How to recreate swapchain after vkAcquireNextImageKHR is VK_SUBOPTIMAL_KHR?

Question

This vulkan tutorial discusses swapchain recreation:

You could also decide to [recreate the swapchain] that if the swap chain is suboptimal, but I've chosen to proceed anyway in that case because we've already acquired an image.

My question is: how would one recreate the swapchain and not proceed in this case of VK_SUBOPTIMAL_KHR?

To see what I mean, let's look at the tutorial's render function:

void drawFrame() {
        vkWaitForFences(device, 1, &inFlightFences[currentFrame], VK_TRUE, UINT64_MAX);

        uint32_t imageIndex;
        VkResult result = vkAcquireNextImageKHR(device, swapChain, UINT64_MAX, imageAvailableSemaphores[currentFrame], VK_NULL_HANDLE, &imageIndex);

        if (result == VK_ERROR_OUT_OF_DATE_KHR) { 
            recreateSwapChain();
            return;
        /* else if (result == VK_SUBOPTIMAL_KHR) { createSwapchain(); ??? } */
        } else if (result != VK_SUCCESS && result != VK_SUBOPTIMAL_KHR) {
            throw std::runtime_error("failed to acquire swap chain image!");
        }

        if (imagesInFlight[imageIndex] != VK_NULL_HANDLE) {
            vkWaitForFences(device, 1, &imagesInFlight[imageIndex], VK_TRUE, UINT64_MAX);
        }
        imagesInFlight[imageIndex] = inFlightFences[currentFrame];

        VkSubmitInfo submitInfo{};
        submitInfo.sType = VK_STRUCTURE_TYPE_SUBMIT_INFO;

        VkSemaphore waitSemaphores[] = {imageAvailableSemaphores[currentFrame]};
        VkPipelineStageFlags waitStages[] = {VK_PIPELINE_STAGE_COLOR_ATTACHMENT_OUTPUT_BIT};
        submitInfo.waitSemaphoreCount = 1;
        submitInfo.pWaitSemaphores = waitSemaphores;
        submitInfo.pWaitDstStageMask = waitStages;

        submitInfo.commandBufferCount = 1;
        submitInfo.pCommandBuffers = &commandBuffers[imageIndex];

        VkSemaphore signalSemaphores[] = {renderFinishedSemaphores[currentFrame]};
        submitInfo.signalSemaphoreCount = 1;
        submitInfo.pSignalSemaphores = signalSemaphores;

        vkResetFences(device, 1, &inFlightFences[currentFrame]);

        if (vkQueueSubmit(graphicsQueue, 1, &submitInfo, inFlightFences[currentFrame]) != VK_SUCCESS) {
            throw std::runtime_error("failed to submit draw command buffer!");
        }

        VkPresentInfoKHR presentInfo{};
        presentInfo.sType = VK_STRUCTURE_TYPE_PRESENT_INFO_KHR;

        presentInfo.waitSemaphoreCount = 1;
        presentInfo.pWaitSemaphores = signalSemaphores;

        VkSwapchainKHR swapChains[] = {swapChain};
        presentInfo.swapchainCount = 1;
        presentInfo.pSwapchains = swapChains;

        presentInfo.pImageIndices = &imageIndex;

        result = vkQueuePresentKHR(presentQueue, &presentInfo);

        if (result == VK_ERROR_OUT_OF_DATE_KHR || result == VK_SUBOPTIMAL_KHR || framebufferResized) {
            framebufferResized = false;
            recreateSwapChain();
        } else if (result != VK_SUCCESS) {
            throw std::runtime_error("failed to present swap chain image!");
        }

        currentFrame = (currentFrame + 1) % MAX_FRAMES_IN_FLIGHT;
    }

The trouble is as follows:

1. vkAcquireImageKHR succeeds, signaling the semaphore and returning a valid, suboptimal image
2. Recreate the swapchain
3. We can't present the image from 1 with the swapchain from 2 due to VUID-VkPresentInfoKHR-pImageIndices-01430. We need to call vkAcquireImageKHR again to get a new image.
4. When we call vkAcquireImageKHR again, the semaphore is in the signaled state which is not allowed (VUID-vkAcquireNextImageKHR-semaphore-01286), we need to 'unsignal' it.

Is the best solution here to destroy and recreate the semaphore?

Answer 1

Ad 3: you can use the old images (and swapchain) if you properly use the oldSwapchain parameter when creating the new swapchain. Which is what I assume the tutorial suggests.

Anyway. What I do is that I paranoidly sanitize that toxic semaphore like this:

// cleanup dangerous semaphore with signal pending from vkAcquireNextImageKHR (tie it to a specific queue)
// https://github.com/KhronosGroup/Vulkan-Docs/issues/1059
void cleanupUnsafeSemaphore( VkQueue queue, VkSemaphore semaphore ){
    const VkPipelineStageFlags psw = VK_PIPELINE_STAGE_BOTTOM_OF_PIPE_BIT;
    VkSubmitInfo submit_info = {};
    submit_info.sType = VK_STRUCTURE_TYPE_SUBMIT_INFO;
    submit_info.waitSemaphoreCount = 1;
    submit_info.pWaitSemaphores = &semaphore;
    submit_info.pWaitDstStageMask;

    vkQueueSubmit( queue, 1, &submit_info, VK_NULL_HANDLE );
}

After that the semaphore can be properly catched with a fence or vkQueueWaitIdle, and then destroyed or reused.

I just destroy them, because the new semaphore count might differ, and I don't really consider swapchain recreation a hotspot (and also I just use vkDeviceWaitIdle in such case).