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Bug #17300
closedThe Fiber scheduler does not work with ConditionVariable
Description
When looking at replacing kernel_sleep
by blocking
, I found an independent bug.
ConditionVariable does not seem to work with the Fiber scheduler currently.
There is an existing test in https://github.com/ruby/ruby/blob/4f8d9b0db84c42c8d37f75de885de1c0a5cb542c/test/fiber/test_mutex.rb#L105-L140 on which I based this reproduction example.
The test should always have signalled==3, but the check is only > 1.
The test is also racy, as ConditionVariable#signal has no effect if no other Thread/Fiber is in ConditionVariable#wait.
Here is the reproduction, by default it runs without the scheduler. Pass it scheduler
as an argument to use the test Scheduler.
I save the script under test/fiber
for convenience.
require_relative 'scheduler'
USE_SCHEDULER = ARGV.delete('scheduler')
mutex = Mutex.new
condition = ConditionVariable.new
signalled = 0
q = Queue.new
a = Thread.new do
Thread.current.scheduler = Scheduler.new if USE_SCHEDULER
body = -> do
mutex.synchronize do
3.times do |i|
q << :ready
p [:wait, i]
condition.wait(mutex)
raise unless mutex.owned?
signalled += 1
end
end
end
USE_SCHEDULER ? Fiber.schedule(&body) : body.call
end
b = Thread.new do
Thread.current.scheduler = Scheduler.new if USE_SCHEDULER
body = -> do
puts "Thread 2 starting"
3.times do |i|
q.pop # Only acquire Mutex once the other thread is in wait
puts "Thread 2 locking Mutex"
mutex.synchronize do
p [:signal, i]
condition.signal
end
sleep 1 # 0.1
end
end
USE_SCHEDULER ? Fiber.schedule(&body) : body.call
end
a.join
b.join
p signalled
$ ruby condvar2.rb
Thread 2 starting
[:wait, 0]
Thread 2 locking Mutex
[:signal, 0]
[:wait, 1]
Thread 2 locking Mutex
[:signal, 1]
[:wait, 2]
Thread 2 locking Mutex
[:signal, 2]
3
ruby condvar2.rb scheduler
Thread 2 starting
Thread 2 locking Mutex
[:wait, 0]
[:signal, 0]
# hangs
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