Fix memory leak when pre-filling MTLCommandBuffers.

Wrap MVKCommandBuffer::prefill() in autoreleasepool.
For non-reusable MVKCommandBuffers, release command instances
once prefilling is done to reduce memory pressure.

Docs/Whats_New.md

@@ -17,16 +17,15 @@ For best results, use a Markdown reader.*
 MoltenVK 1.0.41
 ---------------
 
-Released TBD
+Released 2020/03/30
 
 - Fix issue where immutable samplers are removed during descriptor update.
-- Guard against Metal validation assertion from reuse of query number 
-  within a single `MTLRenderEncoder`.
+- Guard against Metal validation assertion from reuse of query number within a single `MTLRenderEncoder`.
 - Increase value of `VkPhysicalDeviceLimits::minStorageBufferOffsetAlignment` 
   to `16` to avoid Metal validation assertions.
 - Add ability to disable command memory pooling using `MVK_CONFIG_USE_COMMAND_POOLING`
   environment variable.
-
+- Fix memory leak when pre-filling `MTLCommandBuffers` using `MVK_CONFIG_PREFILL_METAL_COMMAND_BUFFERS`.
 
 
 

MoltenVK/MoltenVK/Commands/MVKCommandBuffer.h

@@ -152,6 +152,7 @@ protected:
 	bool canPrefill();
 	void prefill();
 	void clearPrefilledMTLCommandBuffer();
+	void releaseCommands();
 
 	MVKCommand* _head = nullptr;
 	MVKCommand* _tail = nullptr;

MoltenVK/MoltenVK/Commands/MVKCommandBuffer.mm

@@ -54,18 +54,20 @@ VkResult MVKCommandBuffer::begin(const VkCommandBufferBeginInfo* pBeginInfo) {
 	return getConfigurationResult();
 }
 
-VkResult MVKCommandBuffer::reset(VkCommandBufferResetFlags flags) {
+void MVKCommandBuffer::releaseCommands() {
 	MVKCommand* cmd = _head;
 	while (cmd) {
 		MVKCommand* nextCmd = cmd->_next;	// Establish next before returning current to pool.
 		cmd->returnToPool();
 		cmd = nextCmd;
 	}
-
-	clearPrefilledMTLCommandBuffer();
-
 	_head = nullptr;
 	_tail = nullptr;
+}
+
+VkResult MVKCommandBuffer::reset(VkCommandBufferResetFlags flags) {
+	clearPrefilledMTLCommandBuffer();
+	releaseCommands();
 	_doesContinueRenderPass = false;
 	_canAcceptCommands = false;
 	_isReusable = false;
@@ -140,18 +142,24 @@ bool MVKCommandBuffer::canExecute() {
 	return true;
 }
 
-// If we can, prefill a MTLCommandBuffer with the commands in this command buffer
+// If we can, prefill a MTLCommandBuffer with the commands in this command buffer.
+// Wrap in autorelease pool to capture autoreleased Metal encoding activity.
 void MVKCommandBuffer::prefill() {
+	@autoreleasepool {
+		clearPrefilledMTLCommandBuffer();
 
-	clearPrefilledMTLCommandBuffer();
+		if ( !canPrefill() ) { return; }
 
-	if ( !canPrefill() ) { return; }
+		uint32_t qIdx = 0;
+		_prefilledMTLCmdBuffer = _commandPool->newMTLCommandBuffer(qIdx);	// retain
 
-	uint32_t qIdx = 0;
-	_prefilledMTLCmdBuffer = _commandPool->newMTLCommandBuffer(qIdx);	// retain
+		MVKCommandEncoder encoder(this);
+		encoder.encode(_prefilledMTLCmdBuffer);
 
-	MVKCommandEncoder encoder(this);
-	encoder.encode(_prefilledMTLCmdBuffer);
+		// Once encoded onto Metal, if this command buffer is not reusable, we don't need the
+		// MVKCommand instances anymore, so release them in order to reduce memory pressure.
+		if ( !_isReusable ) { releaseCommands(); }
+	}
 }
 
 bool MVKCommandBuffer::canPrefill() {

MoltenVK/MoltenVK/GPUObjects/MVKQueue.mm

@@ -249,7 +249,7 @@ void MVKQueueCommandBufferSubmission::setActiveMTLCommandBuffer(id<MTLCommandBuf
 
 	if (_activeMTLCommandBuffer) { commitActiveMTLCommandBuffer(); }
 
-	_activeMTLCommandBuffer = mtlCmdBuff;	// not retained
+	_activeMTLCommandBuffer = [mtlCmdBuff retain];		// retained to handle prefilled
 	[_activeMTLCommandBuffer enqueue];
 }
 
@@ -276,8 +276,9 @@ void MVKQueueCommandBufferSubmission::commitActiveMTLCommandBuffer(bool signalCo
 
 	// Use temp var because callback may destroy this instance before this function ends.
 	id<MTLCommandBuffer> mtlCmdBuff = _activeMTLCommandBuffer;
-	_activeMTLCommandBuffer = nil;			// not retained
+	_activeMTLCommandBuffer = nil;
 	[mtlCmdBuff commit];
+	[mtlCmdBuff release];		// retained
 }
 
 void MVKQueueCommandBufferSubmission::finish() {