Adds Keep Alive message to SSE

src/HotChocolate/AspNetCore/test/AspNetCore.Tests.Utilities/SubscriptionsExtensions.cs

@@ -32,4 +32,19 @@ public class SubscriptionsExtensions
 
         throw new GraphQLException(ErrorBuilder.New().SetMessage("Foo").Build());
     }
+
+#pragma warning disable CS0618
+    [SubscribeAndResolve]
+#pragma warning restore CS0618
+    public async IAsyncEnumerable<string> Delay(
+        [EnumeratorCancellation] CancellationToken cancellationToken,
+        int delay,
+        int count)
+    {
+        while (!cancellationToken.IsCancellationRequested && count-- > 0)
+        {
+            yield return "next";
+            await Task.Delay(delay, cancellationToken);
+        }
+    }
 }

src/HotChocolate/AspNetCore/test/AspNetCore.Tests/GraphQLOverHttpSpecTests.cs

@@ -451,14 +451,59 @@ public async Task DefferedQuery_NoStreamableAcceptHeader()
                 {""errors"":[{""message"":""The specified operation kind is not allowed.""}]}");
     }
 
+    [Fact]
+    public async Task EventStream_Sends_KeepAlive()
+    {
+        // arrange
+        var server = CreateStarWarsServer();
+        var client = server.CreateClient();
+        client.Timeout = TimeSpan.FromSeconds(30);
+
+        // act
+        using var request = new HttpRequestMessage(HttpMethod.Post, _url)
+        {
+            Content = JsonContent.Create(
+                new ClientQueryRequest { Query = "subscription {delay(count: 2, delay:15000)}" }),
+            Headers = { { "Accept", "text/event-stream" } }
+        };
+
+        using var response = await client.SendAsync(request, ResponseHeadersRead);
+
+        // assert
+        Snapshot
+            .Create()
+            .Add(response)
+            .MatchInline("""
+                Headers:
+                Cache-Control: no-cache
+                Content-Type: text/event-stream; charset=utf-8
+                -------------------------->
+                Status Code: OK
+                -------------------------->
+                event: next
+                data: {"data":{"delay":"next"}}
+
+                :
+
+                event: next
+                data: {"data":{"delay":"next"}}
+
+                :
+
+                event: complete
+
+
+                """);
+    }
+
     private HttpClient GetClient(HttpTransportVersion serverTransportVersion)
     {
         var server = CreateStarWarsServer(
-               configureServices: s => s.AddHttpResponseFormatter(
-                   new HttpResponseFormatterOptions
-                   {
-                       HttpTransportVersion = serverTransportVersion
-                   }));
+            configureServices: s => s.AddHttpResponseFormatter(
+                new HttpResponseFormatterOptions
+                {
+                    HttpTransportVersion = serverTransportVersion
+                }));
 
         return server.CreateClient();
     }

src/HotChocolate/AspNetCore/test/AspNetCore.Tests/__snapshots__/HttpGetSchemaMiddlewareTests.Download_GraphQL_SDL.snap

@@ -92,6 +92,7 @@ type Subscription {
   onReview(episode: Episode!): Review!
   onNext: String!
   onException: String!
+  delay(delay: Int! count: Int!): String!
 }
 
 union SearchResult = Starship | Human | Droid

src/HotChocolate/AspNetCore/test/AspNetCore.Tests/__snapshots__/HttpGetSchemaMiddlewareTests.Download_GraphQL_SDL_Explicit_Route.snap

@@ -92,6 +92,7 @@ type Subscription {
   onReview(episode: Episode!): Review!
   onNext: String!
   onException: String!
+  delay(delay: Int! count: Int!): String!
 }
 
 union SearchResult = Starship | Human | Droid

src/HotChocolate/AspNetCore/test/AspNetCore.Tests/__snapshots__/HttpGetSchemaMiddlewareTests.Download_GraphQL_SDL_Explicit_Route_Explicit_Pattern.snap

@@ -92,6 +92,7 @@ type Subscription {
   onReview(episode: Episode!): Review!
   onNext: String!
   onException: String!
+  delay(delay: Int! count: Int!): String!
 }
 
 union SearchResult = Starship | Human | Droid

src/HotChocolate/Core/src/Execution/Serialization/EventStreamResultFormatter.cs

@@ -1,6 +1,5 @@
 using System;
 using System.IO;
-using System.Text.Encodings.Web;
 using System.Text.Json;
 using System.Threading;
 using System.Threading.Tasks;
@@ -15,19 +14,14 @@ namespace HotChocolate.Execution.Serialization;
 /// </summary>
 public sealed class EventStreamResultFormatter : IExecutionResultFormatter
 {
-    private static readonly byte[] _eventField
-        = { (byte)'e', (byte)'v', (byte)'e', (byte)'n', (byte)'t', (byte)':', (byte)' ' };
-    private static readonly byte[] _dataField
-        = { (byte)'d', (byte)'a', (byte)'t', (byte)'a', (byte)':', (byte)' ' };
-    private static readonly byte[] _nextEvent
-        = { (byte)'n', (byte)'e', (byte)'x', (byte)'t' };
-    private static readonly byte[] _completeEvent
-        =
-        {
-            (byte)'c', (byte)'o', (byte)'m', (byte)'p',
-            (byte)'l', (byte)'e', (byte)'t', (byte)'e'
-        };
-    private static readonly byte[] _newLine = { (byte)'\n' };
+    private static readonly TimeSpan _keepAliveTimeSpan = TimeSpan.FromSeconds(12);
+
+    private static readonly byte[] _eventField = "event: "u8.ToArray();
+    private static readonly byte[] _dataField = "data: "u8.ToArray();
+    private static readonly byte[] _nextEvent = "next"u8.ToArray();
+    private static readonly byte[] _keepAlive = ":\n\n"u8.ToArray();
+    private static readonly byte[] _completeEvent = "complete"u8.ToArray();
+    private static readonly byte[] _newLine = "\n"u8.ToArray();
 
     private readonly JsonResultFormatter _payloadFormatter;
     private readonly JsonWriterOptions _options;
@@ -70,50 +64,121 @@ public EventStreamResultFormatter(JsonResultFormatterOptions options)
     {
         if (result.Kind is SingleResult)
         {
-            await WriteNextMessageAsync((IQueryResult)result, outputStream).ConfigureAwait(false);
+            await WriteNextMessageAsync((IQueryResult)result, outputStream, ct)
+                .ConfigureAwait(false);
             await WriteNewLineAndFlushAsync(outputStream, ct).ConfigureAwait(false);
-            await WriteCompleteMessage(outputStream).ConfigureAwait(false);
+            await WriteCompleteMessage(outputStream, ct).ConfigureAwait(false);
             await WriteNewLineAndFlushAsync(outputStream, ct).ConfigureAwait(false);
         }
         else if (result.Kind is DeferredResult or BatchResult or SubscriptionResult)
         {
             var responseStream = (IResponseStream)result;
 
-            await foreach (var queryResult in responseStream.ReadResultsAsync()
-                .WithCancellation(ct).ConfigureAwait(false))
-            {
-                try
-                {
-                    await WriteNextMessageAsync(queryResult, outputStream)
-                        .ConfigureAwait(false);
-                }
-                finally
-                {
-                    await queryResult.DisposeAsync().ConfigureAwait(false);
-                }
-
-                await WriteNewLineAndFlushAsync(outputStream, ct).ConfigureAwait(false);
-            }
-
-            await WriteCompleteMessage(outputStream).ConfigureAwait(false);
-            await WriteNewLineAndFlushAsync(outputStream, ct).ConfigureAwait(false);
+            // synchronization of the output stream is required to ensure that the messages are not
+            // interleaved.
+            using var synchronization = new SemaphoreSlim(1, 1);
+
+            // we need to keep track if the stream is completed so that we can stop sending keep
+            // alive messages.
+            var completion = new TaskCompletionSource<bool>();
+
+            // we await all tasks so that we can catch all exceptions.
+            await Task.WhenAll(
+                ProcessResponseStreamAsync(
+                    synchronization,
+                    completion,
+                    responseStream,
+                    outputStream,
+                    ct),
+                SendKeepAliveMessagesAsync(synchronization, completion, outputStream, ct));
         }
+
         else
         {
             throw new NotSupportedException();
         }
     }
 
-    private async ValueTask WriteNextMessageAsync(IQueryResult result, Stream outputStream)
+    private static async Task SendKeepAliveMessagesAsync(
+        SemaphoreSlim synchronization,
+        TaskCompletionSource<bool> completion,
+        Stream outputStream,
+        CancellationToken ct)
+    {
+        while (true)
+        {
+            await Task.WhenAny(Task.Delay(_keepAliveTimeSpan, ct), completion.Task);
+
+            if (!ct.IsCancellationRequested && !completion.Task.IsCompleted)
+            {
+                // we do not need try-finally here because we dispose the semaphore in the parent
+                // method.
+                await synchronization.WaitAsync(ct);
+
+                await WriteKeepAliveAndFlush(outputStream, ct);
+
+                synchronization.Release();
+            }
+            else
+            {
+                break;
+            }
+        }
+    }
+
+    private async Task ProcessResponseStreamAsync(
+        SemaphoreSlim synchronization,
+        TaskCompletionSource<bool> completion,
+        IResponseStream responseStream,
+        Stream outputStream,
+        CancellationToken ct)
+    {
+        await foreach (var queryResult in responseStream.ReadResultsAsync()
+                           .WithCancellation(ct)
+                           .ConfigureAwait(false))
+        {
+            // we do not need try-finally here because we dispose the semaphore in the parent
+            // method.
+
+            await synchronization.WaitAsync(ct);
+
+            try
+            {
+                await WriteNextMessageAsync(queryResult, outputStream, ct)
+                    .ConfigureAwait(false);
+            }
+            finally
+            {
+                await queryResult.DisposeAsync().ConfigureAwait(false);
+            }
+
+            await WriteNewLineAndFlushAsync(outputStream, ct).ConfigureAwait(false);
+
+            synchronization.Release();
+        }
+
+        await synchronization.WaitAsync(ct);
+
+        await WriteCompleteMessage(outputStream, ct).ConfigureAwait(false);
+        await WriteNewLineAndFlushAsync(outputStream, ct).ConfigureAwait(false);
+
+        synchronization.Release();
+        completion.SetResult(true);
+    }
+
+    private async ValueTask WriteNextMessageAsync(
+        IQueryResult result,
+        Stream outputStream,
+        CancellationToken ct)
     {
 #if NETCOREAPP3_1_OR_GREATER
-        await outputStream.WriteAsync(_eventField).ConfigureAwait(false);
-        await outputStream.WriteAsync(_nextEvent).ConfigureAwait(false);
-        await outputStream.WriteAsync(_newLine).ConfigureAwait(false);
+        await outputStream.WriteAsync(_eventField, ct).ConfigureAwait(false);
+        await outputStream.WriteAsync(_nextEvent, ct).ConfigureAwait(false);
+        await outputStream.WriteAsync(_newLine, ct).ConfigureAwait(false);
 #else
-        await outputStream.WriteAsync(_eventField, 0, _eventField.Length).ConfigureAwait(false);
-        await outputStream.WriteAsync(_nextEvent, 0, _nextEvent.Length).ConfigureAwait(false);
-        await outputStream.WriteAsync(_newLine, 0, _newLine.Length).ConfigureAwait(false);
+        await outputStream.WriteAsync(_eventField, 0, _eventField.Length, ct).ConfigureAwait(false);
+        await outputStream.WriteAsync(_nextEvent, 0, _nextEvent.Length, ct).ConfigureAwait(false);
+        await outputStream.WriteAsync(_newLine, 0, _newLine.Length, ct).ConfigureAwait(false);
 #endif
 
         using var bufferWriter = new ArrayWriter();
@@ -132,30 +197,31 @@ await using (var writer = new Utf8JsonWriter(bufferWriter, _options))
             }
 
 #if NETCOREAPP3_1_OR_GREATER
-            await outputStream.WriteAsync(_dataField).ConfigureAwait(false);
-            await outputStream.WriteAsync(buffer).ConfigureAwait(false);
-            await outputStream.WriteAsync(_newLine).ConfigureAwait(false);
+            await outputStream.WriteAsync(_dataField, ct).ConfigureAwait(false);
+            await outputStream.WriteAsync(buffer, ct).ConfigureAwait(false);
+            await outputStream.WriteAsync(_newLine, ct).ConfigureAwait(false);
 #else
-            await outputStream.WriteAsync(_dataField, 0, _dataField.Length).ConfigureAwait(false);
-            await outputStream.WriteAsync(bufferWriter.GetInternalBuffer(), read, buffer.Length)
+            await outputStream.WriteAsync(_dataField, 0, _dataField.Length, ct)
                 .ConfigureAwait(false);
-            await outputStream.WriteAsync(_newLine, 0, _newLine.Length).ConfigureAwait(false);
+            await outputStream.WriteAsync(bufferWriter.GetInternalBuffer(), read, buffer.Length, ct)
+                .ConfigureAwait(false);
+            await outputStream.WriteAsync(_newLine, 0, _newLine.Length, ct).ConfigureAwait(false);
 #endif
 
             read += buffer.Length + 1;
         }
     }
 
-    private static async ValueTask WriteCompleteMessage(Stream outputStream)
+    private static async ValueTask WriteCompleteMessage(Stream outputStream, CancellationToken ct)
     {
 #if NETCOREAPP3_1_OR_GREATER
-        await outputStream.WriteAsync(_eventField).ConfigureAwait(false);
-        await outputStream.WriteAsync(_completeEvent).ConfigureAwait(false);
-        await outputStream.WriteAsync(_newLine).ConfigureAwait(false);
+        await outputStream.WriteAsync(_eventField, ct).ConfigureAwait(false);
+        await outputStream.WriteAsync(_completeEvent, ct).ConfigureAwait(false);
+        await outputStream.WriteAsync(_newLine, ct).ConfigureAwait(false);
 #else
-        await outputStream.WriteAsync(_eventField, 0, _eventField.Length).ConfigureAwait(false);
-        await outputStream.WriteAsync(_completeEvent, 0, _completeEvent.Length).ConfigureAwait(false);
-        await outputStream.WriteAsync(_newLine, 0, _newLine.Length).ConfigureAwait(false);
+        await outputStream.WriteAsync(_eventField, 0, _eventField.Length, ct).ConfigureAwait(false);
+        await outputStream.WriteAsync(_completeEvent, 0, _completeEvent.Length, ct).ConfigureAwait(false);
+        await outputStream.WriteAsync(_newLine, 0, _newLine.Length, ct).ConfigureAwait(false);
 #endif
     }
 
@@ -167,6 +233,18 @@ private static async ValueTask WriteCompleteMessage(Stream outputStream)
         await outputStream.WriteAsync(_newLine, ct).ConfigureAwait(false);
 #else
         await outputStream.WriteAsync(_newLine, 0, _newLine.Length, ct).ConfigureAwait(false);
+#endif
+        await outputStream.FlushAsync(ct).ConfigureAwait(false);
+    }
+
+    private static async ValueTask WriteKeepAliveAndFlush(
+        Stream outputStream,
+        CancellationToken ct)
+    {
+#if NETCOREAPP3_1_OR_GREATER
+        await outputStream.WriteAsync(_keepAlive, ct).ConfigureAwait(false);
+#else
+        await outputStream.WriteAsync(_keepAlive, 0, _keepAlive.Length, ct).ConfigureAwait(false);
 #endif
         await outputStream.FlushAsync(ct).ConfigureAwait(false);
     }

src/HotChocolate/Utilities/test/Utilities.Introspection.Tests/__snapshots__/IntrospectionClientTests.Download_Schema_AST.snap

@@ -24,6 +24,7 @@ type Subscription {
   onReview(episode: Episode!): Review!
   onNext: String!
   onException: String!
+  delay(delay: Int! count: Int!): String!
 }
 
 type Human implements Character {

src/HotChocolate/Utilities/test/Utilities.Introspection.Tests/__snapshots__/IntrospectionClientTests.Download_Schema_SDL.snap

@@ -24,6 +24,7 @@ type Subscription {
   onReview(episode: Episode!): Review!
   onNext: String!
   onException: String!
+  delay(delay: Int! count: Int!): String!
 }
 
 type Human implements Character {