Add event for marking the Start/Stop of source generation (#258)

* Add event for marking the Start/Stop of source generation

DllImportGenerator/DllImportGenerator/Diagnostics/Events.cs

@@ -0,0 +1,63 @@
+using System;
+using System.Diagnostics.Tracing;
+
+namespace Microsoft.Interop.Diagnostics
+{
+    [EventSource(Name = "Microsoft-Interop-SourceGeneration-Events")]
+    internal sealed class Events : EventSource
+    {
+        public static class Keywords
+        {
+            public const EventKeywords SourceGeneration = (EventKeywords)1;
+        }
+
+        public static readonly Events Logger = new Events();
+
+        private const int StartSourceGenerationEventId = 1;
+        private const int StopSourceGenerationEventId = StartSourceGenerationEventId + 1;
+
+        private Events()
+        { }
+
+        /// <summary>
+        /// Utility function that wraps emitting start/stop events for the source generation event.
+        /// </summary>
+        /// <param name="methodCount">The number of methods being generated</param>
+        /// <returns>An <see cref="IDisposable"/> instance that will fire the "stop" event when Disposed.</returns>
+        [NonEvent]
+        public static IDisposable SourceGenerationStartStop(int methodCount)
+        {
+            return new StartStopEvent(methodCount);
+        }
+
+        // N.B. The 'Start' and 'Stop' suffixes for event names (i.e. "xxxStart" and "xxxStop")
+        //  have special meaning in EventSource. They enable creating 'activities' if they are
+        //  paired and the Stop event's ID is +1 the Start event's ID.
+        //  See https://blogs.msdn.microsoft.com/vancem/2015/09/14/exploring-eventsource-activity-correlation-and-causation-features/
+
+        /// <summary>
+        /// Indicates the interop's DllImport Roslyn Source Generator has started source generation.
+        /// </summary>
+        /// <param name="methodCount">The number of methods being generated</param>
+        [Event(StartSourceGenerationEventId, Level = EventLevel.Informational, Keywords = Keywords.SourceGeneration)]
+        public void SourceGenerationStart(int methodCount)
+        {
+            this.WriteEvent(StartSourceGenerationEventId, methodCount);
+        }
+
+        /// <summary>
+        /// Indicates the interop's DllImport Roslyn Source Generator has stopped source generation.
+        /// </summary>
+        [Event(StopSourceGenerationEventId, Level = EventLevel.Informational, Keywords = Keywords.SourceGeneration)]
+        public void SourceGenerationStop()
+        {
+            this.WriteEvent(StopSourceGenerationEventId);
+        }
+
+        private class StartStopEvent : IDisposable
+        {
+            public StartStopEvent(int methodCount) => Logger.SourceGenerationStart(methodCount);
+            public void Dispose() => Logger.SourceGenerationStop();
+        }
+    }
+}

DllImportGenerator/DllImportGenerator/DllImportGenerator.cs

@@ -28,6 +28,9 @@ public void Execute(GeneratorExecutionContext context)
                 return;
             }
 
+            // Fire the start/stop pair for source generation
+            using var _ = Diagnostics.Events.SourceGenerationStartStop(synRec.Methods.Count);
+
             // Store a mapping between SyntaxTree and SemanticModel.
             // SemanticModels cache results and since we could be looking at
             // method declarations in the same SyntaxTree we want to benefit from