[RP] Segfault on zero screens (Was: Segfault and faulty focus)

[Martin Samuelsson]

Axel Svensson @ 2019-07-20 (Saturday), 01:20 (+0200)
> The particular segfault I reported originates in the assumption that there 
> exists at least one screen

Ah! That's it! A reasonable expectation prior to xrandr, but having zero 
screens is fully allowed (although arguably useless) with xrandr support.

> See footnote [1] for a MWE that works for me, YMMW.

Once you explained the issue, I managed to reproduce with a normal Xorg 
process merely having launched an xterm and the following commands on a text 
mode console:

export DISPLAY=:0
for S in `xrandr --listmonitors|sed --silent 's/ [0-9]*:.* //p'`
do
 xrandr --output ${S} --off
done
./src/ratpoison # This one segfaults

> The interesting part is what causes the screen removal.

Makes sense. During reconfiguration there's a split second where no screen 
actually exist and ratpoison should cope with that.

> Also note the error message from xrandr.

Without fully understanding them, I wouldn't be surprised if even the tools 
for xrandr itself gets confused when there are zero screens.

> At this point I think I'd appreciate some input.

For most of its life ratpoison knew nothing about xrandr and support for it 
was only added "somewhat recently". The code needs to be gone through to see 
which other functions have this assumption on an existing screen.

I quickly tried adding a few null checks and successfully started ratpoison 
with zero screens, reactivated my screen, launched an xlogo and the new 
window did appear. My interpretation of that little exercise is that fixing 
the bug should be very doable.

Unfortunately I don't know the code well enough to immediately understand 
whether adding all the needed null checks before each use of screens are the 
way to go, or if there could be a smarter way. Won't have more time to look 
at it right now, I'm afraid since my weekend is fully booked.

> Am I simply using the xrandr tool wrong?

Nah. I'd say you're using it right and found a proper bug in ratpoison. Yet 
if you manage to find a workaround to setup your environment in some way 
which doesn't triggering the bug, go for it! Just beware that every time you 
deploy a workaround you increase your technical debt.
--
/Martin