Fix introspection query deduplication

.changesets/fix_geal_introspection_dedup_fix.md

@@ -0,0 +1,5 @@
+### Fix introspection query deduplication ([Issue #6249](https://github.com/apollographql/router/issues/6249))
+
+To reduce CPU usage, query planning and introspection queries are deduplicated. In some cases, deduplicated introspection queries were not receiving their result. This makes sure that answers are sent in all cases.
+
+By [@Geal](https://github.com/Geal) in https://github.com/apollographql/router/pull/6257

apollo-router/src/query_planner/caching_query_planner.rs

@@ -518,7 +518,19 @@ where
             // of restarting the query planner until another timeout
             tokio::task::spawn(
                 async move {
-                    let res = self.delegate.ready().await?.call(request).await;
+                    let service = match self.delegate.ready().await {
+                        Ok(service) => service,
+                        Err(error) => {
+                            let e = Arc::new(error);
+                            let err = e.clone();
+                            tokio::spawn(async move {
+                                entry.insert(Err(err)).await;
+                            });
+                            return Err(CacheResolverError::RetrievalError(e));
+                        }
+                    };
+
+                    let res = service.call(request).await;
 
                     match res {
                         Ok(QueryPlannerResponse { content, errors }) => {
@@ -536,6 +548,10 @@ where
                                     tokio::spawn(async move {
                                         entry.insert(Ok(content)).await;
                                     });
+                                } else {
+                                    tokio::spawn(async move {
+                                        entry.send(Ok(content)).await;
+                                    });
                                 }
                             }