Untitled

class QueueWithTimeout
  def initialize
    @mutex = Mutex.new
    @queue = []
    @recieved = ConditionVariable.new
  end

  def <<(x)
    @mutex.synchronize do
      @queue << x
      @recieved.signal
      puts "Signalled"
    end
  end

  def pop(non_block = false)
    pop_with_timeout(non_block ? 0 : nil)
  end

  def pop_with_timeout(timeout = nil)
    printf "pop_with_timeout in #{Thread.current[:id]}\n"
    @mutex.synchronize do
      if @queue.empty?
        puts "Queue empty in #{Thread.current[:id]}"
        @recieved.wait(@mutex, timeout) if timeout != 0
        puts "Resuming in #{Thread.current[:id]}"
        #if we're still empty after the timeout, raise exception
        raise ThreadError, "queue empty" if @queue.empty?
      end
      @queue.shift
    end
  end
end

queue = QueueWithTimeout.new

Thread.abort_on_exception = true

10.times do |i|
  Thread.new do
    Thread.current[:id] = i
    loop do
      queue.pop_with_timeout(100000.0) # a long timeout, that we'll never reach
      puts i
    end
  end
end

loop do
  queue << "random stuff"
  puts "pushed"
end


__END__
# Example program output (comments on the right after the # char)

$ ruby condition_variable_signal.rb
pop_with_timeout in 2
Queue empty in 2
pop_with_timeout in 0
pop_with_timeout in 1
Signalled                # @recieved.signal called, note that other threads run before condition_variable is actually resumed on line 75
pop_with_timeout in 9
pop_with_timeout in 4
pop_with_timeout in 5
pop_with_timeout in 6
pop_with_timeout in 7
pop_with_timeout in 8
pop_with_timeout in 3
pushed
0                        # Thread 0 popped the value. Note that thread 0 never called @recieved.wait(...)!
pop_with_timeout in 0
Resuming in 2            # Thread 2 wakes up and finds empty queue.
Queue empty in 1
Queue empty in 9
Queue empty in 4
condition_variable_signal.rb:28:in `block in pop_with_timeout': queue empty (ThreadError)
        from condition_variable_signal.rb:22:in `synchronize'
        from condition_variable_signal.rb:22:in `pop_with_timeout'
        from condition_variable_signal.rb:43:in `block (3 levels) in <main>'
        from condition_variable_signal.rb:42:in `loop'
        from condition_variable_signal.rb:42:in `block (2 levels) in <main>'


Conclusions

- I couldn't reproduce "spurious wakeup" behavior.
- I was able to find/reproduce a bug related to interesting `ConditionVariable#signal` behavior, see program output with comments above.
- I do agree using `while` loop with a condition variable predicate solves the problem.