As presented at RubyConf 2014 (San Diego). Lots of live coding, I'll link the video eventually. ABSTRACT: Ever type 'next' into your debugger and then realize you should have used 'step'? Or perhaps invoked a method that you wish you could take back? Regret no more! Just turn the clock back a few ticks and begin again! With only a few restrictions and side-effects we will learn how to construct and use a time machine. WARNING: Time travel may cause zombies. Github: https://github.com/awwaiid/pry-timetravel RubyGems: https://rubygems.org/gems/pry-timetravel

1. For each universe
there are a dozen others;
one must lack my bug?

2. A Partial-Multiverse Model
of Time Travel
for Debugging
RubyConf 2014
Brock Wilcox
awwaiid@thelackthereof.org
bwilcox@optoro.com

3. awwaiid@rbconf:~$ whoami

4. Debuggers

5. pry-byebug

6. How does THAT work‽

7. Sweet

8. OK. What could be better?

9. MAD SCIENCE

10. pry-timetravel

11. And we're done!

12. The Idea
(nothing new under the sun)

13. fork() the universe during 'next'

14. Suspend the child fork

15. Keep track of all created child forks

16. Upon 'back', grab the most recent child fork

17. Resume (time-travel!) the previous child fork

18. Wha?

19. Two essential ingredients.

20. Unix fork()

21. New child process:
new (child) PID
copy of memory
share file handles (sockets)
... other stuff
runs separately

22. Copy-On-Write!

23. Signals

24. Registered OS callbacks

25. SIGINT
SIGTERM
SIGKILL
SIGHUP
SIGUSR1
SIGALRM
SIGTSTP

26. SIGINT (^C)
SIGTERM (kill)
SIGKILL (kill -9)
SIGHUP (daemon reload)
SIGUSR1 (... things ...)
SIGALRM (just another 5 minutes!)

27. Signal handling / sending in Ruby

28. SIGTSTP (^Z)
SIGSTOP
SIGCONT

30. def snap
if child_pid = fork
# I am the parent!
Process.kill 'SIGSTOP', $$
else
# I am the child!
@snap_pid.push child_pid
end
end

31. def restore
Process.kill 'SIGCONT', @snap_pid[-1]
Process.kill 'SIGSTOP', $$
end

32. Limitation:
Shared file handles (inc. sockets!)

33. Limitation:
External state.

34. Limitation:
Threads. Not so much.

35. Limitation:
Can only travel back to checkpoints
(see also: Primer)

36. Limitation:
Might crash the multiverse through
memory exhaustion. I guess.

37. Wrap in a pry plugin
and we're done, right?

38. Complication:
Shell detects STOP

39. awwaiid@floyd:~$ pry
[1] pry(main)>
zsh: suspended (signal) pry

40. Fix:
Make a dummy parent process
that just sleeps (or sig-pause)

41. Complication:
Killing parent snapshots

42. Fix:
Let the One True (parent) snapshot
kill all of its children (via SIGUSR1).
(also: nope. totally wrong.)

43. Complication:
Every snapshot wants to clean up

44. Fix:
If you are not the current snapshot,
terminate yourself without cleanup!

45. Complication:
ZOMBIES

46. Zombie:
Child process that has exited nicely
but then the parent exited not-nicely.
The process is dead, yet remains.

47. Great. Now we can go back!

48. I WANT MOAR

49. Tree of snapshots

50. TODO: auto-snapshot

51. References:
https://github.com/TomOnTime/timetravelpdb
http://programmers.stackexchange.com
/questions/181527
/why-is-reverse-debugging-rarely-used
http://debug.elm-lang.org/

52. If I had the chance
I'd write a better haiku
oh wait, I HAVE A TIME MACHINE

53. THE END