fix(GraphQL): Disallow Subscription typename. (#6077) (#6173)

This PR disallow Subscription typename so that it doesn't conflict with Subscriptions generated.

(cherry picked from commit 601e837)

graphql/schema/gqlschema_test.yml

@@ -2387,6 +2387,18 @@ invalid_schemas:
      "locations":[{"line":6, "column":5}]}
     ]
 
+  -
+    name: "Subscription typename should return error"
+    input: |
+      type Subscription {
+              name: String
+      }
+
+    errlist: [
+    {"message": "Subscription is a reserved word, so you can't declare a type with this name. Pick a different name for the type.", "locations": [{"line":1, "column":6}]},
+    ]
+
+
 valid_schemas:
   - name: "@auth on interface implementation"
     input: |
@@ -2677,4 +2689,4 @@ valid_schemas:
         id: ID!
         name: String
         owns: [Object] @dgraph(pred: "~Object.owner")
-      }
+      }

graphql/schema/rules.go

@@ -1781,7 +1781,13 @@ func isReservedArgument(name string) bool {
 }
 
 func isReservedKeyWord(name string) bool {
-	if isScalar(name) || isQueryOrMutation(name) || name == "uid" {
+	reservedTypeNames := map[string]bool{
+		// Reserved Type names
+		"uid":          true,
+		"Subscription": true,
+	}
+
+	if isScalar(name) || isQueryOrMutation(name) || reservedTypeNames[name] {
 		return true
 	}