class Pry::InputLock

def self.for(input)
  # XXX This method leaks memory, as we never unregister an input once we
  # are done with it. Fortunately, the leak is tiny (or so we hope).  In
  # usual scenarios, we would leak the StringIO that is passed to be
  # evaluated from the command line.
  global_lock.synchronize do
    input_locks[input] ||= Pry::InputLock.new
  end
end

def __with_ownership
  @mutex.synchronize do
    # Three cases:
    # 1) There are no owners, in this case we are good to go.
    # 2) The current owner of the input is not reading the input (it might
    #    just be evaluating some ruby that the user typed).
    #    The current owner will figure out that it cannot go back to reading
    #    the input since we are adding ourselves to the @owners list, which
    #    in turns makes us the current owner.
    # 3) The owner of the input is in the interruptible region, reading from
    #    the input. It's safe to send an Interrupt exception to interrupt
    #    the owner. It will then proceed like in case 2).
    #    We wait until the owner sets the interruptible flag back
    #    to false, meaning that he's out of the interruptible region.
    #    Note that the owner may receive multiple interrupts since, but that
    #    should be okay (and trying to avoid it is futile anyway).
    while @interruptible
      @owners.last.raise Interrupt
      @cond.wait(@mutex)
    end
    @owners << Thread.current
  end
  yield
ensure
  @mutex.synchronize do
    # We are releasing any desire to have the input ownership by removing
    # ourselves from the list.
    @owners.delete(Thread.current)
    # We need to wake up the thread at the end of the @owners list, but
    # sadly Ruby doesn't allow us to choose which one we wake up, so we wake
    # them all up.
    @cond.broadcast
  end
end

def enter_interruptible_region
  @mutex.synchronize do
    # We patiently wait until we are the owner. This may happen as another
    # thread calls with_ownership() because of a binding.pry happening in
    # another thread.
    @cond.wait(@mutex) until @owners.last == Thread.current
    # We are the legitimate owner of the input. We mark ourselves as
    # interruptible, so other threads can send us an Interrupt exception
    # while we are blocking from reading the input.
    @interruptible = true
  end
end

def initialize
  @mutex = Mutex.new
  @cond = ConditionVariable.new
  @owners = []
  @interruptible = false
end

def interruptible_region
  enter_interruptible_region
  # XXX Note that there is a chance that we get the interrupt right after
  # the readline call succeeded, but we'll never know, and we will retry the
  # call, discarding that piece of input.
  yield
rescue Interrupt
  # We were asked to back off. The one requesting the interrupt will be
  # waiting on the conditional for the interruptible flag to change to false.
  # Note that there can be some inefficiency, as we could immediately
  # succeed in enter_interruptible_region(), even before the one requesting
  # the ownership has the chance to register itself as an owner.
  # To mitigate the issue, we sleep a little bit.
  leave_interruptible_region
  sleep 0.01
  retry
ensure
  leave_interruptible_region
end

def leave_interruptible_region
  @mutex.synchronize do
    # We check if we are still the owner, because we could have received an
    # Interrupt right after the following @cond.broadcast, making us retry.
    @interruptible = false if @owners.last == Thread.current
    @cond.broadcast
  end
rescue Interrupt
  # We need to guard against a spurious interrupt delivered while we are
  # trying to acquire the lock (the rescue block is no longer in our scope).
  retry
end

def with_ownership(&block)
  # If we are in a nested with_ownership() call (nested pry context), we do nothing.
  nested = @mutex.synchronize { @owners.include?(Thread.current) }
  nested ? yield : __with_ownership(&block)
end