Subscription data fetcher return type should be checked

[vojtapol]

When you add a subscription resolver:

public class ImportantEventSubscription implements GraphQLSubscriptionResolver {

    public String importantEvent(String id, DataFetchingEnvironment environment) {

        return "hi";
    }

}

extend type Subscription {
  importantEvent(id: String!): String
}

The schema is parsed and built by graphql-java-tools without errors but then it fails on subscribe with:

graphql.AssertException: You data fetcher must return a Publisher of events when using graphql subscriptions
	at graphql.Assert.assertTrue(Assert.java:76) ~[graphql-java-13.0.jar:na]
	at graphql.execution.SubscriptionExecutionStrategy.lambda$createSourceEventStream$2(SubscriptionExecutionStrategy.java:74) ~[graphql-java-13.0.jar:na]
	at java.util.concurrent.CompletableFuture.uniApply(CompletableFuture.java:602) ~[na:1.8.0_152-ea]
	at java.util.concurrent.CompletableFuture.uniApplyStage(CompletableFuture.java:614) ~[na:1.8.0_152-ea]
	at java.util.concurrent.CompletableFuture.thenApply(CompletableFuture.java:1983) ~[na:1.8.0_152-ea]
	at graphql.execution.SubscriptionExecutionStrategy.createSourceEventStream(SubscriptionExecutionStrategy.java:71) ~[graphql-java-13.0.jar:na]
	at graphql.execution.SubscriptionExecutionStrategy.execute(SubscriptionExecutionStrategy.java:37) ~[graphql-java-13.0.jar:na]
	at graphql.execution.Execution.executeOperation(Execution.java:161) ~[graphql-java-13.0.jar:na]
	at graphql.execution.Execution.execute(Execution.java:102) ~[graphql-java-13.0.jar:na]
	at graphql.GraphQL.execute(GraphQL.java:605) ~[graphql-java-13.0.jar:na]
	at graphql.GraphQL.parseValidateAndExecute(GraphQL.java:538) ~[graphql-java-13.0.jar:na]

I think we could be checking that subscription data fetchers return Publisher during the schema validation step.

[edudar-chwy]

@oryan-block, I think this was a little bit overdone. SubscriptionExecutionStrategy supports CompletionStage<Publisher<X>> or Publisher<X> return types.

protected Object /*CompletableFuture<FetchedValue> | FetchedValue>*/
    fetchField(ExecutionContext executionContext, ExecutionStrategyParameters parameters) {

This change prevents any kind of CompletionStage from being used as a return type, as it must be either Publisher or ReceiveChannel

[oryan-block]

@edudar-chwy is it good enough to allow for CompletableFuture or does it have to be all CompletionStage implementations? It's a bit of a problem because of generics..

[edudar-chwy]

I bet CompletableFuture would be enough

[oryan-block]

@edudar-chwy fix for that issue is here: #797
Sorry for the late response.