Remove mutex around command buffer completion handler

src/Veldrid/MTL/MTLGraphicsDevice.cs

@@ -25,13 +25,6 @@ private static readonly Dictionary<IntPtr, MTLGraphicsDevice> s_aotRegisteredBlo
         private readonly bool[] _supportedSampleCounts;
         private BackendInfoMetal _metalInfo;
 
-        private readonly object _submittedCommandsLock = new object();
-        private readonly Dictionary<MTLCommandBuffer, MTLCommandList> _submittedCLs = new Dictionary<MTLCommandBuffer, MTLCommandList>();
-        private MTLCommandBuffer _latestSubmittedCB;
-
-        private readonly object _resetEventsLock = new object();
-        private readonly List<ManualResetEvent[]> _resetEvents = new List<ManualResetEvent[]>();
-
         private const string UnalignedBufferCopyPipelineMacOSName = "MTL_UnalignedBufferCopy_macOS";
         private const string UnalignedBufferCopyPipelineiOSName = "MTL_UnalignedBufferCopy_iOS";
         private readonly object _unalignedBufferCopyPipelineLock = new object();
@@ -201,22 +194,7 @@ public MTLGraphicsDevice(
 
         public override GraphicsDeviceFeatures Features { get; }
 
-        private void OnCommandBufferCompleted(IntPtr block, MTLCommandBuffer cb)
-        {
-            lock (_submittedCommandsLock)
-            {
-                MTLCommandList cl = _submittedCLs[cb];
-                _submittedCLs.Remove(cb);
-                cl.OnCompleted(cb);
-
-                if (_latestSubmittedCB.NativePtr == cb.NativePtr)
-                {
-                    _latestSubmittedCB = default(MTLCommandBuffer);
-                }
-            }
-
-            ObjectiveCRuntime.release(cb.NativePtr);
-        }
+        private void OnCommandBufferCompleted(IntPtr block, MTLCommandBuffer cb) => ObjectiveCRuntime.release(cb.NativePtr);
 
         // Xamarin AOT requires native callbacks be static.
         [MonoPInvokeCallback(typeof(MTLCommandBufferHandler))]
@@ -240,12 +218,7 @@ private protected override void SubmitCommandsCore(CommandList commandList, Fenc
                 mtlCL.CommandBuffer.encodeSignalEvent(mtlFence.SharedEvent.NativePtr, MTLFence.SIGNALED);
 
             mtlCL.CommandBuffer.addCompletedHandler(_completionBlockLiteral);
-
-            lock (_submittedCommandsLock)
-            {
-                _submittedCLs.Add(mtlCL.CommandBuffer, mtlCL);
-                _latestSubmittedCB = mtlCL.Commit();
-            }
+            mtlCL.Commit();
         }
 
         private protected override void WaitForNextFrameReadyCore()
@@ -438,20 +411,8 @@ private protected override void UpdateTextureCore(
 
         private protected override void WaitForIdleCore()
         {
-            MTLCommandBuffer lastCB = default(MTLCommandBuffer);
-
-            lock (_submittedCommandsLock)
-            {
-                lastCB = _latestSubmittedCB;
-                ObjectiveCRuntime.retain(lastCB.NativePtr);
-            }
-
-            if (lastCB.NativePtr != IntPtr.Zero && lastCB.status != MTLCommandBufferStatus.Completed)
-            {
-                lastCB.waitUntilCompleted();
-            }
-
-            ObjectiveCRuntime.release(lastCB.NativePtr);
+            // todo: this may not work as it's not guaranteed that _latestSubmittedCB hasn't already been released yet.
+            throw new NotSupportedException("WaitForIdle is not supported on Metal currently. Use a fence and spin on the signal (because WaitForFence is also unsupported).");
         }
 
         protected override MappedResource MapCore(MappableResource resource, MapMode mode, uint subresource)