Fix EventSource never receiving the SSE complete event

src/HotChocolate/AspNetCore/src/Transport.Formatters/EventStreamResultFormatter.cs

@@ -404,7 +404,7 @@ public void Dispose()
     private static class MessageHelper
     {
         private static ReadOnlySpan<byte> NextEvent => "event: next\ndata: "u8;
-        private static ReadOnlySpan<byte> CompleteEvent => "event: complete\n\n"u8;
+        private static ReadOnlySpan<byte> CompleteEvent => "event: complete\ndata:\n\n"u8;
         private static ReadOnlySpan<byte> NewLine2 => "\n\n"u8;
 
         public static void FormatNextMessage(

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

@@ -238,6 +238,7 @@ ... @defer {
                 data: {"incremental":[{"id":"2","data":{"description":"Abc desc"}}],"completed":[{"id":"2"}],"hasNext":false}
 
                 event: complete
+                data:
 
 
                 """);
@@ -349,6 +350,7 @@ ... @defer {
                 data: {"incremental":[{"data":{"description":"Abc desc"},"path":["product"]}],"hasNext":false}
 
                 event: complete
+                data:
 
 
                 """);
@@ -401,6 +403,7 @@ ... @defer(label: "productDescription") {
                 data: {"incremental":[{"data":{"description":"Abc desc"},"path":["product"],"label":"productDescription"}],"hasNext":false}
 
                 event: complete
+                data:
 
 
                 """);
@@ -455,6 +458,7 @@ ... @defer(label: "productDescription") {
                 data: {"incremental":[{"id":"2","data":{"description":"Abc desc"}}],"completed":[{"id":"2"}],"hasNext":false}
 
                 event: complete
+                data:
 
 
                 """);
@@ -509,6 +513,7 @@ ... on Droid @defer(label: "droid_details") {
                 data: {"incremental":[{"id":"2","data":{"primaryFunction":"Astromech"}}],"completed":[{"id":"2"}],"hasNext":false}
 
                 event: complete
+                data:
 
 
                 """);

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

@@ -186,6 +186,7 @@ public async Task SingleResult_Should_WriteOneNextEvent_When_EventStreamIsAccept
                 data: {"data":{"__typename":"Query"}}
 
                 event: complete
+                data:
 
 
                 """);
@@ -421,6 +422,7 @@ public async Task EventStream_Sends_KeepAlive()
                 :
 
                 event: complete
+                data:
 
 
                 """);
@@ -471,6 +473,7 @@ public async Task EventStream_When_Accept_Is_All()
                 :
 
                 event: complete
+                data:
 
 
                 """);
@@ -521,6 +524,7 @@ public async Task EventStream_When_Accept_Is_All_And_Subscription_Directive()
                 :
 
                 event: complete
+                data:
 
 
                 """);

src/HotChocolate/AspNetCore/test/AspNetCore.Tests/Serialization/DeferredResultFormatterTests.cs

@@ -83,6 +83,7 @@ public async Task EventStream_Formats_New_Defer_Structure()
                 data: {{Payload2New}}
 
                 event: complete
+                data:
                 """),
             content);
     }
@@ -104,6 +105,7 @@ public async Task EventStream_Formats_Legacy_Defer_Structure()
                 data: {{Payload2Legacy}}
 
                 event: complete
+                data:
                 """),
             content);
     }

src/HotChocolate/AspNetCore/test/Transport.Http.Tests/GraphQLHttpClientTests.cs

@@ -2010,6 +2010,54 @@ public async Task ReadAsResultStream_Text_Event_Stream_Unknown_Event_With_Data_I
         Assert.Equal(0, count);
     }
 
+    [Fact]
+    public async Task ReadAsResultStream_Text_Event_Stream_Complete_With_Empty_Data_Ends_Stream()
+    {
+        // arrange
+        // A complete event with an empty data field ends the stream before the next event.
+        var ms = new MemoryStream();
+        var sw = new StreamWriter(ms);
+        sw.Write("event: next");
+        sw.Write('\n');
+        sw.Write("data: {\"data\":{\"number\":0}}");
+        sw.Write('\n');
+        sw.Write('\n');
+        sw.Write("event: complete");
+        sw.Write('\n');
+        sw.Write("data:");
+        sw.Write('\n');
+        sw.Write('\n');
+        sw.Write("event: next");
+        sw.Write('\n');
+        sw.Write("data: {\"data\":{\"number\":1}}");
+        sw.Write('\n');
+        sw.Write('\n');
+        sw.Flush();
+        ms.Position = 0;
+
+        var handler = new MockHttpMessageHandler(ms, "text/event-stream");
+        using var client = new DefaultGraphQLHttpClient(new HttpClient(handler));
+
+        var operationRequest = new OperationRequest("{ number }");
+        var request = new GraphQLHttpRequest(
+            operationRequest,
+            new Uri("http://localhost:5000/graphql"));
+
+        // act
+        using var result = await client.SendAsync(request, TestContext.Current.CancellationToken);
+        var stream = result.ReadAsResultStreamAsync();
+
+        // assert
+        var numbers = new List<int>();
+
+        await foreach (var document in stream)
+        {
+            numbers.Add(document.Data.GetProperty("number").GetInt32());
+        }
+
+        Assert.Equal(0, Assert.Single(numbers));
+    }
+
     [Fact]
     public async Task Post_Variables_Do_Not_Escape_Apostrophe_To_Unicode()
     {

website/content/docs/fusion/subscriptions.md

@@ -757,6 +757,7 @@ event: next
 data: {"data":{"onReviewAdded":{"body":"Works as described"}}}
 
 event: complete
+data:
 ```
 
 For details on defining subscriptions in a subgraph (the `[Subscribe]` and `[Topic]`

website/content/docs/hotchocolate/migrating/migrate-from-15-to-16.md

@@ -1687,3 +1687,14 @@ The default is **64**. Operations that arrive while the gate is full queue up an
 Every execution is bounded by the `ExecutionTimeout` option (default 30 seconds). This applies uniformly to queries, mutations, subscription handshakes, and each subscription event. The budget covers both the time an execution spends waiting for a concurrency slot and the time it spends running. When the budget is exceeded, the execution is cancelled and the caller receives a clean timeout error. `ExecutionTimeout` is the single setting that controls cancellation for every execution.
 
 Subscriptions participate in the limit like any other operation. The initial subscribe consumes a slot while the subscribe resolver runs, and each emitted event consumes a slot while its result is being produced. Idle subscriptions (waiting on the next event) cost nothing. The slot is released between events.
+
+## SSE complete event carries an empty data field
+
+Hot Chocolate 16.0 through 16.6.7 ended every `text/event-stream` response with an `event: complete` message that had no `data:` field. The browser `EventSource` API does not dispatch such an event, so an `EventSource` client that did not close the connection itself reconnected when the response ended, and the server executed the operation again. The message now carries an empty `data:` field, and `EventSource` receives `complete`. This change lands in **16.6.8**.
+
+Tests that compare SSE response bodies as strings expect the extra line:
+
+```text
+event: complete
+data:
+```

website/content/docs/hotchocolate/server/http-transport.md

@@ -331,7 +331,7 @@ Results are delivered as SSE events. This transport works well with browser `Eve
 Accept: text/event-stream
 ```
 
-Each result is sent as an `event: next` message with the JSON payload in the `data:` field. A final `event: complete` message signals the end of the stream.
+Each result is sent as an `event: next` message with the JSON payload in the `data:` field. A final `event: complete` message with an empty `data:` field signals the end of the stream.
 
 ## JSON Lines (`application/jsonl`)
 
@@ -520,7 +520,7 @@ event: complete
 data:
 ```
 
-Each result is delivered as an `event: next` message with the JSON payload in the `data:` field. A final `event: complete` message signals the end of the stream.
+Each result is delivered as an `event: next` message with the JSON payload in the `data:` field. A final `event: complete` message with an empty `data:` field signals the end of the stream.
 
 ## SSE for Single Results