Fixed VmaDeviceMemoryBlock::Map for mapping hysteresis to work correctly when the mapping fails

Also added test for it.
Fixes #407 - thanks @matusfedorko

Author: adam-sawicki-a

include/vk_mem_alloc.h

@@ -10443,11 +10443,11 @@ VkResult VmaDeviceMemoryBlock::Map(VmaAllocator hAllocator, uint32_t count, void
 
     VmaMutexLock lock(m_MapAndBindMutex, hAllocator->m_UseMutex);
     const uint32_t oldTotalMapCount = m_MapCount + m_MappingHysteresis.GetExtraMapping();
-    m_MappingHysteresis.PostMap();
     if (oldTotalMapCount != 0)
     {
-        m_MapCount += count;
         VMA_ASSERT(m_pMappedData != VMA_NULL);
+        m_MappingHysteresis.PostMap();
+        m_MapCount += count;
         if (ppData != VMA_NULL)
         {
             *ppData = m_pMappedData;
@@ -10465,11 +10465,13 @@ VkResult VmaDeviceMemoryBlock::Map(VmaAllocator hAllocator, uint32_t count, void
             &m_pMappedData);
         if (result == VK_SUCCESS)
         {
+            VMA_ASSERT(m_pMappedData != VMA_NULL);
+            m_MappingHysteresis.PostMap();
+            m_MapCount = count;
             if (ppData != VMA_NULL)
             {
                 *ppData = m_pMappedData;
             }
-            m_MapCount = count;
         }
         return result;
     }

src/Tests.cpp

@@ -8079,6 +8079,37 @@ static void TestMappingHysteresis()
 
         vmaDestroyPool(g_hAllocator, pool);
     }
+
+    // Test hysteresis working currectly in case the mapping fails. See issue #407.
+    {
+        VkBufferCreateInfo bufCreateInfo = {VK_STRUCTURE_TYPE_BUFFER_CREATE_INFO};
+        bufCreateInfo.size = 1 * MEGABYTE;
+        bufCreateInfo.usage = VK_BUFFER_USAGE_VERTEX_BUFFER_BIT;
+
+        VmaAllocationCreateInfo allocCreateInfo = {};
+        allocCreateInfo.usage = VMA_MEMORY_USAGE_AUTO;
+
+        VkBuffer buf;
+        VmaAllocation alloc;
+        TEST(vmaCreateBuffer(g_hAllocator, &bufCreateInfo, &allocCreateInfo,
+            &buf, &alloc, nullptr) == VK_SUCCESS);
+
+        VkMemoryPropertyFlags memProps = 0;
+        vmaGetAllocationMemoryProperties(g_hAllocator, alloc, &memProps);
+
+        // It makes sense to test only if this buffer ended up in a non-HOST_VISIBLE memory,
+        // which may not be the case on some integrated graphics.
+        if((memProps & VK_MEMORY_PROPERTY_HOST_VISIBLE_BIT) == 0)
+        {
+            void* ptr;
+            for (size_t i = 0; i < 10; ++i)
+            {
+                TEST(vmaMapMemory(g_hAllocator, alloc, &ptr) == VK_ERROR_MEMORY_MAP_FAILED);
+            }
+        }
+
+        vmaDestroyBuffer(g_hAllocator, buf, alloc);
+    }
 }
 
 void Test()