Don't close connection when Content-Length set but no bytes written.

src/Microsoft.AspNetCore.Server.Kestrel/Internal/Http/Frame.cs

@@ -695,7 +695,13 @@ namespace Microsoft.AspNetCore.Server.Kestrel.Internal.Http
                 responseHeaders.HeaderContentLengthValue.HasValue &&
                 _responseBytesWritten < responseHeaders.HeaderContentLengthValue.Value)
             {
-                _keepAlive = false;
+                // We need to close the connection if any bytes were written since the client
+                // cannot be certain of how many bytes it will receive.
+                if (_responseBytesWritten > 0)
+                {
+                    _keepAlive = false;
+                }
+
                 ReportApplicationError(new InvalidOperationException(
                     $"Response Content-Length mismatch: too few bytes written ({_responseBytesWritten} of {responseHeaders.HeaderContentLengthValue.Value})."));
             }

test/Microsoft.AspNetCore.Server.Kestrel.FunctionalTests/ResponseTests.cs

@@ -668,7 +668,7 @@ namespace Microsoft.AspNetCore.Server.Kestrel.FunctionalTests
         }
 
         [Fact]
-        public async Task WhenAppSetsContentLengthButDoesNotWriteBody500ResponseSentAndConnectionCloses()
+        public async Task WhenAppSetsContentLengthButDoesNotWriteBody500ResponseSentAndConnectionDoesNotClose()
         {
             var testLogger = new TestApplicationErrorLogger();
             var serviceContext = new TestServiceContext { Log = new TestKestrelTrace(testLogger) };
@@ -682,12 +682,17 @@ namespace Microsoft.AspNetCore.Server.Kestrel.FunctionalTests
                 using (var connection = server.CreateConnection())
                 {
                     await connection.Send(
+                        "GET / HTTP/1.1",
+                        "",
                         "GET / HTTP/1.1",
                         "",
                         "");
-                    await connection.ReceiveForcedEnd(
+                    await connection.Receive(
-                        $"HTTP/1.1 500 Internal Server Error",
+                        "HTTP/1.1 500 Internal Server Error",
-                        "Connection: close",
+                        $"Date: {server.Context.DateHeaderValue}",
+                        "Content-Length: 0",
+                        "",
+                        "HTTP/1.1 500 Internal Server Error",
                         $"Date: {server.Context.DateHeaderValue}",
                         "Content-Length: 0",
                         "",
@@ -695,10 +700,9 @@ namespace Microsoft.AspNetCore.Server.Kestrel.FunctionalTests
                 }
             }
 
-            var errorMessage = Assert.Single(testLogger.Messages, message => message.LogLevel == LogLevel.Error);
+            var error = testLogger.Messages.Where(message => message.LogLevel == LogLevel.Error);
-            Assert.Equal(
+            Assert.Equal(2, error.Count());
-                $"Response Content-Length mismatch: too few bytes written (0 of 5).",
+            Assert.All(error, message => message.Equals("Response Content-Length mismatch: too few bytes written (0 of 5)."));
-                errorMessage.Exception.Message);
         }
 
         [Theory]

test/Microsoft.AspNetCore.Server.KestrelTests/EngineTests.cs

@@ -853,12 +853,10 @@ namespace Microsoft.AspNetCore.Server.KestrelTests
         [MemberData(nameof(ConnectionAdapterData))]
         public async Task ThrowingInOnStartingResultsInFailedWritesAnd500Response(ListenOptions listenOptions)
         {
+            var callback1Called = false;
+            var callback2CallCount = 0;
+
             var testContext = new TestServiceContext();
-
-            var onStartingCallCount1 = 0;
-            var onStartingCallCount2 = 0;
-            var failedWriteCount = 0;
-
             var testLogger = new TestApplicationErrorLogger();
             testContext.Log = new KestrelTrace(testLogger);
 
@@ -869,19 +867,17 @@ namespace Microsoft.AspNetCore.Server.KestrelTests
                 var response = httpContext.Response;
                 response.OnStarting(_ =>
                 {
-                    onStartingCallCount1++;
+                    callback1Called = true;
                     throw onStartingException;
                 }, null);
                 response.OnStarting(_ =>
                 {
-                    onStartingCallCount2++;
+                    callback2CallCount++;
                     throw onStartingException;
                 }, null);
 
                 var writeException = await Assert.ThrowsAsync<ObjectDisposedException>(async () => await response.Body.FlushAsync());
                 Assert.Same(onStartingException, writeException.InnerException);
-
-                failedWriteCount++;
             }, testContext, listenOptions))
             {
                 using (var connection = server.CreateConnection())
@@ -892,7 +888,7 @@ namespace Microsoft.AspNetCore.Server.KestrelTests
                         "GET / HTTP/1.1",
                         "",
                         "");
-                    await connection.Receive(
+                    await connection.ReceiveEnd(
                         "HTTP/1.1 500 Internal Server Error",
                         $"Date: {testContext.DateHeaderValue}",
                         "Content-Length: 0",
@@ -905,10 +901,10 @@ namespace Microsoft.AspNetCore.Server.KestrelTests
                 }
             }
 
-            // The first registered OnStarting callback should not be called,
+            // The first registered OnStarting callback should have been called,
-            // since they are called LIFO and the other one failed.
+            // since they are called LIFO order and the other one failed.
-            Assert.Equal(0, onStartingCallCount1);
+            Assert.False(callback1Called);
-            Assert.Equal(2, onStartingCallCount2);
+            Assert.Equal(2, callback2CallCount);
             Assert.Equal(2, testLogger.ApplicationErrorsLogged);
         }