Check HasStarted before calling Flush/Start in StartAsync (#8041)

Author: jkotalik

src/Http/Http/src/Internal/DefaultHttpResponse.cs

@@ -151,6 +151,11 @@ public override void Redirect(string location, bool permanent)
 
         public override Task StartAsync(CancellationToken cancellationToken = default)
         {
+            if (HasStarted)
+            {
+                return Task.CompletedTask;
+            }
+
             if (HttpResponseStartFeature == null)
             {
                 return HttpResponseFeature.Body.FlushAsync(cancellationToken);

src/Http/Http/test/Internal/DefaultHttpResponseTests.cs

@@ -110,6 +110,10 @@ public async Task ResponseStart_CallsFeatureIfSet()
             mock.Setup(o => o.StartAsync(It.IsAny<CancellationToken>())).Returns(Task.CompletedTask);
             features.Set(mock.Object);
 
+            var responseMock = new Mock<IHttpResponseFeature>();
+            responseMock.Setup(o => o.HasStarted).Returns(false);
+            features.Set(responseMock.Object);
+
             var context = new DefaultHttpContext(features);
             await context.Response.StartAsync();
 
@@ -126,12 +130,35 @@ public async Task ResponseStart_CallsFeatureIfSetWithProvidedCancellationToken()
             mock.Setup(o => o.StartAsync(It.Is<CancellationToken>((localCt) => localCt.Equals(ct)))).Returns(Task.CompletedTask);
             features.Set(mock.Object);
 
+            var responseMock = new Mock<IHttpResponseFeature>();
+            responseMock.Setup(o => o.HasStarted).Returns(false);
+            features.Set(responseMock.Object);
+
             var context = new DefaultHttpContext(features);
             await context.Response.StartAsync(ct);
 
             mock.Verify(m => m.StartAsync(default), Times.Once());
         }
 
+        [Fact]
+        public async Task ResponseStart_DoesNotCallStartIfHasStartedIsTrue()
+        {
+            var features = new FeatureCollection();
+
+            var startMock = new Mock<IHttpResponseStartFeature>();
+            startMock.Setup(o => o.StartAsync(It.IsAny<CancellationToken>())).Returns(Task.CompletedTask);
+            features.Set(startMock.Object);
+
+            var responseMock = new Mock<IHttpResponseFeature>();
+            responseMock.Setup(o => o.HasStarted).Returns(true);
+            features.Set(responseMock.Object);
+
+            var context = new DefaultHttpContext(features);
+            await context.Response.StartAsync();
+
+            startMock.Verify(m => m.StartAsync(default), Times.Never());
+        }
+
         [Fact]
         public async Task ResponseStart_CallsResponseBodyFlushIfNotSet()
         {