Fork off a search thread before sending back fetched responses

This is just a temporary fix until #23048 is fixed. FieldCollapsing
is executing blocking calls on a network thread which causes potential deadlocks
and trips assertions.

Relates to #23048

core/src/main/java/org/elasticsearch/action/search/AbstractSearchAsyncAction.java

@@ -558,13 +558,25 @@ abstract class AbstractSearchAsyncAction<FirstResult extends SearchPhaseResult>
         private void sendResponse(SearchPhaseController searchPhaseController, ScoreDoc[] sortedDocs,
                                 String scrollId, SearchPhaseController.ReducedQueryPhase reducedQueryPhase,
                                 AtomicArray<? extends QuerySearchResultProvider> fetchResultsArr) {
+            // this is only a temporary fix since field collapsing executes a blocking call on response
+            // which could be a network thread. we are fixing this but for now we just fork off again.
+            // this should be removed once https://github.com/elastic/elasticsearch/issues/23048 is fixed
+            getExecutor().execute(new ActionRunnable<SearchResponse>(listener) {
+                @Override
+                public void doRun() throws IOException {
                     final boolean isScrollRequest = request.scroll() != null;
                     final InternalSearchResponse internalResponse = searchPhaseController.merge(isScrollRequest, sortedDocs, reducedQueryPhase,
                         fetchResultsArr);
                     listener.onResponse(new SearchResponse(internalResponse, scrollId, expectedSuccessfulOps, successfulOps.get(),
                         buildTookInMillis(), buildShardFailures()));
                 }
+
+                @Override
+                public void onFailure(Exception e) {
+                    raisePhaseFailure(new ReduceSearchPhaseException("fetch", "", e, buildShardFailures()));
+                }
+            });
         }
 
-
+    }
 }