add changelog

Signed-off-by: Kaushal Kumar <ravi.kaushal97@gmail.com>

Author: kaushalmahi12

CHANGELOG.md

@@ -11,6 +11,7 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 ### Fixed
 - Fix unnecessary refreshes on update preparation failures ([#15261](https://github.com/opensearch-project/OpenSearch/issues/15261))
 - Fix NullPointerException in segment replicator ([#18997](https://github.com/opensearch-project/OpenSearch/pull/18997))
+- Fix OOM due to large number of shard result buffering ([#19066](https://github.com/opensearch-project/OpenSearch/pull/19066))
 
 ### Dependencies
 

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

@@ -348,7 +348,7 @@ public void consume(QuerySearchResult result, Runnable next) throws CircuitBreak
                 checkCircuitBreaker();
                 if (hasFailure() || result.isNull()) {
                     result.consumeAll();
-                    if (result.isNull() && !hasFailure()) {
+                    if (result.isNull()) {
                         SearchShardTarget target = result.getSearchShardTarget();
                         emptyResults.add(new SearchShard(target.getClusterAlias(), target.getShardId()));
                     }
@@ -388,7 +388,7 @@ private void checkCircuitBreaker() throws CircuitBreakingException {
                 // force the CircuitBreaker eval to ensure during buffering we did not hit the circuit breaker limit
                 addEstimateAndMaybeBreak(0);
             } catch (CircuitBreakingException e) {
-                resetCircuitBreaker();
+                resetCircuitBreakerForCurrentRequest();
                 // onPartialMergeFailure should only be invoked once since this is responsible for cancelling the
                 // search task
                 if (!hasFailure()) {
@@ -405,7 +405,7 @@ private synchronized void onMergeFailure(Exception exc) {
                 return;
             }
             assert circuitBreakerBytes >= 0;
-            resetCircuitBreaker();
+            resetCircuitBreakerForCurrentRequest();
             failure.compareAndSet(null, exc);
             MergeTask task = runningTask.get();
             runningTask.compareAndSet(task, null);
@@ -422,9 +422,8 @@ private synchronized void onMergeFailure(Exception exc) {
             }
         }
 
-        private void resetCircuitBreaker() {
+        private void resetCircuitBreakerForCurrentRequest() {
             if (circuitBreakerBytes > 0) {
-                // make sure that we reset the circuit breaker
                 circuitBreaker.addWithoutBreaking(-circuitBreakerBytes);
                 circuitBreakerBytes = 0;
             }