Fix thread safety issues around the cursor and cursor sprites in the OpenGL backends #8993
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michicc
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Think commit message probably needs amending, as this does not "fix" the issue that _fio exists (in its current form) |
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Motivation / Problem
The main/OpenGL thread unnecessarily gets the cursor sprites from the global sprite cache without holding any locks.
The OpenGL does cache the cursor sprites along with the associated sprite data, but it still looks up some of the sprite properties form the sprite cache each time.
The sprite cache, sprite processing and everything to do with fio cannot be used from more than one thread at once.
In the case where the cursor sprites are already in the cache this results in the race conditions listed in #8870.
In the case where the cursor sprites need to be added to the cache the additional race conditions listed below occur.
In the case where two threads end up writing sprites into the sprite cache at the same time, such as after the sprite cache has been cleared, the two threads writing over each other could cause the sort of invalid state induced crash seen in #8977.
Additionally, a number of cursor state properties are accessed without holding the required lock.
Additional race conditions when OpenGLBackend::DrawMouseCursor inserts a sprite into the cache
Description
Remove all direct accesses to _cursor and the sprite cache from the parts of the OpenGL code where the lock is not held.
Cache additional cursor properties as required.
Retrieve the cursor sprite properties from the local copy of the sprites, not the sprite cache sprite.
Limitations
This doesn't address the other race conditions, however the other ones that I've found so far are less important.
Checklist for review
Some things are not automated, and forgotten often. This list is a reminder for the reviewers.