Correctly set doc_count when MovAvg "predicts" values on existing buckets (#24892)

If the bucket already exists, due to non-overlapping series or missing data, the
MovAvg creates a merged bucket with the existing aggs + the new prediction.  This
fixes a small bug where the doc_count was not being set correctly.

Relates to #24327

core/src/main/java/org/elasticsearch/search/aggregations/pipeline/movavg/MovAvgPipelineAggregator.java

@@ -161,7 +161,7 @@ public class MovAvgPipelineAggregator extends PipelineAggregator {
                     }).collect(Collectors.toList());
                     aggs.add(new InternalSimpleValue(name(), predictions[i], formatter, new ArrayList<PipelineAggregator>(), metaData()));
 
-                    Bucket newBucket = factory.createBucket(newKey, 0, new InternalAggregations(aggs));
+                    Bucket newBucket = factory.createBucket(newKey, bucket.getDocCount(), new InternalAggregations(aggs));
 
                     // Overwrite the existing bucket with the new version
                     newBuckets.set(lastValidPosition + i + 1, newBucket);

core/src/test/java/org/elasticsearch/search/aggregations/pipeline/moving/avg/MovAvgIT.java

@@ -19,9 +19,12 @@
 
 package org.elasticsearch.search.aggregations.pipeline.moving.avg;
 
+import org.elasticsearch.action.bulk.BulkRequestBuilder;
+import org.elasticsearch.action.bulk.BulkResponse;
 import org.elasticsearch.action.index.IndexRequestBuilder;
 import org.elasticsearch.action.search.SearchPhaseExecutionException;
 import org.elasticsearch.action.search.SearchResponse;
+import org.elasticsearch.action.support.WriteRequest;
 import org.elasticsearch.common.collect.EvictingQueue;
 import org.elasticsearch.search.aggregations.bucket.histogram.Histogram;
 import org.elasticsearch.search.aggregations.bucket.histogram.Histogram.Bucket;
@@ -41,6 +44,7 @@ import org.elasticsearch.search.aggregations.support.ValuesSourceAggregationBuil
 import org.elasticsearch.test.ESIntegTestCase;
 import org.hamcrest.Matchers;
 
+import java.io.IOException;
 import java.util.ArrayList;
 import java.util.Arrays;
 import java.util.Collection;
@@ -67,6 +71,7 @@ import static org.hamcrest.core.IsNull.nullValue;
 public class MovAvgIT extends ESIntegTestCase {
     private static final String INTERVAL_FIELD = "l_value";
     private static final String VALUE_FIELD = "v_value";
+    private static final String VALUE_FIELD2 = "v_value2";
 
     static int interval;
     static int numBuckets;
@@ -1204,6 +1209,68 @@ public class MovAvgIT extends ESIntegTestCase {
         }
     }
 
+    public void testPredictWithNonEmptyBuckets() throws Exception {
+
+        createIndex("predict_non_empty");
+        BulkRequestBuilder bulkBuilder = client().prepareBulk().setRefreshPolicy(WriteRequest.RefreshPolicy.IMMEDIATE);
+
+        for (int i = 0; i < 10; i++) {
+            bulkBuilder.add(client().prepareIndex("predict_non_empty", "type").setSource(
+                jsonBuilder().startObject().field(INTERVAL_FIELD, i)
+                    .field(VALUE_FIELD, 10)
+                    .field(VALUE_FIELD2, 10)
+                    .endObject()));
+        }
+        for (int i = 10; i < 20; i++) {
+            // Extra so there is a bucket that only has second field
+            bulkBuilder.add(client().prepareIndex("predict_non_empty", "type").setSource(
+                jsonBuilder().startObject().field(INTERVAL_FIELD, i).field(VALUE_FIELD2, 10).endObject()));
+        }
+
+        bulkBuilder.execute().actionGet();
+        ensureSearchable();
+
+        SearchResponse response = client()
+            .prepareSearch("predict_non_empty")
+            .setTypes("type")
+            .addAggregation(
+                histogram("histo")
+                    .field(INTERVAL_FIELD)
+                    .interval(1)
+                    .subAggregation(max("max").field(VALUE_FIELD))
+                    .subAggregation(max("max2").field(VALUE_FIELD2))
+                    .subAggregation(
+                        movingAvg("movavg_values", "max")
+                            .window(windowSize)
+                            .modelBuilder(new SimpleModel.SimpleModelBuilder())
+                            .gapPolicy(BucketHelpers.GapPolicy.SKIP).predict(5))).execute().actionGet();
+
+        assertSearchResponse(response);
+
+        Histogram histo = response.getAggregations().get("histo");
+        assertThat(histo, notNullValue());
+        assertThat(histo.getName(), equalTo("histo"));
+        List<? extends Bucket> buckets = histo.getBuckets();
+        assertThat("Size of buckets array is not correct.", buckets.size(), equalTo(20));
+
+        SimpleValue current = buckets.get(0).getAggregations().get("movavg_values");
+        assertThat(current, nullValue());
+
+        for (int i = 1; i < 20; i++) {
+            Bucket bucket = buckets.get(i);
+            assertThat(bucket, notNullValue());
+            assertThat(bucket.getKey(), equalTo((double)i));
+            assertThat(bucket.getDocCount(), equalTo(1L));
+            SimpleValue movAvgAgg = bucket.getAggregations().get("movavg_values");
+            if (i < 15) {
+                assertThat(movAvgAgg, notNullValue());
+                assertThat(movAvgAgg.value(), equalTo(10d));
+            } else {
+                assertThat(movAvgAgg, nullValue());
+            }
+        }
+    }
+
     private void assertValidIterators(Iterator expectedBucketIter, Iterator expectedCountsIter, Iterator expectedValuesIter) {
         if (!expectedBucketIter.hasNext()) {
             fail("`expectedBucketIter` iterator ended before `actual` iterator, size mismatch");