Fix threading bug in Vulkan rendering device

[joemarshall]

This is a fix for the following bug:

#78786

The bug was that:
draw_list_switch_to_next_pass was not holding the lock, and was calling
_draw_list_free(&viewport);
and draw_list_allocate with stuff in between, all mid a single multiple pass draw-list.

The free and allocate functions release and acquire the lock. So in between the two calls, other stuff was happening (other draw lists being started and ended), which meant bad things happened because the vulkan state goes bad (segfaults in graphics drivers on my machine).

This is I think a pretty trivial bugfix - deffo for 4.2, would be great for 4.1 if poss?

• Bugsquad edit, fixes: Game quits after rendering calls on main rendering device if multithreaded rendering is enabled #78786

[clayjohn]

Looks good. From reviewing related code, it seems like everything called during a draw list needs to be made thread safe.

[joemarshall]

Looks good. From reviewing related code, it seems like everything called during a draw list needs to be made thread safe.

The draw_list_* calls mostly don't need thread safety adding because the lock is taken during draw_list_begin, and released on draw_list_end. It makes sense because there is only one draw list tracked, so it makes no sense for drawing calls from another thread to get put in during creation of a draw list in one thread.

Incidentally I don't think holding this lock is a major issue, because it's vulkan, everything is deferred, the actual draw_list calls are super fast. The only downside of it is that it means if people call draw_list_begin without a matching draw_list_end, or try to add things to the same draw_list across multiple threads, bad things will happen. But really there isn't any non-stupid use case for that until or unless we add support for multiple draw lists on one renderingdevice.

This bug was the only thing I caught when I went through it, I think I was quite thorough - I stepped through things in debugger also.

[RandomShaper]

If this fixes threading bugs, it's fine I guess. In any case, we have to reassess the whole thread safety topic in RD to determine and enforce specific design goals.

[akien-mga]

Thanks! And congrats for your first merged Godot contribution 🎉

[YuriSizov]

Cherry-picked for 4.1.1.