Potential race condition in DescriptorPool::freeDescriptorSet

[dariuszwas]

Describe the bug
I believe following code contains a race condition.
https://github.com/vsg-dev/VulkanSceneGraph/blob/master/src/vsg/vk/DescriptorPool.cpp#L139

    {
        std::scoped_lock<std::mutex> lock(mutex);
        _recyclingList.push_back(dsi);
        ++_availableDescriptorSet;
        accumulateBindings(_recycledDescriptorPoolSizes, dsi->_descriptorSetLayout, +1);
    }
    dsi->_descriptorPool = {};

dsi->_descriptorPool can be overwritten by thread_2 inside allocateDescriptorSet() while thread_1 has left the mutex but didn't yet perform:
dsi->_descriptorPool = {};

To Reproduce
I got one crash. Reproducing this is not easy.

Possible solution

    auto keepAlive = dsi->_descriptorPool;
    {
        std::scoped_lock<std::mutex> lock(mutex);
        _recyclingList.push_back(dsi);
        ++_availableDescriptorSet;
        accumulateBindings(_recycledDescriptorPoolSizes, dsi->_descriptorSetLayout, +1);
        dsi->_descriptorPool = {};
    }

[robertosfield]

Your suggested fix looks reasonable. Use of auto hides the fact that it's take a reference though, so explicitly specifying the type would be more helpful to developers reviewing the code.

BTW, what was the usage case that tripped up on this race condition?

[dariuszwas]

I have a tool for generating large volume of terrain renders. It's based upon worldviewer app from vsgCs repository.