[Bug 1280665] [NEW] libsdl1.2 always uses backingstore which causes tearing in modern X servers

[Launchpad Bug Tracker]

You have been subscribed to a public bug by Timo Jyrinki (timo-jyrinki):

[Impact]
Tearing in SDL applications regardless of vsync setting.

[Test Case]
Use eg. XBMC to play any video, or a particular video that more easily shows up the tearing of the images.

[Regression Potential]
This is a regression fix in itself, and restores old behavior.

---

Fedora 21 has a similar patch applied
https://bugzilla.redhat.com/show_bug.cgi?id=1073057

libsdl1.2 always enables backingstore when it is present in the X
server. For modern X servers, backingstore is implemented by redirecting
through the composite extension. This causes tearing on SDL applications
such as xbmc because the redirected rendering is not guaranteed to be
synchronized.

A discussion of the problem by Nvidia is here:
https://devtalk.nvidia.com/default/topic/679688/linux/tearing-in-sdl-1-2
-apps-unless-composite-extension-is-completely-
disabled-/post/4111287/#4111287

The upstream bug is here:
https://bugzilla.libsdl.org/show_bug.cgi?id=2383

I have a patch that restores a check for the SDL_VIDEO_X11_BACKINGSTORE
environment variable before trying to enable backingstore.

** Affects: libsdl
     Importance: Unknown
         Status: Unknown

** Affects: libsdl1.2 (Ubuntu)
     Importance: Medium
         Status: Fix Released


** Tags: patch
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libsdl1.2 always uses backingstore which causes tearing in modern X servers
https://bugs.launchpad.net/bugs/1280665
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