bug#45541: 28.0.50; Frequent crashes on ARM macOS

[Philipp Stephani]

Am Mi., 30. Dez. 2020 um 15:49 Uhr schrieb Philipp Stephani
<p.stephani2@gmail.com>:
>
> Am Mi., 30. Dez. 2020 um 15:42 Uhr schrieb Philipp Stephani
> <p.stephani2@gmail.com>:
> >
> > Am Mi., 30. Dez. 2020 um 15:05 Uhr schrieb Alan Third <alan@idiocy.org>:
> > >
> > > On Wed, Dec 30, 2020 at 02:01:18PM +0100, Philipp Stephani wrote:
> > > > Am Mi., 30. Dez. 2020 um 01:10 Uhr schrieb Alan Third <alan@idiocy.org>:
> > > > >
> > > > > On Tue, Dec 29, 2020 at 11:32:43PM +0100, Philipp wrote:
> > > > > >
> > > > > > With an Emacs built from the emacs-27 branch I get frequent crashes 
> > > > > > on
> > > > > > ARM64 macOS (Big Sur).  I haven't managed to produce an Elisp 
> > > > > > stacktrace
> > > > > > yet, but here's the crash report:
> > > > > <snip>
> > > > > > Looks like a crash in ns_mouse_position.
> > > > >
> > > > > Can you check that the build includes this?
> > > > >
> > > > > 6aa9fe3e1b4052b2acde86404a90e35893ebfa00?
> > > > > Fix crash in ns_mouse_position (bug#44313)
> > > >
> > > > Yes, according to "git branch --contains" the commit is present.  The
> > > > exception report makes me think that NSApp is null or invalid, but how
> > > > should that be possible?
> > >
> > > I have no idea... What's happening when the crash occurs? I take it
> > > the mouse is being moved, but are frames opening and closing or
> > > anything?
> >
> > I'm now able to reproduce this consistently. It happens when a tooltip
> > appears and you hover over that tooltip (try M-x locate and move the
> > mouse around).
> > The crash is in the line
> >   if (f && FRAME_NS_P (f))
> > in nsterm.m in ns_mouse_position, so apparently "f" is not NULL, but
> > also not valid.
>
> Looks like "f" isn't initialized to NULL in that function?

This was probably fixed with commit
b4911a6f0da0bfae3832b3aa0c111db4bb2f49d5 on master. I guess we should
backport the one-line initialization change onto the release branch,
since the crash is nasty and the fix is trivial.