Fix synchronization issue with locking MTLArgumentEncoder.

Move mutex lock for using MTLArgumentEncoder to MVKMTLArgumentEncoder
to be tracked along with the MTLArgumentEncoder it guards.

Docs/Whats_New.md

@@ -26,6 +26,7 @@ Released TBD
   to find a cached shader, by only considering resources from the current shader stage.
 - Rename `kMVKShaderStageMax` to `kMVKShaderStageCount`.
 - Fix crash when requesting `MTLCommandBuffer` logs in runtime debug mode on older OS versions.
+- Fix synchronization issue with locking `MTLArgumentEncoder` for Metal Argument Buffers.
 - Protect against crash when retrieving `MTLTexture` when `VkImage` has no `VkDeviceMemory` bound.
 - Adjust some `VkPhysicalDeviceLimits` values for Vulkan and Metal compliance. 
 - Fix internal reference from `SPIRV_CROSS_NAMESPACE_OVERRIDE` to `SPIRV_CROSS_NAMESPACE`.

MoltenVK/MoltenVK/Commands/MVKCommandEncoderState.mm

@@ -488,27 +488,28 @@ void MVKResourcesCommandEncoderState::bindDescriptorSet(uint32_t descSetIndex,
 void MVKResourcesCommandEncoderState::encodeMetalArgumentBuffer(MVKShaderStage stage) {
 	if ( !_cmdEncoder->isUsingMetalArgumentBuffers() ) { return; }
 
-	// The Metal arg encoder can only write to one arg buffer at a time (it holds the arg buffer),
-	// so we need to lock out other access to it while we are writing to it.
-	MVKPipeline* pipeline = getPipeline();
-	lock_guard<mutex> lock(pipeline->_mtlArgumentEncodingLock);
+	bool useDescSetArgBuff = _cmdEncoder->isUsingDescriptorSetMetalArgumentBuffers();
 
+	MVKPipeline* pipeline = getPipeline();
 	uint32_t dsCnt = pipeline->getDescriptorSetCount();
 	for (uint32_t dsIdx = 0; dsIdx < dsCnt; dsIdx++) {
 		auto* descSet = _boundDescriptorSets[dsIdx];
 		if ( !descSet ) { continue; }
 
-		id<MTLArgumentEncoder> mtlArgEncoder = nil;
+		auto* dsLayout = descSet->getLayout();
+
+		// The Metal arg encoder can only write to one arg buffer at a time (it holds the arg buffer),
+		// so we need to lock out other access to it while we are writing to it.
+		auto& mvkArgEnc = useDescSetArgBuff ? dsLayout->getMTLArgumentEncoder() : pipeline->getMTLArgumentEncoder(dsIdx, stage);
+		lock_guard<mutex> lock(mvkArgEnc.mtlArgumentEncodingLock);
+
 		id<MTLBuffer> mtlArgBuffer = nil;
 		NSUInteger metalArgBufferOffset = 0;
-
-		auto* dsLayout = descSet->getLayout();
-		if (dsLayout->isUsingDescriptorSetMetalArgumentBuffers()) {
-			mtlArgEncoder = dsLayout->getMTLArgumentEncoder().getMTLArgumentEncoder();
+		id<MTLArgumentEncoder> mtlArgEncoder = mvkArgEnc.getMTLArgumentEncoder();
+		if (useDescSetArgBuff) {
 			mtlArgBuffer = descSet->getMetalArgumentBuffer();
 			metalArgBufferOffset = descSet->getMetalArgumentBufferOffset();
 		} else {
-			mtlArgEncoder = pipeline->getMTLArgumentEncoder(dsIdx, stage).getMTLArgumentEncoder();
 			// TODO: Source a different arg buffer & offset for each pipeline-stage/desccriptors set
 			// Also need to only encode the descriptors that are referenced in the shader.
 			// MVKMTLArgumentEncoder could include an MVKBitArray to track that and have it checked below.

MoltenVK/MoltenVK/GPUObjects/MVKDescriptorSet.h

@@ -34,8 +34,13 @@ class MVKResourcesCommandEncoderState;
 #pragma mark -
 #pragma mark MVKDescriptorSetLayout
 
-/** Holds and manages the lifecycle of a MTLArgumentEncoder. The encoder can only be set once. */
+/**
+ * Holds and manages the lifecycle of a MTLArgumentEncoder. The encoder can
+ * only be set once, and copying this object results in an uninitialized
+ * empty object, since mutex and MTLArgumentEncoder can/should not be copied.
+ */
 struct MVKMTLArgumentEncoder {
+	std::mutex mtlArgumentEncodingLock;
 	NSUInteger mtlArgumentEncoderSize = 0;
 
 	id<MTLArgumentEncoder> getMTLArgumentEncoder() { return _mtlArgumentEncoder; }
@@ -44,6 +49,10 @@ struct MVKMTLArgumentEncoder {
 		_mtlArgumentEncoder = mtlArgEnc;		// takes ownership
 		mtlArgumentEncoderSize = mtlArgEnc.encodedLength;
 	}
+
+	MVKMTLArgumentEncoder(const MVKMTLArgumentEncoder& other) {}
+	MVKMTLArgumentEncoder& operator=(const MVKMTLArgumentEncoder& other) { return *this; }
+	MVKMTLArgumentEncoder() {}
 	~MVKMTLArgumentEncoder() { [_mtlArgumentEncoder release]; }
 
 private:

MoltenVK/MoltenVK/GPUObjects/MVKPipeline.h

@@ -188,9 +188,6 @@ public:
 	/** Returns the number of descriptor sets in this pipeline layout. */
 	uint32_t getDescriptorSetCount() { return _descriptorSetCount; }
 
-	/** A mutex lock to protect access to the Metal argument encoders. */
-	std::mutex _mtlArgumentEncodingLock;
-
 	/** Constructs an instance for the device. layout, and parent (which may be NULL). */
 	MVKPipeline(MVKDevice* device, MVKPipelineCache* pipelineCache, MVKPipelineLayout* layout, MVKPipeline* parent);