Probably it's not a bug, but just a hidden behaviour. Please write your comments.
When a thread waits on ConditionVariable, it releases the mutex. And killing the thread during this state assumes no need to lock the mutex again because there's no ruby code to be executed. It results to problems:
the killed thread remains alive until the mutex is released by another thread
joining the killed thread from another thread locked on the mutex brings to deadlock
@m= Mutex.new
@r = ConditionVariable.new
t = Thread.new {
@m.synchronize do
@r.wait(@m)
end
}
sleep 0.01
@m.synchronize do
t.kill
sleep 0.01
puts "alive?: #{t.alive?}" # the thread should not be alive here
t.join # at this point we're getting dealock
end
I don't think this is a bug. Killing a thread requires running ensure blocks until the thread exits, the ensure block for ConditionVariable#wait needs to reacquire the mutex. If it didn't, the method would exit with the mutex not locked, which would cause a ThreadError: Attempt to unlock a mutex which is not locked in Mutex#synchronize. Even if we added code to both ConditionVariable#wait and Mutex#synchronize to recognize killed threads and try to avoid this issue, it wouldn't result in correct behavior for manual ensure blocks. Consider:
This issue is not specific to ConditionVariable, it can happen anytime a thread tries to lock a mutex in an ensure block. For example, replacing the definition of t in your example with the following code will also trigger it:
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