multiscreen logging

[Sebastian Kügler]

Hey,

[Please keep both lists addressed.]

Debugging multiscreen issues is a nightmare:

- there are at least 4 different processes involved (kded, kcmshell / 
  systemsettings, kscreen_backend_launcher and plasmashell)
- some are critical during log in
- they IPC with each other
- especially the backend launcher's debug is really hard to get at

This means that:
- it's hard (almost impossible) for users to get us good and useful logs
- it's hard for ourselves to debug and find out what's exactly going on, 
especially when multiple components need to play in tune

Yesternight, after debugging the so-many-th issue, it occurred to me that we 
need to make this way easier to debug. Q(C)Debug falls short in that we get 
logs of individual components, if we're lucky. If we're really lucky, we get 
timestamps, so we can get a rough idea of what is going down when.

All of these problems can be solved with a relatively simple, shared log file.

So I'd like to switch most of (lib)kscreen's debug output to logging to a 
file. The files has then logs from multiple processes and will be much easier 
to go through.

API-wise, my main thing is having log messages and a bit of context, so it'd 
look like this:

Log::instance()->setContext("handling resume event");
// ...
Log::log("Enabled output" + output->name());

In the log, we'd then get

[TIMESTAMP] (kded: handling resume event) Enabled output eDP-1


The key here is that we get a mostly correct sequence of things, that all the 
info is there right away, and that it's easier for the user to annotate the 
log ("Now plugging in my external display as HDMI-2").

The file is simple enough that even plasmashell could log to the file (at 
least until we've fixed the interaction between screen handling and 
plasmashell), so we get the full picture.

libkscreen[sebas/log] has a basic implementation. It's incomplete in the sense 
that it needs a bit of autotesting (just haven't gotten to it yet), review and 
then switching over the code-base. (For now it's on by default, but can be 
disabled using a env var.)

I hope that this makes it much more straight-forward for us (and even users) 
to figure out where their multiscreen headaches are coming from, and that it 
gives us a one-stop shop (pretty much) to tell users what info we need to fix 
their bugs. ("Just send me the logfile in ~/.local/share/kscreen.log").

What do you think? If you like the idea, I'll polish up my branch and will 
post it for review, so we can discuss the actual implementation.

Cheers,
-- 
sebas

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