Method: Futex#open

Defined in:
lib/futex.rb

#open(exclusive = true) ⇒ Object

Open the file. By default the file will be locked for exclusive access, which means that absolutely no other process will be able to do the same. This type of access (exclusive) is supposed to be used when you are making changes to the file. However, very often you may need just to read it and it’s OK to let many processes do the reading at the same time, provided none of them do the writing. In that case you should call this method open() with false first argument, which will mean “shared” access. Many threads and processes may have shared access to the same lock file, but they all will stop and wait if one of them will require an “exclusive” access. This mechanism is inherited from POSIX, read about it <a href=“man7.org/linux/man-pages/man2/flock.2.html”>here</a>.



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# File 'lib/futex.rb', line 95

def open(exclusive = true)
  FileUtils.mkdir_p(File.dirname(@lock))
  step = (1 / @sleep).to_i
  start = Time.now
  prefix = exclusive ? '' : 'non-'
  b = badge(exclusive)
  Thread.current.thread_variable_set(:futex_lock, @lock)
  Thread.current.thread_variable_set(:futex_badge, b)
  open_synchronized(@lock) do |f|
    cycle = 0
    loop do
      if f.flock((exclusive ? File::LOCK_EX : File::LOCK_SH) | File::LOCK_NB)
        Thread.current.thread_variable_set(:futex_cycle, nil)
        Thread.current.thread_variable_set(:futex_time, nil)
        break
      end
      sleep(@sleep)
      cycle += 1
      Thread.current.thread_variable_set(:futex_cycle, cycle)
      Thread.current.thread_variable_set(:futex_time, Time.now - start)
      if Time.now - start > @timeout
        raise CantLock.new("#{b} can't get #{prefix}exclusive access \
to the file #{@path} because of the lock at #{@lock}, after #{age(start)} \
of waiting: #{IO.read(@lock)} (modified #{age(File.mtime(@lock))} ago)",
        File.mtime(@lock))
      end
      next unless (cycle % step).zero? && Time.now - start > @timeout / 2
      debug("#{b} still waiting for #{prefix}exclusive \
access to #{@path}, #{age(start)} already: #{IO.read(@lock)} \
(modified #{age(File.mtime(@lock))} ago)")
    end
    debug("Locked by #{b} in #{age(start)}, #{prefix}exclusive: \
#{@path} (attempt no.#{cycle})")
    IO.write(@lock, b)
    acq = Time.now
    res = block_given? ? yield(@path) : nil
    debug("Unlocked by #{b} in #{age(acq)}, #{prefix}exclusive: #{@path}")
    res
  end
ensure
  Thread.current.thread_variable_set(:futex_cycle, nil)
  Thread.current.thread_variable_set(:futex_time, nil)
  Thread.current.thread_variable_set(:futex_lock, nil)
  Thread.current.thread_variable_set(:futex_badge, nil)
end