mutex.rb

# Mutexes provide a mechanism for multiply threads to synchronize access to a
# critical portion of code. It helps bring some order and some guaranty to the
# world of multi-threaded chaos.
# In this program, since any thread has to lock the mutex before it can push to
# the Array, there's a guarantee that no two threads will be performing this
# operation at the same time. In other words, this operation can no longer be
# interrupted before it's completed. Once one thread begins pushing to the
# Array, no other threads will be able to enter that portion of code until the
# first thread is finished. This operation is now thread-safe.

shared_array = []
# Notice that the mutex is shared among all the threads. The guarantee only
# works if the threads are sharing the same Mutex instance. In this way, when
# one thread locks a mutex, others have to wait for it to be unlocked.
mutex = Mutex.new

10.times.map do
  Thread.new do
    1000.times do
      # Thread lock mutex and become owner, could be one one owner
      # Now only one thread can run wrapper code and update
      mutex.lock
      shared_array << nil
      mutex.unlock
      # After unlock mutex other threads can lock mutex

      # Other convenience method to do same
      # mutex.synchronize { shared_array << nil }
    end
  end
end.each(&:join)

# Here you can see that we have 10 * 10000 elements in array

puts shared_array.size

# Now all are same, because of mutex/
# The mutex sets up same boundaries for the thread. The first thread that hits
# this bit of code will lock the mutex. it then becomes the owner of that mutex.
# Until the owning thread unlocks the mutex, no other thread can lock it.

# $ ruby  => 10000
# $ jruby => 10000
# $ rbx => 1000