Siarhei Siamashka wrote:
I would suggest to generally forbid using the same pixman_image_t from multiple threads, but also add a function for creating pixman_image_t clone, which would share the pixel buffer with the original image and track pixel buffer lifetime via thread-safe refcounting.
I don't see any usefulness in that. All it will do is force a multithreaded application to make their own pixman_image_t that checks if the thread is different and encapsulates the ref.
You should be able to use the same source surface from any number of cairo_t in any number of threads. Without it programmers will just be forced to make a per-thread copy of all their source surfaces. However there is NO need for more than one thread or more than one cairo_t to write to a surface at a time.
The problem is that this is likely to work in many cases, producing unreliable code. Enforcing it may make cairo programs work better. What might work is any attempt to create a cairo_t to draw on a surface will fail if there is another one drawing to that surface or using it as a source, and any attempt to use a surface as a source will fail if there is a cairo_t to draw into it.
If this would break existing single-threaded software then perhaps it could not fail if the conflicting cairo_t's all belong to the same thread.
I would not do blocking (ie attempts to use a cairo_t as a source would block until the writing cairo_t is destroyed).
I also think the testing should be put on the code where a cairo_t is attached to a surface as a source or destination, not on the drawing operations themselves, to reduce the overhead of the testing.
Using the same cairo_t in different threads is not allowed but probably should not be tested for. It is almost certain to crash soon enough that the programmer will realize and fix it.
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