Segfault when cursor moves after monitor plugged in

[AlanGriffiths]

(There may be simpler, but this works for me.)

Run up miral-app with two screens L and R. Terminal appears on R.

Use mouse to move the terminal window to L, leaving mouse over titlebar.

Unplug L, wait for screen to update (terminal appears on R as it's now at 0,0).

Replug L, wait for screen to update (terminal appears on L as it's now at 0,0).

Move cursor off window.

Expect: system continues to work.
Actual: segfault at src/platforms/mesa/server/kms/cursor.cpp:296

[AlanGriffiths]

Some more detail on the scenario that provokes this:

Qt (at least in this scenario) uses a single surface and updates the buffer. The error is occurring when a bufferstream has been used to draw the pointer and WlPointer::set_cursor() is called immediately prior to the new (e.g. sizing) image being sent. (I'm still not clear why the previous buffer is invalid, but the existence of "log_warning("Attempt to read from WlShmBuffer after the wl_buffer has been destroyed");" suggests that this can indeed happen.)

[AlanGriffiths]

OK, the problem is that CursorImageFromBuffer assumes that PixelSource::read() will execute the do_with_pixels block. But this is not true of the WlShmBuffer::read() implementation.