Thread didn't execute? #6142
Description
koyoko% jruby -v
jruby 9.2.9.0 (2.5.7) 2019-10-30 458ad3e OpenJDK 64-Bit Server VM 25.242-b08 on 1.8.0_242-b08 +jit [linux-x86_64]
koyoko% uname -a
Linux koyoko.local 5.5.9-arch1-2 #1 SMP PREEMPT Thu, 12 Mar 2020 23:01:33 +0000 x86_64 GNU/Linux
In my live stream, talking about thread safety no less, I encountered some kind of issue in JRuby when using threads in code that should have been fine.
https://youtu.be/Wu9LRNOc5pQ?t=630
It appears that while we did construct a thread, it never actually executed any code.
Here is the code from the live stream above:
#!/usr/bin/env ruby
def test(repeats = 1000)
mutex = Mutex.new
mutexes = {}
threads = []
threads = repeats.times.map do |i|
Thread.new do
mutex.synchronize do
mutexes[i] ||= Mutex.new
end
end
end
threads.each(&:join)
if threads.size != mutexes.size
puts "There were #{threads.size} threads and #{mutexes.size} mutexes."
repeats.times do |i|
unless mutexes.include?(i)
puts "Missing #{i}!"
end
end
end
missing = repeats.times.to_a - mutexes.keys
unless missing.empty?
puts "Missing: #{missing.inspect}"
end
end
100.times{test}I have been discussing the issue with @headius and he suggested it might be a problem with the native thread being GCed before starting:
public void start() {
Thread thread = getThread();
if (thread != null) thread.start();
else throw new RuntimeException("WTF");
}and he was able to get it to fail by increasing GC frequency.
[] ~/projects/jruby $ jruby thread_mutexes.rb
Unhandled Java exception: java.lang.RuntimeException: WTF
However I am not able to reproduce the issue, it seems very infrequent.