Looking for a good way to break out of Journal#watch

[kiyoto]

I am currently writing a plugin for Fluentd using this gem. The issue that I ran into is, I couldn't find a good way to break out of Journal#watch. In other words:

def start
  @journal = Systemd::Journal.new
  @journal.watch do |entry|
    # do stuff ehre
  end
end

and this "start" method is run in a different event loop (using Cool.io). What I want to do is, in the corresponding "shutdown" method, stop the watch loop like this (shutdown is invoked when there is a INT/KILL signal.

def shutdown
  # somehow stop @journal.watch
end

Is this something possible? If there is a better way to go about it, pointers is hugely appreciated.

[ledbettj]

Sorry for the late response.

Yes, watch does not return and in particular uses a systemd journal API that blocks inside C code, and so cannot be interrupted from ruby land. In particular watch is implemented something like this:

loop do
    if journal.wait # something happened - wake up!
      yield journal.current_entry while journal.move_next
    end
end

You can do something similar and pass a timeout value to wait so that it will wake up even if nothing changed every so often, so you can check if you need to exit (untested example below):

while should_i_keep_running?
    if journal.wait(3_000_000) # wake up at least every 3 seconds
      yield journal.current_entry while journal.move_next
    end
end

Additionally, if you need to handle signals interrupting wait, you can pass select: true as a second parameter. This uses IO.select to wait for things to change from the ruby side, so it can be interrupted, unlike normal wait. Note that this won't work on JRuby.

[kiyoto]

Thanks! I will examine this further. For now, consider this to be closed.