D7870: Don't crash if a client (legally) uses a deleted global

[David Edmundson]

davidedmundson created this revision.
davidedmundson added a reviewer: Plasma.
Restricted Application added projects: Plasma on Wayland, Frameworks.
Restricted Application added subscribers: Frameworks, plasma-devel.

REVISION SUMMARY
  There is a race condition in the following situation:
  
  - Server creates a global
  - Client binds to that global (making a new resource for that global)
  
  Simultaneously:
  
  - The client uses this resource
  - The server deletes the global
  
  We then process an event for a resource linked to a deleted global.
  
  This is noted in the specification, the client documentation says:
  "The object remains valid and requests to the object will be
  ignored until the client destroys it, to avoid races between the global
  going away and a client sending a request to it. "
  
  KWayland does not handle this at all.
  
  The global's user data refer to our C++ wrapper
  The resource's user data refer to *the same* C++ wrapper
  
  When the global is deleted the resource user data now refers to garbage.
  
  To fix the issue, instead of setting the resource userdata to the
  global, we set it to a smartpointer to the global stored on the heap.
  
  We can then validate if our global is still valid.
  
  Theoretically this applies to every global
  
  Practically there are only 3 globals that don't have the lifespan of the
  server. Output (which is read only and doesn't matter), Blur and
  BackgroundContrast.
  
  Blur resets it's global when a screen geometry changes.
  
  Unfotunately this exactly at the same time that Plasmashell is 
  doing a lot of processing and creating some blurs.

TEST PLAN
  See unit test

REPOSITORY
  R127 KWayland

BRANCH
  globalref

REVISION DETAIL
  https://phabricator.kde.org/D7870

AFFECTED FILES
  autotests/client/test_wayland_registry.cpp
  src/server/blur_interface.cpp

To: davidedmundson, #plasma
Cc: plasma-devel, #frameworks, leezu, ZrenBot, alexeymin, progwolff, lesliezhai, ali-mohamed, jensreuterberg, abetts, eliasp, sebas, apol, mart, hein
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