Xenial's gdb 7.11 breaks rr

Kyle Huey me at kylehuey.com
Sat May 14 00:09:02 UTC 2016


1. Build tip rr (8566e3038892)
2. ./bin/rr record ./bin/reverse_continue_process_signal
3. ./bin/rr replay
4. c
5. Once, the program has reached the final SIGKILL, b 22
6. rc

Broken versions of gdb appear to forget which direction execution is
heading in when an intermediate (and uninteresting) signal is reported by
rr to gdb and resume with a vCont instead of a bc which sends us back to
the SIGKILL at the end.

This is fixed on the gdb tip, but building gdb is sufficiently annoying,
and there is enough intermediate breakage, that I did not pin down a fix
range.  Downgrading gdb on Ubuntu is non-trivial as well, so I ended up
having to build from source :/

I encountered this debugging Firefox, where the SIGKILL was the shutdown
hang timeout killer and the intermediate signal is a SIGCHLD.

- Kyle
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