Fixes array out of bounds for value count agg (#26038)

https://github.com/elastic/elasticsearch/pull/17379 fixed many metric aggs so that if the parent aggregation does not collect any documents an empty bucket value is returned instead of an ArrayOutOfBoundsException being thrown. Unfortunately the value count aggregation was mised from this fix.

This change applies this fix from #17379 for the value count aggregation.

core/src/main/java/org/elasticsearch/search/aggregations/metrics/valuecount/ValueCountAggregator.java

@@ -83,7 +83,7 @@ public class ValueCountAggregator extends NumericMetricsAggregator.SingleValue {
 
     @Override
     public double metric(long owningBucketOrd) {
-        return valuesSource == null ? 0 : counts.get(owningBucketOrd);
+        return (valuesSource == null || owningBucketOrd >= counts.size()) ? 0 : counts.get(owningBucketOrd);
     }
 
     @Override

core/src/test/java/org/elasticsearch/search/aggregations/metrics/ValueCountIT.java

@@ -18,24 +18,31 @@
  */
 package org.elasticsearch.search.aggregations.metrics;
 
-import java.util.Collection;
-import java.util.Collections;
-import java.util.Map;
-
 import org.elasticsearch.action.search.SearchResponse;
 import org.elasticsearch.common.settings.Settings;
 import org.elasticsearch.plugins.Plugin;
 import org.elasticsearch.script.Script;
 import org.elasticsearch.script.ScriptType;
+import org.elasticsearch.search.aggregations.BucketOrder;
 import org.elasticsearch.search.aggregations.InternalAggregation;
+import org.elasticsearch.search.aggregations.bucket.filter.Filter;
 import org.elasticsearch.search.aggregations.bucket.global.Global;
+import org.elasticsearch.search.aggregations.bucket.terms.Terms;
 import org.elasticsearch.search.aggregations.metrics.valuecount.ValueCount;
 import org.elasticsearch.test.ESIntegTestCase;
 
+import java.util.Collection;
+import java.util.Collections;
+import java.util.List;
+import java.util.Map;
+
 import static org.elasticsearch.common.xcontent.XContentFactory.jsonBuilder;
 import static org.elasticsearch.index.query.QueryBuilders.matchAllQuery;
+import static org.elasticsearch.index.query.QueryBuilders.termQuery;
 import static org.elasticsearch.search.aggregations.AggregationBuilders.count;
+import static org.elasticsearch.search.aggregations.AggregationBuilders.filter;
 import static org.elasticsearch.search.aggregations.AggregationBuilders.global;
+import static org.elasticsearch.search.aggregations.AggregationBuilders.terms;
 import static org.elasticsearch.search.aggregations.metrics.MetricAggScriptPlugin.METRIC_SCRIPT_ENGINE;
 import static org.elasticsearch.search.aggregations.metrics.MetricAggScriptPlugin.SUM_FIELD_PARAMS_SCRIPT;
 import static org.elasticsearch.search.aggregations.metrics.MetricAggScriptPlugin.SUM_VALUES_FIELD_SCRIPT;
@@ -243,4 +250,33 @@ public class ValueCountIT extends ESIntegTestCase {
         assertThat(client().admin().indices().prepareStats("cache_test_idx").setRequestCache(true).get().getTotal().getRequestCache()
                 .getMissCount(), equalTo(1L));
     }
+
+    public void testOrderByEmptyAggregation() throws Exception {
+        SearchResponse searchResponse = client().prepareSearch("idx").setQuery(matchAllQuery())
+                .addAggregation(terms("terms").field("value").order(BucketOrder.compound(BucketOrder.aggregation("filter>count", true)))
+                        .subAggregation(filter("filter", termQuery("value", 100)).subAggregation(count("count").field("value"))))
+                .get();
+
+        assertHitCount(searchResponse, 10);
+
+        Terms terms = searchResponse.getAggregations().get("terms");
+        assertThat(terms, notNullValue());
+        List<? extends Terms.Bucket> buckets = terms.getBuckets();
+        assertThat(buckets, notNullValue());
+        assertThat(buckets.size(), equalTo(10));
+
+        for (int i = 0; i < 10; i++) {
+            Terms.Bucket bucket = buckets.get(i);
+            assertThat(bucket, notNullValue());
+            assertThat(bucket.getKeyAsNumber(), equalTo((long) i + 1));
+            assertThat(bucket.getDocCount(), equalTo(1L));
+            Filter filter = bucket.getAggregations().get("filter");
+            assertThat(filter, notNullValue());
+            assertThat(filter.getDocCount(), equalTo(0L));
+            ValueCount count = filter.getAggregations().get("count");
+            assertThat(count, notNullValue());
+            assertThat(count.value(), equalTo(0.0));
+
+        }
+    }
 }