Interfacing rr to gdb in GNU Emacs

First of all, for those who haven't tried rr yet, stop everything and go check it out right now. It's revolutionary. I'll wait.

OK. Back? I generally talk to gdb from inside GNU Emacs. This has two modes:

GUD. This is the "old" mode, and you launch it via M-x gud-gdb. GUD expects gdb to be running with annotation level 1. This annotation level can be set in several ways:
- gdb --fullname
- gdb -f
- gdb --annotate 1
- Execute set annotate 1 within gdb
When you M-x gud-gdb, emacs suggests gdb --fullname, so by taking the suggestion the user doesn't have to remember any of these.
gdb-mi. This is the "new" mode, launched via M-x gdb. It also needs a special communications mode, but this one is entered with any of
- gdb -i=mi
- gdb --ui=mi
- gdb --interpreter=mi
As with GUD, M-x gdb suggests something: gdb -i=mi so once again the user doesn't need to remember any of this.

Now, when I rr replay, I'm also talking to gdb, and I need to pass the same communication options. But rr doesn't support these, so the user needs to do something to turn them on, which at the very least means learning and remembering trivia about these options. Well, I fixed this, so now I can now go back to being blissfully ignorant:

https://mail.mozilla.org/pipermail/rr-dev/2017-February/000452.html

https://github.com/mozilla/rr/commit/684045339af9905e4ae432d237ba2cfb98ad0df3

Now, when invoking rr replay from Emacs, you simply take the suggested command (gdb --fullname or gdb -i=mi), replace gdb with rr replay, add the log we're replaying, and things then work as expected.

Note that if you're stuck with an older version of rr then at least for GUD there's a simple workaround:

echo set annotate 1 >> ~/.gdbinit