Bug #21158: Ractor using 'receive_if' sits in busy wait when there are skipped messages in the incoming queue

• Status: Open
• Assignee: -
• Target version: -
• 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

[NeilW (Neil Wilson)]

When using the conditional receive example from the Ractor documentation (either as a script, or within irb)

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

r.send(1)
r.send('test')

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

[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.