Fix introspection query deduplication (#6257)

Introspection queries are not supposed to be cached, but their result still needs to be sent to deduplicated queries. This also fixes a case where errors returned by `poll_ready` were not sent to deduplicated queries

.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;
+                                    });
                                 }
                             }