Implement an XShmGetImage-based backend #431
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This only adds the support for the XCB MIT-SHM extension to mss's internal xcb libraries. The actual usage of shared memory for screenshots will be done in a future commit.
This is close to complete, but there's a few things that need to be chased down: notably, the test_thread_safety test is failing, for some reason. It also currently doesn't work correctly if the root window size increases. That said, this is quite promising: on my computer, the new backend can take 4k screenshots at 30-34 fps, while the XGetImage backend could only run at 11-14 fps.
Previously, I had been trying to close the memfd in _shutdown_shm, and ignoring EBADF. It turns out that XCB will close the memfd when you send it to the X server. I think this was one potential cause of the issues I saw in test_thread_safety: the two threads would be reallocated each others' fds, leading to thread A closing an fd that thread B was using, thinking that it was thread A's memfd. Fix so that the memfd is only explicitly closed in an error situation.
Under X11, when the last client disconnects, the server resets. If a new client tries to connect before the reset is complete, it may fail. Since we often run the tests under Xvfb, they're frequently the only clients. Since our tests run in rapid succession, this combination can lead to intermittent failures. To avoid this, we open a connection at the start of the test session and keep it open until the end.
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Changes proposed in this PR
This is close to complete, but there's a few things that need to be
chased down: notably, the test_thread_safety test is failing, for some
reason. It also currently doesn't work correctly if the root window
size increases.
That said, this is quite promising: on my computer, the new backend
can take 4k screenshots at 30-34 fps, while the XGetImage backend
could only run at 11-14 fps.
It is very important to keep up to date tests and documentation.
Is your code right?
./check.shpassed