Runs the given block synchronized with the argument given; i.e. for the same argument the block is never run in multiple threads simultaneously, while for different arguments the block can run in parallel in multiple threads.

synchronize_with_unique_argument.rb

# synchronizes the given block, but only when called with the same argument
def synchronize_with_unique(arg, mutex, map, &block)
  # get the lock for the supplied argument while increasing the refcount
  lock_info = mutex.synchronize do
     info = (map[arg] ||= { lock: Mutex.new, count: 0 })
     info[:count] += 1
     info
  end
  
  # do the actual locking
  begin
    lock_info[:lock].synchronize &block
  ensure
    # decrease the refcount and remove the lock from the hash if this was the last ref
    mutex.synchronize do
      map.delete(arg) if (lock_info[:count] -= 1) == 0
    end
  end
end