Fix deletion of GPU counter MTLFence while it is being used by MTLCommandBuffer.

Move release of GPU counter MTLFence from MVKCommandEncoder destructor
to MTLCommandBuffer completion handler.

Docs/Whats_New.md

@@ -22,6 +22,7 @@ Released TBD
 - Support base vertex instance support in shader conversion.
 - Fix alignment between outputs and inputs between shader stages when using nested structures.
 - Fix issue where the depth component of a stencil-only renderpass attachment was incorrectly attempting to be stored.
+- Fix deletion of GPU counter `MTLFence` while it is being used by `MTLCommandBuffer`.
 - `MoltenVKShaderConverter` tool defaults to the highest MSL version supported on runtime OS.
 - Update *glslang* version, to use `python3` in *glslang* scripts, to replace missing `python` on *macOS 12.3*.
 - Update to latest SPIRV-Cross:

MoltenVK/MoltenVK/Commands/MVKCommandBuffer.h

@@ -29,7 +29,6 @@
 #include <unordered_map>
 
 class MVKCommandPool;
-class MVKQueue;
 class MVKQueueCommandBufferSubmission;
 class MVKCommandEncoder;
 class MVKCommandEncodingPool;
@@ -492,8 +491,6 @@ public:
 
 	MVKCommandEncoder(MVKCommandBuffer* cmdBuffer);
 
-	~MVKCommandEncoder() override;
-
 protected:
     void addActivatedQueries(MVKQueryPool* pQueryPool, uint32_t query, uint32_t queryCount);
     void finishQueries();

MoltenVK/MoltenVK/Commands/MVKCommandBuffer.mm

@@ -830,7 +830,14 @@ void MVKCommandEncoder::encodeTimestampStageCounterSamples() {}
 #endif
 
 id<MTLFence> MVKCommandEncoder::getStageCountersMTLFence() {
-	if ( !_stageCountersMTLFence ) { _stageCountersMTLFence = [getMTLDevice() newFence]; }		//retained
+	if ( !_stageCountersMTLFence ) {
+		// Create MTLFence as local ref and pass to completion handler
+		// block to release once MTLCommandBuffer no longer needs it.
+		id<MTLFence> mtlFence = [getMTLDevice() newFence];
+		[_mtlCmdBuffer addCompletedHandler: ^(id<MTLCommandBuffer> mcb) { [mtlFence release]; }];
+
+		_stageCountersMTLFence = mtlFence;		// retained
+	}
 	return _stageCountersMTLFence;
 }
 
@@ -899,10 +906,6 @@ MVKCommandEncoder::MVKCommandEncoder(MVKCommandBuffer* cmdBuffer) : MVKBaseDevic
 			_stageCountersMTLFence = nil;
 }
 
-MVKCommandEncoder::~MVKCommandEncoder() {
-	[_stageCountersMTLFence release];
-}
-
 
 #pragma mark -
 #pragma mark Support functions