[ruby-core:121182] [Ruby master Bug#21158] Ractor using 'receive_if' sits in busy wait when there are skipped messages in the incoming queue

["NeilW (Neil Wilson) via ruby-core" <ruby-core@ml.ruby-lang.org>]

Issue #21158 has been reported by NeilW (Neil Wilson).

----------------------------------------
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

* 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
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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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