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Signals in Ruby / "Rescue Exception" Considered Harmful

Yesterday we had an issue with the different behaviour of “kill “ and “kill -9 “ and in the process I had to refresh my knowledge of Unix signals, learn how you handle them in Ruby and properly learn Rubys exception hierarchy.

To -9 or not to -9?

The unix kill command is perhaps strangely named as it actually sends signals to processes (see “man signal” for a full list). It defaults to sending SIGTERM to the process and the application writer can decide how to treat it by “trapping” it, allowing for a safe shutdown or debug dumps etc. “kill -9” sends SIGKILL and the signals SIGKILL and SIGSTOP cannot be caught, blocked, or ignored by your programs. I think in the first instance you should just use “kill”, give the app the chance to do the right thing then get -9 on its ass if you need to.

Catching signals in Ruby

puts "I have PID #{Process.pid}"

Signal.trap("USR1") {puts "prodded me"}

loop do 
  sleep 5
  puts "doing stuff"
end

Is about the simplest code that will trap the “USR1” signal (which you can send with “kill -USR1 “). The USR1 and USR2 signals are left free for you to use however you wish in your applications.

If you look at the image below you can see that it responds to the USR1 signal I send it and kill (ie sending SIGTERM) works also.

The following two code snippets are the same except one takes the default and the other catches Exception (ie any exception)

puts "I have PID #{Process.pid}"

Signal.trap("USR1") {puts "prodded me"}

loop do 
  begin
  puts "doing stuff"
  sleep 10
  rescue => e
    puts e.inspect
  end
end

So that still works as before and errors in our “do stuff” loop would get caught.

puts "I have PID #{Process.pid}"

Signal.trap("USR1") {puts "prodded me"}

loop do 
  begin
  puts "doing stuff"
  sleep 10
  rescue Exception => e
    puts e.inspect
  end
end

This fails though. You can see that SIGTERM no longer works and CTRL-C from the terminal does not work also. This is because we are catching the SignalException when we do “rescue Exception”. Kill -9 worked though, as it will kill any application as the signal cannot be caught.

Rubys Exception Heirachy

The full exception heirachy (from the excellent cheat gem) is

exceptions:
  Exception
   NoMemoryError
   ScriptError
     LoadError
     NotImplementedError
     SyntaxError
   SignalException
     Interrupt
       Timeout::Error    # require 'timeout' for Timeout::Error
   StandardError         # caught by rescue if no type is specified
     ArgumentError
     IOError
       EOFError
     IndexError
     LocalJumpError
     NameError
       NoMethodError
     RangeError
       FloatDomainError
     RegexpError
     RuntimeError
     SecurityError
     SocketError
     SystemCallError
     SystemStackError
     ThreadError
     TypeError
     ZeroDivisionError
   SystemExit
   fatal

I think you should only catch StandardError or its children, possibly some of its siblings and avoid catching Exception as you probably dont want to change how the process deals with signals (you could trap them if you need to)

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