Re: MPS: a random backtrace while toying with gdb

[Eli Zaretskii <eliz@gnu.org>]

> Date: Sun, 30 Jun 2024 10:16:42 +0000
> From: Pip Cet <pipcet@protonmail.com>
> Cc: gerd.moellmann@gmail.com, yantar92@posteo.net, emacs-devel@gnu.org, eller.helmut@gmail.com
> 
> > > We could always "steal" the SIGSEGV handler and reinstall it with a sigmask (without modifying the MPS source). My guess is that's equivalent to what the macOS code does, essentially, by using a different class of signal that blocks POSIX signals...
> > 
> > 
> > And this is less "ugly" (let alone more safe) than using sigblock?
> > What am I missing?
> 
> Mostly, I think, the race condition where the barrier is installed (by MPS) some time before dflt_scan is even called (and, symmetrically, we'd unblock it too early while the barrier is still in effect). IOW, sigblock wouldn't work.

Then we should block signals earlier, e.g., in igc_alloc (and any
other place which could cause our objects moved).

> (I should point out that you've convinced me as far as SIGPROF goes. At least for the initial stage, it makes more sense to count hits in GC (which we do, with false positives, using the current scratch/igc code) than to delay signal processing until GC is over).

SIGPROF is not the only one.  We have other useful signals, and we
need to think about a solution that will cover most if not all of
them.  I've listed the signals that came to mind a few messages ago.

I still cannot believe MPS doesn't have a canonical solution for these
cases.