[Swan-dev] rr: debugging using replays

[D. Hugh Redelmeier]

I don't use gdb much.  Consequently I'm clumsy using it.  Consequently I 
don't use gdb much.

This tool looks like it might make gdb a lot more useful:
	<https://rr-project.org/>
It is a Mozilla research project.  But at this point it just works.
Have a look at the videos on that page.

Idea:

	(with limitation) you run a program under rr to record that run.

	When it is captured, you can replay it (forward and back!) as many
	times as you want, from within GDB.  Breakpoints and all work.

It supports with multithreaded programs, but it only allows one core to be 
used. (I have no idea whether it supports fork.)

For a non-deterministic bug, you keep rerunning the recording step until
you observe the problem.  You can replay that execution as much as you
want without the non-determinism shifting on you.

Some kinds of output are lost: in demos of debugging Firefox, Firefox
windows were not created during replay.  But anything that the program
actually examines comes from the recording.

The overhead comes in two parts:

- run time during recording.  That seems fine: less than a factor of
  two.

- RAM requirements.  That can get large.  It mostly records everything
  that passes into and out of each system call.

If they can debug Firefox with this, I imagine that we could debug
Pluto.

Pluto *might* use some system calls that they don't yet support.
Apparently they will implement them if there is a need.