Gracefully handle uncaught exceptions in user code when possible

src/Microsoft.AspNet.Server.Kestrel/Http/Frame.cs

@@ -317,6 +317,28 @@ namespace Microsoft.AspNet.Server.Kestrel.Http
 
         public void ProduceEnd(Exception ex)
         {
+            if (ex != null)
+            {
+                if (_resultStarted)
+                {
+                    // We can no longer respond with a 500, so we simply close the connection.
+                    ConnectionControl.End(ProduceEndType.SocketDisconnect);
+                    return;
+                }
+                else
+                {
+                    StatusCode = 500;
+                    ReasonPhrase = null;
+
+                    // If OnStarting hasn't been triggered yet, we don't want to trigger it now that
+                    // the app func has failed. https://github.com/aspnet/KestrelHttpServer/issues/43
+                    _onStarting = null;
+
+                    ResponseHeaders.Clear();
+                    ResponseHeaders["Content-Length"] = new[] { "0" };
+                }
+            }
+
             ProduceStart();
 
             if (!_keepAlive)

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

@@ -18,7 +18,7 @@ namespace Microsoft.AspNet.Server.KestrelTests
     /// <summary>
     /// Summary description for EngineTests
     /// </summary>
-    internal class EngineTests
+    public class EngineTests
     {
         private async Task App(Frame frame)
         {
@@ -57,6 +57,20 @@ namespace Microsoft.AspNet.Server.KestrelTests
             }
         }
 
+        private async Task AppThatThrows(Frame frame)
+        {
+            // Anything added to the ResponseHeaders dictionary is ignored
+            frame.ResponseHeaders["Content-Length"] = new[] { "11" };
+            throw new Exception();
+        }
+
+        private async Task AppThatThrowsAfterWrite(Frame frame)
+        {
+            frame.ResponseHeaders["Content-Length"] = new[] { "11" };
+            await frame.ResponseBody.WriteAsync(Encoding.UTF8.GetBytes("Hello World"), 0, 11);
+            throw new Exception();
+        }
+
         private async Task AppChunked(Frame frame)
         {
             var data = new MemoryStream();
@@ -343,5 +357,52 @@ namespace Microsoft.AspNet.Server.KestrelTests
                 }
             }
         }
+
+        [Fact]
+        public async Task ThrowingResultsIn500Response()
+        {
+            using (var server = new TestServer(AppThatThrows))
+            {
+                using (var connection = new TestConnection())
+                {
+                    await connection.SendEnd(
+                        "GET / HTTP/1.1",
+                        "",
+                        "GET / HTTP/1.1",
+                        "Connection: close",
+                        "",
+                        "");
+                    await connection.ReceiveEnd(
+                        "HTTP/1.1 500 Internal Server Error",
+                        "Content-Length: 0",
+                        "",
+                        "HTTP/1.1 500 Internal Server Error",
+                        "Content-Length: 0",
+                        "Connection: close",
+                        "",
+                        "");
+                }
+            }
+        }
+
+        [Fact]
+        public async Task ThrowingAfterWritingKillsConnection()
+        {
+            using (var server = new TestServer(AppThatThrowsAfterWrite))
+            {
+                using (var connection = new TestConnection())
+                {
+                    await connection.Send(
+                        "GET / HTTP/1.1",
+                        "",
+                        "");
+                    await connection.ReceiveEnd(
+                        "HTTP/1.1 200 OK",
+                        "Content-Length: 11",
+                        "",
+                        "Hello World");
+                }
+            }
+        }
     }
 }