Gracefully handle exceptions thrown from OnStarting callbacks

- If OnStarting is being called after the app func has completed, return a 500.
- If Onstarting is being called due to a call to write, throw from write.

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

@@ -25,7 +25,6 @@ namespace Microsoft.AspNet.Server.Kestrel.Http
 
         static Encoding _ascii = Encoding.ASCII;
         Mode _mode;
-        private bool _resultStarted;
         private bool _responseStarted;
         private bool _keepAlive;
         private readonly FrameRequestHeaders _requestHeaders = new FrameRequestHeaders();
@@ -246,6 +245,14 @@ namespace Microsoft.AspNet.Server.Kestrel.Http
             try
             {
                 await Application.Invoke(this).ConfigureAwait(false);
+
+                // Trigger FireOnStarting if ProduceStart hasn't been called yet.
+                // We call it here, so it can go through our normal error handling
+                // and respond with a 500 if an OnStarting callback throws.
+                if (!_responseStarted)
+                {
+                    FireOnStarting();
+                }
             }
             catch (Exception ex)
             {
@@ -284,7 +291,7 @@ namespace Microsoft.AspNet.Server.Kestrel.Http
 
         public void ProduceContinue()
         {
-            if (_resultStarted) return;
+            if (_responseStarted) return;
 
             string[] expect;
             if (HttpVersion.Equals("HTTP/1.1") &&
@@ -306,11 +313,9 @@ namespace Microsoft.AspNet.Server.Kestrel.Http
 
         public void ProduceStart(bool immediate = true)
         {
-            if (_resultStarted) return;
+            // ProduceStart shouldn't no-op in the future just b/c FireOnStarting throws.
-            _resultStarted = true;
+            if (_responseStarted) return;
-
             FireOnStarting();
-
             _responseStarted = true;
 
             var status = ReasonPhrases.ToStatus(StatusCode, ReasonPhrase);
@@ -334,7 +339,7 @@ namespace Microsoft.AspNet.Server.Kestrel.Http
         {
             if (ex != null)
             {
-                if (_resultStarted)
+                if (_responseStarted)
                 {
                     // We can no longer respond with a 500, so we simply close the connection.
                     ConnectionControl.End(ProduceEndType.SocketDisconnect);

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

@@ -535,5 +535,92 @@ namespace Microsoft.AspNet.Server.KestrelTests
                 }
             }
         }
+
+        [Fact]
+        public async Task ThrowingInOnStartingResultsIn500Response()
+        {
+            using (var server = new TestServer(frame =>
+            {
+                frame.OnStarting(_ =>
+                {
+                    throw new Exception();
+                }, null);
+
+                frame.ResponseHeaders.Clear();
+                frame.ResponseHeaders["Content-Length"] = new[] { "11" };
+
+                // If we write to the response stream, we will not get a 500.
+
+                return Task.FromResult<object>(null);
+            }))
+            {
+                using (var connection = new TestConnection())
+                {
+                    await connection.SendEnd(
+                        "GET / HTTP/1.1",
+                        "",
+                        "GET / HTTP/1.1",
+                        "Connection: close",
+                        "",
+                        "");
+                    await connection.Receive(
+                        "HTTP/1.1 500 Internal Server Error",
+                        "");
+                    await connection.ReceiveStartsWith("Date:");
+                    await connection.Receive(
+                        "Content-Length: 0",
+                        "Server: Kestrel",
+                        "",
+                        "HTTP/1.1 500 Internal Server Error",
+                        "");
+                    await connection.ReceiveStartsWith("Date:");
+                    await connection.ReceiveEnd(
+                        "Content-Length: 0",
+                        "Server: Kestrel",
+                        "Connection: close",
+                        "",
+                        "");
+                }
+            }
+        }
+
+        [Fact]
+        public async Task ThrowingInOnStartingResultsInFailedWrites()
+        {
+            using (var server = new TestServer(async frame =>
+            {
+                var onStartingException = new Exception();
+
+                frame.OnStarting(_ =>
+                {
+                    throw onStartingException;
+                }, null);
+
+                frame.ResponseHeaders.Clear();
+                frame.ResponseHeaders["Content-Length"] = new[] { "11" };
+
+                var writeException = await Assert.ThrowsAsync<Exception>(async () =>
+                    await frame.ResponseBody.WriteAsync(Encoding.ASCII.GetBytes("Hello World"), 0, 11));
+
+                Assert.Same(onStartingException, writeException);
+
+                // The second write should succeed since the OnStarting callback will not be called again
+                await frame.ResponseBody.WriteAsync(Encoding.ASCII.GetBytes("Exception!!"), 0, 11);
+            }))
+            {
+                using (var connection = new TestConnection())
+                {
+                    await connection.Send(
+                        "GET / HTTP/1.1",
+                        "",
+                        "");
+                    await connection.Receive(
+                        "HTTP/1.1 200 OK",
+                        "Content-Length: 11",
+                        "",
+                        "Exception!!"); ;
+                }
+            }
+        }
     }
 }