Fix incorrect mutex used in QueryPhaseResultConsumer (#116171)

Everything synchronizes on PendingMerges, this was just a typo.
closes #115716

Author: original-brownbear

muted-tests.yml

@@ -223,9 +223,6 @@ tests:
 - class: org.elasticsearch.search.functionscore.QueryRescorerIT
   method: testScoring
   issue: https://github.com/elastic/elasticsearch/issues/116050
-- class: org.elasticsearch.indexing.IndexActionIT
-  method: testAutoGenerateIdNoDuplicates
-  issue: https://github.com/elastic/elasticsearch/issues/115716
 - class: org.elasticsearch.xpack.application.connector.ConnectorIndexServiceTests
   issue: https://github.com/elastic/elasticsearch/issues/116087
 - class: org.elasticsearch.compute.operator.FilterOperatorTests

server/src/main/java/org/elasticsearch/action/search/QueryPhaseResultConsumer.java

@@ -480,7 +480,7 @@ protected void doRun() {
                             onMergeFailure(t);
                             return;
                         }
-                        synchronized (QueryPhaseResultConsumer.this) {
+                        synchronized (QueryPhaseResultConsumer.PendingMerges.this) {
                             if (hasFailure()) {
                                 return;
                             }
@@ -501,7 +501,7 @@ protected void doRun() {
                             }
                         }
                         Runnable r = mergeTask.consumeListener();
-                        synchronized (QueryPhaseResultConsumer.this) {
+                        synchronized (QueryPhaseResultConsumer.PendingMerges.this) {
                             while (true) {
                                 mergeTask = queue.poll();
                                 runningTask.set(mergeTask);