monitor_reentrant_mutex.md

🕹 Reentrant Locks with Monitor

Ruby's built-in Mutex is not reentrant: a thread cannot lock the same mutex twice without deadlocking. Monitor (or MonitorMixin) solves this by allowing recursive locking within the same thread. This is useful when you have nested synchronized methods.

require 'monitor'

class ThreadSafeCounter
  include MonitorMixin

  def initialize
    super()  # initialize the MonitorMixin
    @count = 0
  end

  def increment
    synchronize do
      @count += 1
      nested_increment
    end
  end

  def nested_increment
    synchronize do
      # Reentrant lock allows us to enter again
      @count += 1
    end
  end

  def value
    synchronize { @count }
  end
end

counter = ThreadSafeCounter.new
threads = 10.times.map do
  Thread.new { 1000.times { counter.increment } }
end
threads.each(&:join)
puts counter.value  # => 20000