Assert subscription field is not introspection.

Author: benjie

src/validation/__tests__/SingleFieldSubscriptionsRule-test.js

@@ -83,4 +83,32 @@ describe('Validate: Subscriptions with single field', () => {
       },
     ]);
   });
+
+  it('fails with introspection field', () => {
+    expectErrors(`
+      subscription ImportantEmails {
+        __typename
+      }
+    `).to.deep.equal([
+      {
+        message:
+          'Subscription "ImportantEmails" must not select an introspection top level field.',
+        locations: [{ line: 3, column: 9 }],
+      },
+    ]);
+  });
+
+  it('fails with introspection field in anonymous subscription', () => {
+    expectErrors(`
+      subscription {
+        __typename
+      }
+    `).to.deep.equal([
+      {
+        message:
+          'Anonymous Subscription must not select an introspection top level field.',
+        locations: [{ line: 3, column: 9 }],
+      },
+    ]);
+  });
 });

src/validation/rules/SingleFieldSubscriptionsRule.js

@@ -6,9 +6,10 @@ import type { OperationDefinitionNode } from '../../language/ast';
 import type { ASTValidationContext } from '../ValidationContext';
 
 /**
- * Subscriptions must only include one field.
+ * Subscriptions must only include a non-introspection field.
  *
- * A GraphQL subscription is valid only if it contains a single root field.
+ * A GraphQL subscription is valid only if it contains a single root field and
+ * that root field is not an introspection field.
  */
 export function SingleFieldSubscriptionsRule(
   context: ASTValidationContext,
@@ -25,6 +26,22 @@ export function SingleFieldSubscriptionsRule(
               node.selectionSet.selections.slice(1),
             ),
           );
+        } else {
+          const selection = node.selectionSet.selections[0];
+          if (selection.kind === 'Field') {
+            const fieldName = selection.name.value;
+            // fieldName represents an introspection field if it starts with `__`
+            if (fieldName[0] === '_' && fieldName[1] === '_') {
+              context.reportError(
+                new GraphQLError(
+                  node.name
+                    ? `Subscription "${node.name.value}" must not select an introspection top level field.`
+                    : 'Anonymous Subscription must not select an introspection top level field.',
+                  node.selectionSet.selections,
+                ),
+              );
+            }
+          }
         }
       }
     },