Externally synchronized parameter lists

[kvark]

In Vulkan, there is a section for "Externally Synchronized Parameter Lists":

• [x] The buffer member of each element of the pBufferBinds member of each element of the pBindInfo parameter in vkQueueBindSparse
• [x] The image member of each element of the pImageOpaqueBinds member of each element of the pBindInfo parameter in vkQueueBindSparse
• [x] The image member of each element of the pImageBinds member of each element of the pBindInfo parameter in vkQueueBindSparse
• [x] Each element of the pFences parameter in vkResetFences
• [x] Each element of the pDescriptorSets parameter in vkFreeDescriptorSets
• [x] The dstSet member of each element of the pDescriptorWrites parameter in vkUpdateDescriptorSets
• [x] The dstSet member of each element of the pDescriptorCopies parameter in vkUpdateDescriptorSets
• [x] Each element of the pCommandBuffers parameter in vkFreeCommandBuffers
• [x] Each element of the pWaitSemaphores member of the pPresentInfo parameter in vkQueuePresentKHR
• [x] Each element of the pSwapchains member of the pPresentInfo parameter in vkQueuePresentKHR
• [x] The semaphore parameter in vkAcquireNextImageKHR
• [x] The fence parameter in vkAcquireNextImageKHR
• [x] The fence parameter in vkQueueSubmit
• [x] The surface member of each element of the pCreateInfos parameter in vkCreateSharedSwapchainsKHR
• [x] The oldSwapchain member of each element of the pCreateInfos parameter in vkCreateSharedSwapchainsKHR
• [x] The memory parameter in vkMapMemory
• [x] The memory parameter in vkUnmapMemory
• [x] The event parameter in vkSetEvent
• [x] The event parameter in vkResetEvent
• [x] The queue parameter in vkQueueWaitIdle

There are also a few instances where a command can take in a user allocated list whose contents are externally synchronized parameters. In these cases, the caller must guarantee that at most one thread is using a given element within the list at a given time.

Rust semantics for this may be a little bit tricky. It can't be &mut strictly speaking. Could they be BorrowMut<>?

For example, descriptor set writes should be:

struct DescriptorSetWrite {
  set: &'a mut B::DescriptorSet,
  ...
}

[kvark]

This looks more complicated than I hoped...

If descriptor write is &mut DescriptorSet, then the user can no longer have multiple writes into the same descriptor in this API. But it's fine in Vulkan. So unfortunately sounds like we can't have &mut Descriptor Set. We basically need something that locks an element in place while it's being processed, like a user-visible Arc<Mutex<..>>, or a trait that hides it.

Or, more concretely, the user should be able to pass one of the following:

• &mut T
• impl BorrowMut<T>
• &RefCell<T>
• &Mutex<T>

[kvark]

I'm thinking about just making it so write_descriptor_sets works with a single set (and is renamed). That would mean &mut clearly, without any borrowing crap. Any concerns - @msiglreith @grovesNL @cwfitzgerald ?