <ruby-core@ml.ruby-lang.org>]

Issue #21158 has been updated by luke-gru (Luke Gruber).


Thank you for the report, I'm able to reproduce it.

This is happening because if the proc doesn't match (`receive_if` fails), the basket value is changed from `reserved` to `ref`
so it never goes back to sleep (`ractor_queue_empty_p` always return false and we don't get into the `ractor_sleep` function.)

I'll take a closer look and try to get a patch out soon.


----------------------------------------
Bug #21158: Ractor using 'receive_if' sits in busy wait when there are skipped messages in the incoming queue 
https://bugs.ruby-lang.org/issues/21158#change-112154

* Author: NeilW (Neil Wilson)
* Status: Open
* ruby -v: ruby 3.4.2 (2025-02-15 revision d2930f8e7a) +PRISM [x86_64-linux]
* Backport: 3.1: UNKNOWN, 3.2: UNKNOWN, 3.3: UNKNOWN, 3.4: UNKNOWN
----------------------------------------
When using the conditional receive example from the Ractor documentation (either as a script, or within irb)

<pre>
r = Ractor.new do
  val = Ractor.receive_if{|msg| msg.is_a?(Array)}
  puts "Received successfully: #{val}"
end

r.send(1)
r.send('test')
</pre>

ruby will sit in busy wait, pegging the processor at 100%, until it gets a message that matches the condition. Within irb it will often stop the REPL responding.

The same problem occurs on ruby 3.02, 3.3.7 and 3.4.1




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