Throw an error on asynchronous introspection query behavior.

packages/apollo-server-core/src/utils/schemaHash.ts

@@ -5,11 +5,23 @@ import stableStringify from 'json-stable-stringify';
 import { GraphQLSchema } from 'graphql/type';
 import { createHash } from 'crypto';
 
+const hasOwn = Object.prototype.hasOwnProperty;
+
 export function generateSchemaHash(schema: GraphQLSchema): string {
   const introspectionQuery = getIntrospectionQuery();
   const documentAST = parse(introspectionQuery);
   const result = execute(schema, documentAST) as ExecutionResult;
 
+  // If the execution of an introspection query results in a then-able, it
+  // indicates that one or more of its resolvers is behaving in an asynchronous
+  // manner.  This is not the expected behavior of a introspection query
+  // which does not have any asynchronous resolvers.
+  if (hasOwn.call(result, 'then')) {
+    throw new Error(
+      'The introspection query is resolving asynchronously; execution of an introspection query is not expected to return a `Promise`.',
+    );
+  }
+
   if (!result || !result.data || !result.data.__schema) {
     throw new Error('Unable to generate server introspection document.');
   }