D10816: Add platform detection to KWorkspace library to adjust QT_QPA_PLATFORM

[Martin Flöser]

graesslin created this revision.
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REVISION SUMMARY
  This is a preparation step to unset QT_QPA_PLATFORM from the wayland
  startup session script. Setting QT_QPA_PLATFORM breaks 3rd-party Qt
  software which does not bundle QtWayland. Most prominent example is
  the Qt installer itself (see
  https://bugreports.qt.io/browse/QTBUG-60222).
  
  On the other hand our Plasma workspace applications need to be forced to
  Wayland on a Wayland system. So we have a conflict between we want to
  set QT_QPA_PLATFORM and we don't want to set QT_QPA_PLATFORM.
  
  This change adds new API to KWorkspace to address this problem. The new
  method adjusts the QT_QPA_PLATFORM based on the XDG_SESSION_TYPE
  enviornment variable. It is completely opt-in. Meaning applications need
  to explicitly add the call prior to creating the QGuiApplication and if
  the user specifies either QT_QPA_PLATFORM env variable or any of the
  -platform command line argument variants, the platform detection is
  skipped.
  
  The change also adjusts all plasma-workspace applications which should
  use Wayland on Wayland to use the new API. The startup script on the
  other hand still sets QT_QPA_PLATFORM. We also have applications outside
  of plasma-workspace which needs this detection. Examples are:
  
  - powerdevil
  - systemsettings
  - kinfocenter
  
  Once this change is merged those applications can be adjusted by linking
  against PW::KWorkspace and afterwards QT_QPA_PLATFORM can be unset from
  startplasmacompositor.

TEST PLAN
  See added autotest

REPOSITORY
  R120 Plasma Workspace

BRANCH
  platform-detection

REVISION DETAIL
  https://phabricator.kde.org/D10816

AFFECTED FILES
  krunner/CMakeLists.txt
  krunner/main.cpp
  ksmserver/logout-greeter/main.cpp
  ksmserver/switchuser-greeter/main.cpp
  ksplash/ksplashqml/CMakeLists.txt
  ksplash/ksplashqml/main.cpp
  kuiserver/CMakeLists.txt
  kuiserver/main.cpp
  libkworkspace/CMakeLists.txt
  libkworkspace/autotests/CMakeLists.txt
  libkworkspace/autotests/testPlatformDetection.cpp
  libkworkspace/kworkspace.cpp
  libkworkspace/kworkspace.h
  shell/CMakeLists.txt
  shell/main.cpp
  startkde/kcminit/CMakeLists.txt
  startkde/kcminit/main.cpp
  systemmonitor/CMakeLists.txt
  systemmonitor/main.cpp

To: graesslin, #plasma
Cc: plasma-devel, ZrenBot, lesliezhai, ali-mohamed, jensreuterberg, abetts, sebas, apol, mart
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