Don't fire DiagnosticSource.StopActivity in some cases (#9595)

- Don't fire the event if DiagnosticSource.StartActivity was never called. This avoids allocating the anonymous object and extra strings.

src/Hosting/Hosting/src/Internal/HostingApplication.cs

@@ -62,6 +62,7 @@ namespace Microsoft.AspNetCore.Hosting.Internal
             public long StartTimestamp { get; set; }
             public bool EventLogEnabled { get; set; }
             public Activity Activity { get; set; }
+            internal bool HasDiagnosticListener { get; set; }
         }
     }
 }

src/Hosting/Hosting/src/Internal/HostingApplicationDiagnostics.cs

@@ -48,7 +48,8 @@ namespace Microsoft.AspNetCore.Hosting.Internal
 
             if (loggingEnabled || (diagnosticListenerEnabled && _diagnosticListener.IsEnabled(ActivityName, httpContext)))
             {
-                context.Activity = StartActivity(httpContext);
+                context.Activity = StartActivity(httpContext, out var hasDiagnosticListener);
+                context.HasDiagnosticListener = hasDiagnosticListener;
             }
 
             if (diagnosticListenerEnabled)
@@ -132,7 +133,7 @@ namespace Microsoft.AspNetCore.Hosting.Internal
             // Always stop activity if it was started
             if (activity != null)
             {
-                StopActivity(httpContext, activity);
+                StopActivity(httpContext, activity, context.HasDiagnosticListener);
             }
 
             if (context.EventLogEnabled && exception != null)
@@ -229,9 +230,10 @@ namespace Microsoft.AspNetCore.Hosting.Internal
         }
 
         [MethodImpl(MethodImplOptions.NoInlining)]
-        private Activity StartActivity(HttpContext httpContext)
+        private Activity StartActivity(HttpContext httpContext, out bool hasDiagnosticListener)
         {
             var activity = new Activity(ActivityName);
+            hasDiagnosticListener = false;
 
             var headers = httpContext.Request.Headers;
             if (!headers.TryGetValue(HeaderNames.TraceParent, out var requestId))
@@ -266,6 +268,7 @@ namespace Microsoft.AspNetCore.Hosting.Internal
 
             if (_diagnosticListener.IsEnabled(ActivityStartKey))
             {
+                hasDiagnosticListener = true;
                 _diagnosticListener.StartActivity(activity, new { HttpContext = httpContext });
             }
             else
@@ -277,9 +280,16 @@ namespace Microsoft.AspNetCore.Hosting.Internal
         }
 
         [MethodImpl(MethodImplOptions.NoInlining)]
-        private void StopActivity(HttpContext httpContext, Activity activity)
+        private void StopActivity(HttpContext httpContext, Activity activity, bool hasDiagnosticListener)
         {
-            _diagnosticListener.StopActivity(activity, new { HttpContext = httpContext });
+            if (hasDiagnosticListener)
+            {
+                _diagnosticListener.StopActivity(activity, new { HttpContext = httpContext });
+            }
+            else
+            {
+                activity.Stop();
+            }
         }
     }
 }

src/Hosting/Hosting/test/HostingApplicationTests.cs

@@ -56,6 +56,51 @@ namespace Microsoft.AspNetCore.Hosting.Tests
             Assert.Equal(Activity.Current.Id, pairs["ActivityId"].ToString());
         }
 
+        [Fact]
+        public void ActivityStopDoesNotFireIfNoListenerAttachedForStart()
+        {
+            // Arrange
+            var diagnosticSource = new DiagnosticListener("DummySource");
+            var logger = new LoggerWithScopes(isEnabled: true);
+            var hostingApplication = CreateApplication(out var features, diagnosticSource: diagnosticSource, logger: logger);
+            var startFired = false;
+            var stopFired = false;
+
+            diagnosticSource.Subscribe(new CallbackDiagnosticListener(pair =>
+            {
+                // This should not fire
+                if (pair.Key == "Microsoft.AspNetCore.Hosting.HttpRequestIn.Start")
+                {
+                    startFired = true;
+                }
+
+                // This should not fire
+                if (pair.Key == "Microsoft.AspNetCore.Hosting.HttpRequestIn.Stop")
+                {
+                    stopFired = true;
+                }
+            }),
+            (s, o, arg3) =>
+            {
+                // The events are off
+                return false;
+            });
+
+
+            // Act
+            var context = hostingApplication.CreateContext(features);
+
+            Assert.Single(logger.Scopes);
+            var pairs = ((IReadOnlyList<KeyValuePair<string, object>>)logger.Scopes[0]).ToDictionary(p => p.Key, p => p.Value);
+            Assert.Equal(Activity.Current.Id, pairs["ActivityId"].ToString());
+
+            hostingApplication.DisposeContext(context, exception: null);
+
+            Assert.False(startFired);
+            Assert.False(stopFired);
+            Assert.Null(Activity.Current);
+        }
+
         [Fact]
         public void ActivityIsNotCreatedWhenIsEnabledForActivityIsFalse()
         {