bug#72692: Emacs 31.05 (40eecd594ac) get SIGSEGV on Linux (Linux 6.6.45 Kde Wayland)

[Eli Zaretskii <eliz@gnu.org>]

> Cc: 72692@debbugs.gnu.org
> Date: Sun, 18 Aug 2024 12:43:06 +0000
> From:  Pip Cet via "Bug reports for GNU Emacs,
>  the Swiss army knife of text editors" <bug-gnu-emacs@gnu.org>
> 
> Thanks. That has a different fontset, so it looks like a fontset was
> prematurely freed while still being referred to by a face.  I think the
> assumption made in xfaces.c, that it's always safe to free a fontset if
> we're freeing the realized ASCII face, is incorrect.

Why do you think that?  free_realized_face frees a face, so what other
face can still use the same fontset, if it's a so-called "ASCII face"?

> I can confirm that we're sometimes leaving a frame's fontset field
> invalid by running this code:

If face2->ascii_face is equal to face->ascii_face, then I see no
reason not to free the fontset because of that other face.  The
comment in dispextern.h says:

  /* Fontset ID if for this face's fontset.  Non-ASCII faces derived
     from the same ASCII face have the same fontset.  */
  int fontset;

And how did you see that a frame's fontset was left invalid here?  A
frame doesn't have a fontset, AFAIK.

I don't object to using this code as a means of investigation, of
course, but I think we should have a clear idea of what the diagnostic
it emits means.

I also think it might be unsafe to call 'message' from this place,
because 'message' might trigger redisplay.  It is better to call
message_dolog or add_to_log (and ask the user to look at *Messages*).