Add empty "data:" field for SSE complete event

Prior to this commit, the "event:complete" event would have no "data:"
field when completing the stream of data. This can cause problems with
browsers, as the `EventSource` callback might not get called without it.

This commit ensures that an empty data field is present in all cases.

Fixes gh-940

spring-graphql/src/main/java/org/springframework/graphql/server/webflux/GraphQlSseHandler.java

@@ -54,7 +54,7 @@ public class GraphQlSseHandler {
 
     private static final Log logger = LogFactory.getLog(GraphQlSseHandler.class);
 
-    private static final Mono<ServerSentEvent<Map<String, Object>>> COMPLETE_EVENT = Mono.just(ServerSentEvent.<Map<String, Object>>builder().event("complete").build());
+    private static final Mono<ServerSentEvent<Map<String, Object>>> COMPLETE_EVENT = Mono.just(ServerSentEvent.<Map<String, Object>>builder(Collections.emptyMap()).event("complete").build());
 
     private final WebGraphQlHandler graphQlHandler;
 

spring-graphql/src/main/java/org/springframework/graphql/server/webmvc/GraphQlSseHandler.java

@@ -147,7 +147,7 @@ private void writeNext(Map<String, Object> value) {
         @Override
         protected void hookOnComplete() {
             try {
-                this.sseBuilder.event("complete").send();
+                this.sseBuilder.event("complete").data("");
             } catch (IOException exc) {
                 throw new RuntimeException(exc);
             }

spring-graphql/src/test/java/org/springframework/graphql/server/webflux/GraphQlSseHandlerTests.java

@@ -76,6 +76,7 @@ void shouldRejectQueryOperations() {
                         data:{"errors":[{"message":"SSE transport only supports Subscription operations","locations":[],"extensions":{"classification":"OperationNotSupported"}}]}
                         
                         event:complete
+                        data:{}
                         
                         """);
     }
@@ -96,7 +97,8 @@ void shouldWriteMultipleEventsForSubscription() {
                         data:{"data":{"bookSearch":{"id":"5","name":"Animal Farm"}}}
                         
                         event:complete
-
+                        data:{}
+                        
                         """);
     }
 
@@ -118,6 +120,7 @@ void shouldWriteEventsAndTerminalError() {
                         data:{"errors":[{"message":"Subscription error","locations":[],"extensions":{"classification":"INTERNAL_ERROR"}}]}
                         
                         event:complete
+                        data:{}
 
                         """);
     }

spring-graphql/src/test/java/org/springframework/graphql/server/webmvc/GraphQlSseHandlerTests.java

@@ -71,6 +71,7 @@ void shouldRejectQueryOperations() throws Exception {
                         data:{"errors":[{"message":"SSE transport only supports Subscription operations","locations":[],"extensions":{"classification":"OperationNotSupported"}}]}
                         
                         event:complete
+                        data:
                         
                         """);
     }
@@ -95,7 +96,8 @@ void shouldWriteMultipleEventsForSubscription() throws Exception {
                         data:{"data":{"bookSearch":{"id":"5","name":"Animal Farm"}}}
                         
                         event:complete
-
+                        data:
+                        
                         """);
     }
 
@@ -121,7 +123,8 @@ void shouldWriteEventsAndTerminalError() throws Exception {
                         data:{"errors":[{"message":"Subscription error","locations":[],"extensions":{"classification":"INTERNAL_ERROR"}}]}
                         
                         event:complete
-
+                        data:
+                        
                         """);
     }