Cancelling does not cancel until next interval

[jar349]

Consider the following app:

require "timers"


class Application
  TEN_SECONDS = 10

  def initialize
    @should_continue = nil
    @timer_group = Timers::Group.new
  end

  def start
    puts "Starting on thread #{Thread.current.object_id}"
    @should_continue = true
    @timer_group.now_and_every(TEN_SECONDS) {
      puts "Running on thread #{Thread.current.object_id}"
    }

    while @should_continue
      puts "Waiting on thread #{Thread.current.object_id}"
      @timer_group.wait
    end
  end

  def stop
    puts "Stopping on thread #{Thread.current.object_id}"
    @should_continue = false
    @timer_group.cancel
  end
end


app = Application.new
Signal.trap("INT") { app.stop }
app.start

When I run this program and then press CTRL+C, I immediately see Stopping on thread xxxxxx but the program does not actually terminate until 10 seconds has elapsed.

$ bundle exec ruby app.rb
Starting on thread 70127555125280
Running on thread 70127555125280
Waiting on thread 70127555125280
Running on thread 70127555125280
Waiting on thread 70127555125280
^CStopping on thread 70127555125280
--- hangs here for 10 seconds ---
$

At first, I suspected that this was due to everything running on one thread but I ran a test where I called @timer_group.cancel on a separate thread and still the program did not terminate until the wait period elapsed. This will be a problem for me because my real program's interval will be 30 minutes.

Is there a way for me to have the call to cancel take effect right away?

[ioquatix]

Timers::Group is not thread safe so don't try calling methods from a different thread.

@timer_group.wait is like calling sleep x.

@timer_group.wait do |duration|
  sleep duration
end

is the same.

I made a test program:

require 'timers'

group = Timers::Group.new

group.now_and_every(5) do
	puts "Running on thread #{Thread.current.object_id}"
end

while true
	puts "Waiting on thread #{Thread.current.object_id}"
	group.wait
end

Signal.trap("INT") {puts "interrupted"}

It exits with Interrupt when I press Ctrl-C:

Running on thread 60
Waiting on thread 60
^CTraceback (most recent call last):
        2: from /tmp/d891193c-b95e-4af8-9fe4-2483db895759:12:in `<main>'
        1: from /home/samuel/.gem/ruby/2.7.1/gems/timers-4.3.0/lib/timers/group.rb:81:in `wait'
/home/samuel/.gem/ruby/2.7.1/gems/timers-4.3.0/lib/timers/group.rb:81:in `sleep': Interrupt
Exited with signal 2 after 2.4 seconds

[ioquatix]

Oh, sorry, I made a mistake, the interrupt handler was not registered.

[ioquatix]

Hmm, I tried this:

Signal.trap("INT") do
	puts "interrupted"
end

while true
	puts "Waiting on thread #{Thread.current.object_id}"
	duration = sleep(2)
	puts "duration: #{duration}"
end

But the interrupt does not cancel sleep. So this is probably not something we can solve with Timers alone. You need to raise an exception from your interrupt handler.

[jar349]

Thanks to your advice regarding the need to raise an exception, the following program now does what I want:

require "timers"


class Cancelled < Exception
end


class Application
  TEN_SECONDS = 10

  def initialize
    @should_continue = nil
    @timer_group = Timers::Group.new
  end

  def start
    @should_continue = true
    puts "Starting on thread #{Thread.current.object_id}"

    while @should_continue
      @timer_group.after(TEN_SECONDS) {
        puts "Running on thread #{Thread.current.object_id}"
      }
      begin
        puts "Waiting on thread #{Thread.current.object_id}"
        @timer_group.wait
      rescue Cancelled
        @should_continue = false
      rescue Exception => ex
        puts "Caught unhandled exception #{ex.message}"
      end
    end
  end

  def stop
    puts "Stopping on thread #{Thread.current.object_id}"
    raise Cancelled.new
  end
end


app = Application.new
Signal.trap("INT") { app.stop }
app.start

When running this program, sending CTRL+C immediately stops the program:

$ bundle exec ruby app.rb
Starting on thread 70358631907280
Waiting on thread 70358631907280
Running on thread 70358631907280
Waiting on thread 70358631907280
Running on thread 70358631907280
Waiting on thread 70358631907280
^CStopping on thread 70358631907280

[ioquatix]

In C, I believe sleep would return with some kind of errno if interrupted early. At least, that's what I'd check first, but I guess Ruby does not propagate this.