Validate subscriptions in Operation function (#5926)

We realised that as a result of b40c632, introspection queries started failing in Insomnia because subscription didn't exist in the types. Instead of adding more hacks there, we add validation for this inside Operation. We already do something similar for Queries/Mutations.

graphql/e2e/directives/schema.graphql

@@ -139,7 +139,7 @@ type Student implements People {
         taughtBy: [Teacher] @hasInverse(field: "teaches")
 }
 
-type Message {
+type Message @withSubscription {
     content: String! @dgraph(pred: "post")
     author: String @dgraph(pred: "<职业>")
 }

graphql/resolve/resolver_error_test.go

@@ -399,6 +399,13 @@ func TestManyMutationsWithError(t *testing.T) {
 	}
 }
 
+func TestSubscriptionErrorWhenNoneDefined(t *testing.T) {
+	gqlSchema := test.LoadSchemaFromString(t, testGQLSchema)
+	resp := resolveWithClient(gqlSchema, `subscription { foo }`, nil, nil)
+	test.RequireJSONEq(t, x.GqlErrorList{{Message: "Not resolving subscription because schema" +
+		" doesn't have any fields defined for subscription operation."}}, resp.Errors)
+}
+
 func resolve(gqlSchema schema.Schema, gqlQuery string, dgResponse string) *schema.Response {
 	return resolveWithClient(gqlSchema, gqlQuery, nil, &executor{resp: dgResponse})
 }

graphql/schema/request.go

@@ -55,6 +55,12 @@ func (s *schema) Operation(req *Request) (Operation, error) {
 		return nil, listErr
 	}
 
+	if len(doc.Operations) == 1 && doc.Operations[0].Operation == ast.Subscription &&
+		s.schema.Subscription == nil {
+		return nil, errors.Errorf("Not resolving subscription because schema doesn't have any " +
+			"fields defined for subscription operation.")
+	}
+
 	if len(doc.Operations) > 1 && req.OperationName == "" {
 		return nil, errors.Errorf("Operation name must by supplied when query has more " +
 			"than 1 operation.")

graphql/schema/wrappers.go

@@ -568,12 +568,6 @@ func customMappings(s *ast.Schema) map[string]map[string]*ast.Directive {
 
 // AsSchema wraps a github.com/vektah/gqlparser/ast.Schema.
 func AsSchema(s *ast.Schema) (Schema, error) {
-
-	// vektah/gqlparser library doesn't validate subscriptions properly if s.Subscription == nil.
-	//s.Subscription is nil when there is no type with @withSubscription true, so we are handling that case.
-	if s.Subscription == nil {
-		s.Subscription = &ast.Definition{Name: "Subscription"}
-	}
 	// Auth rules can't be effectively validated as part of the normal rules -
 	// because they need the fully generated schema to be checked against.
 	authRules, err := authRules(s)