[Differential] [Request, 417 lines] D2295: improve logging for kscreen

[sebas (Sebastian Kügler)]

sebas created this revision.
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REVISION SUMMARY
  Debugging kscreen is generally really hard since it's spread across different 
  processes, not all of which are easy to look into.
  
  With this change, we collect all of kscreen's debug output (from kcm, kded 
  and backendlauncher) and write them to a log file.
  
  This means that we can look into one log file, and see exactly what's going 
  on, and in which component. It makes collecting debug info much easier.
  
  This is implemented in the form of a QMessageHandler, hooking into the 
  qCDebug calls. This is a pretty neat solution, since:
  
  - we can make use of the stream operators from qDebug
  - we don't need to rewrite all the debug statements
  - normal debug is otherwise still working
  
  The general idea is that we cache file names and initialization from the
  env vars on first creation, allow subsequent context changes. The log file 
  is opened and closed right away, since other processes may want to also 
  write to it.
  
  We don't do any file-locking to keep things simple. The logging isn't
  mission-critical, but rather a debugging tool for users. If we end up
  with corrupt logs, that's bad luck (but can happen due to multi-process
  race conditions). More advanced multi-process locking and delayed
  writing is just going to give us headaches, so we avoid that at the
  price of logs not being 100% reliably formatted.
  
  Also:
  
  - disable logging for most autotests
  - all messages are then passed to the previously installed messagehandler.

TEST PLAN
  new code comes with autotests

REPOSITORY
  rLIBKSCREEN KScreen Library

BRANCH
  sebas/log

REVISION DETAIL
  https://phabricator.kde.org/D2295

AFFECTED FILES
  autotests/CMakeLists.txt
  autotests/testbackendloader.cpp
  autotests/testconfigmonitor.cpp
  autotests/testinprocess.cpp
  autotests/testkwaylandbackend.cpp
  autotests/testkwaylandconfig.cpp
  autotests/testlog.cpp
  autotests/testqscreenbackend.cpp
  autotests/testscreenconfig.cpp
  src/CMakeLists.txt
  src/backendlauncher/main.cpp
  src/backendmanager.cpp
  src/getconfigoperation.cpp
  src/log.cpp
  src/log.h

EMAIL PREFERENCES
  https://phabricator.kde.org/settings/panel/emailpreferences/

To: sebas, #plasma
Cc: plasma-devel, jensreuterberg, abetts, sebas
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