Fix handling of terminate_after when size is 0 (#58212)

`terminate_after` is ignored on search requests that don't return top hits (`size` set to 0)
and do not tracked the number of hits accurately (`track_total_hits`).
We use early termination when the number of hits to track is reached during collection
but this breaks the hard termination of `terminate_after` if it happens before we reached
the `terminate_after` value.
This change ensures that we continue to check `terminate_after` even if the tracking of total
hits has reached the provided value.

Closes #57624

server/src/main/java/org/elasticsearch/search/query/QueryCollectorContext.java

@@ -24,6 +24,7 @@
 import org.apache.lucene.search.MultiCollector;
 import org.apache.lucene.search.Query;
 import org.apache.lucene.search.ScoreMode;
+import org.apache.lucene.search.SimpleCollector;
 import org.apache.lucene.search.Weight;
 import org.elasticsearch.common.lucene.MinimumScoreCollector;
 import org.elasticsearch.common.lucene.search.FilteredCollector;
@@ -41,6 +42,17 @@
 import static org.elasticsearch.search.profile.query.CollectorResult.REASON_SEARCH_TERMINATE_AFTER_COUNT;
 
 abstract class QueryCollectorContext {
+    private static final Collector EMPTY_COLLECTOR = new SimpleCollector() {
+        @Override
+        public void collect(int doc) {
+        }
+
+        @Override
+        public ScoreMode scoreMode() {
+            return ScoreMode.COMPLETE_NO_SCORES;
+        }
+    };
+
     private String profilerName;
 
     QueryCollectorContext(String profilerName) {
@@ -124,15 +136,15 @@ Collector create(Collector in ) throws IOException {
     static QueryCollectorContext createMultiCollectorContext(Collection<Collector> subs) {
         return new QueryCollectorContext(REASON_SEARCH_MULTI) {
             @Override
-            Collector create(Collector in) throws IOException {
+            Collector create(Collector in) {
                 List<Collector> subCollectors = new ArrayList<> ();
                 subCollectors.add(in);
                 subCollectors.addAll(subs);
                 return MultiCollector.wrap(subCollectors);
             }
 
             @Override
-            protected InternalProfileCollector createWithProfiler(InternalProfileCollector in) throws IOException {
+            protected InternalProfileCollector createWithProfiler(InternalProfileCollector in) {
                 final List<InternalProfileCollector> subCollectors = new ArrayList<> ();
                 subCollectors.add(in);
                 if (subs.stream().anyMatch((col) -> col instanceof InternalProfileCollector == false)) {
@@ -152,12 +164,20 @@ protected InternalProfileCollector createWithProfiler(InternalProfileCollector i
      */
     static QueryCollectorContext createEarlyTerminationCollectorContext(int numHits) {
         return new QueryCollectorContext(REASON_SEARCH_TERMINATE_AFTER_COUNT) {
-            private EarlyTerminatingCollector collector;
+            private Collector collector;
 
+            /**
+             * Creates a {@link MultiCollector} to ensure that the {@link EarlyTerminatingCollector}
+             * can terminate the collection independently of the provided <code>in</code> {@link Collector}.
+             */
             @Override
-            Collector create(Collector in) throws IOException {
+            Collector create(Collector in) {
                 assert collector == null;
-                this.collector = new EarlyTerminatingCollector(in, numHits, true);
+
+                List<Collector> subCollectors = new ArrayList<> ();
+                subCollectors.add(new EarlyTerminatingCollector(EMPTY_COLLECTOR, numHits, true));
+                subCollectors.add(in);
+                this.collector = MultiCollector.wrap(subCollectors);
                 return collector;
             }
         };

server/src/test/java/org/elasticsearch/search/query/QueryPhaseTests.java

@@ -452,6 +452,40 @@ public void testTerminateAfterEarlyTermination() throws Exception {
             assertThat(context.queryResult().topDocs().topDocs.scoreDocs.length, equalTo(0));
             assertThat(collector.getTotalHits(), equalTo(1));
         }
+
+        // tests with trackTotalHits and terminateAfter
+        context.terminateAfter(10);
+        context.setSize(0);
+        for (int trackTotalHits : new int[] { -1, 3, 76, 100}) {
+            context.trackTotalHitsUpTo(trackTotalHits);
+            TotalHitCountCollector collector = new TotalHitCountCollector();
+            context.queryCollectors().put(TotalHitCountCollector.class, collector);
+            QueryPhase.executeInternal(context);
+            assertTrue(context.queryResult().terminatedEarly());
+            if (trackTotalHits == -1) {
+                assertThat(context.queryResult().topDocs().topDocs.totalHits.value, equalTo(0L));
+            } else {
+                assertThat(context.queryResult().topDocs().topDocs.totalHits.value, equalTo((long) Math.min(trackTotalHits, 10)));
+            }
+            assertThat(context.queryResult().topDocs().topDocs.scoreDocs.length, equalTo(0));
+            assertThat(collector.getTotalHits(), equalTo(10));
+        }
+
+        context.terminateAfter(7);
+        context.setSize(10);
+        for (int trackTotalHits : new int[] { -1, 3, 75, 100}) {
+            context.trackTotalHitsUpTo(trackTotalHits);
+            EarlyTerminatingCollector collector = new EarlyTerminatingCollector(new TotalHitCountCollector(), 1, false);
+            context.queryCollectors().put(EarlyTerminatingCollector.class, collector);
+            QueryPhase.executeInternal(context);
+            assertTrue(context.queryResult().terminatedEarly());
+            if (trackTotalHits == -1) {
+                assertThat(context.queryResult().topDocs().topDocs.totalHits.value, equalTo(0L));
+            } else {
+                assertThat(context.queryResult().topDocs().topDocs.totalHits.value, equalTo(7L));
+            }
+            assertThat(context.queryResult().topDocs().topDocs.scoreDocs.length, equalTo(7));
+        }
         reader.close();
         dir.close();
     }