Fix Gateway / Playground Query Plan view (#3418)

This commit revisits recent changes from PR #3376 which broke
Playground's ability to detect support for the Query Plan view.

Playground's internals decide that QP is supported based on the
response from the first introspection query. The previously mentioned
PR broke this by not providing any response.extensions.__queryPlanExperimental
at all. This makes sense for debug mode, but Playground still needs
some form of hint to know that it's supported and continue sending
the Apollo-Query-Plan-Experimental header.

Author: trevor-scheer

packages/apollo-gateway/CHANGELOG.md

@@ -4,7 +4,7 @@
 
 > The changes noted within this `vNEXT` section have not been released yet.  New PRs and commits which introduce changes should include an entry in this `vNEXT` section as part of their development.  When a release is being prepared, a new header will be (manually) created below and the the appropriate changes within that release will be moved into the new section.
 
-* _Nothing yet! Stay tuned!_
+* Fix Gateway / Playground Query Plan view [#3418](https://github.com/apollographql/apollo-server/pull/3418)
 
 # v.0.10.7
 

packages/apollo-gateway/src/index.ts

@@ -539,11 +539,17 @@ export class ApolloGateway implements GraphQLService {
       this.logger.debug(serializedQueryPlan);
     }
 
-    if (shouldShowQueryPlan && serializedQueryPlan) {
+    if (shouldShowQueryPlan) {
       // TODO: expose the query plan in a more flexible JSON format in the future
       // and rename this to `queryPlan`. Playground should cutover to use the new
       // option once we've built a way to print that representation.
-      response.extensions = { __queryPlanExperimental: serializedQueryPlan };
+
+      // In the case that `serializedQueryPlan` is null (on introspection), we
+      // still want to respond to Playground with something truthy since it depends
+      // on this to decide that query plans are supported by this gateway.
+      response.extensions = {
+        __queryPlanExperimental: serializedQueryPlan || true,
+      };
     }
     return response;
   };