Avoid zero-byte send in WebSockets (#3326)

Author: BrennanConroy

src/Common/WebSocketExtensions.cs

@@ -39,22 +39,28 @@ public static ValueTask SendAsync(this WebSocket webSocket, ReadOnlySequence<byt
         private static async ValueTask SendMultiSegmentAsync(WebSocket webSocket, ReadOnlySequence<byte> buffer, WebSocketMessageType webSocketMessageType, CancellationToken cancellationToken = default)
         {
             var position = buffer.Start;
+            // Get a segment before the loop so we can be one segment behind while writing
+            // This allows us to do a non-zero byte write for the endOfMessage = true send
+            buffer.TryGet(ref position, out var prevSegment);
             while (buffer.TryGet(ref position, out var segment))
             {
 #if NETCOREAPP3_0
-                await webSocket.SendAsync(segment, webSocketMessageType, endOfMessage: false, cancellationToken);
+                await webSocket.SendAsync(prevSegment, webSocketMessageType, endOfMessage: false, cancellationToken);
 #else
-                var isArray = MemoryMarshal.TryGetArray(segment, out var arraySegment);
+                var isArray = MemoryMarshal.TryGetArray(prevSegment, out var arraySegment);
                 Debug.Assert(isArray);
                 await webSocket.SendAsync(arraySegment, webSocketMessageType, endOfMessage: false, cancellationToken);
 #endif
+                prevSegment = segment;
             }
 
-            // Empty end of message frame
+            // End of message frame
 #if NETCOREAPP3_0
-            await webSocket.SendAsync(Memory<byte>.Empty, webSocketMessageType, endOfMessage: true, cancellationToken);
+            await webSocket.SendAsync(prevSegment, webSocketMessageType, endOfMessage: true, cancellationToken);
 #else
-            await webSocket.SendAsync(new ArraySegment<byte>(Array.Empty<byte>()), webSocketMessageType, endOfMessage: true, cancellationToken);
+            var isArrayEnd = MemoryMarshal.TryGetArray(prevSegment, out var arraySegmentEnd);
+            Debug.Assert(isArrayEnd);
+            await webSocket.SendAsync(arraySegmentEnd, webSocketMessageType, endOfMessage: true, cancellationToken);
 #endif
         }
     }

test/Microsoft.AspNetCore.Http.Connections.Tests/WebSocketsTests.cs

@@ -396,5 +396,36 @@ public async Task SubProtocolSelectorIsUsedToSelectSubProtocol()
                 }
             }
         }
+
+        [Fact]
+        public async Task MultiSegmentSendWillNotSendEmptyEndOfMessageFrame()
+        {
+            using (var feature = new TestWebSocketConnectionFeature())
+            {
+                var serverSocket = await feature.AcceptAsync();
+                var sequence = ReadOnlySequenceFactory.CreateSegments(new byte[] { 1 }, new byte[] { 15 });
+                Assert.False(sequence.IsSingleSegment);
+
+                await serverSocket.SendAsync(sequence, WebSocketMessageType.Text);
+
+                // Run the client socket
+                var client = feature.Client.ExecuteAndCaptureFramesAsync();
+
+                await serverSocket.CloseAsync(WebSocketCloseStatus.NormalClosure, "", default);
+
+                var messages = await client.OrTimeout();
+                Assert.Equal(2, messages.Received.Count);
+
+                // First message: 1 byte, endOfMessage false
+                Assert.Single(messages.Received[0].Buffer);
+                Assert.Equal(1, messages.Received[0].Buffer[0]);
+                Assert.False(messages.Received[0].EndOfMessage);
+
+                // Second message: 1 byte, endOfMessage true
+                Assert.Single(messages.Received[1].Buffer);
+                Assert.Equal(15, messages.Received[1].Buffer[0]);
+                Assert.True(messages.Received[1].EndOfMessage);
+            }
+        }
     }
 }