Review Request 127387: Refactor the backend loading code

[Sebastian Kügler]

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https://git.reviewboard.kde.org/r/127387/
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(Updated March 16, 2016, 2:04 p.m.)


Review request for Plasma, Solid, Daniel Vrátil, and Martin Gräßlin.


Changes
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Addressed Martin's comments.


Repository: libkscreen


Description
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Refactor the backend loading code
    
    Untangle the large plugin loading logic in the BackendManager static
    into  separate bits. This makes the code clearer and easier to auto-test.
    
    - listBackends() compiles a list of backends from the plugin paths
    - preferredBackend() picks the backend, in this priority:
        - if KSCREEN_BACKEND is set in the environment, this is the only
          used method to find the backend plugin
        - if platform is X11, the XRandR backend is picked
            - if platform is wayland, KWayland backend is picked
            - if neither is the case, QScreen backend is picked
    
    It does introduce a slight behavioral change: The mechanism was based on
    falling through, so it would consider another backend if the logically
    picked on fails to load. This is undesired behavior, however, since the
    backendloader may be able to load the plugin, but that doesn't mean that
    the plugin actually work.
    
    Parsing of the KSCREEN_BACKEND variable is kept case-insensitive.


    autotests for new backend loading logic
    
    - makes sure we find plugins installed
    - pick plugins from env var
    
    We can't sensible test all the runtime cases yet, but this at least
    covers the code paths around those few lines that do runtime detection
    of x11 and wayland.


Diffs (updated)
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  autotests/CMakeLists.txt 26c7952 
  autotests/testbackendloader.cpp PRE-CREATION 
  src/backendmanager.cpp 382ae71 
  src/backendmanager_p.h 150883b 

Diff: https://git.reviewboard.kde.org/r/127387/diff/


Testing
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manual runtime tests and autotests pass


Thanks,

Sebastian Kügler

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