[7.x] Fix bug with significant terms background count (#76730) (#76744)

* Fix bug with significant terms background count (#76730)

When building empty responses for shards that don't have the term field in question,
significant terms ignored the background filter.

This commit fixes this bug by respecting the background filter count, even
when building empty results.

closes #76729

* fixing bwc yaml test version

rest-api-spec/src/yamlRestTest/resources/rest-api-spec/test/search.aggregation/30_sig_terms.yml

@@ -73,6 +73,87 @@
   - match: {aggregations.class.buckets.1.sig_terms.buckets.0.key: "good"}
 
 ---
+"Test background filter count ":
+  - skip:
+      version: " - 7.14.99"
+      reason: bugfix introduced in 7.15.0
+
+  - do:
+      indices.create:
+        index:  goodbad
+        body:
+          settings:
+            number_of_shards: "1"
+          mappings:
+            properties:
+              text:
+                type: text
+                fielddata: true
+              class:
+                type: keyword
+  - do:
+      indices.create:
+        index: goodbad-2
+        body:
+          settings:
+            number_of_shards: "1"
+          mappings:
+            properties:
+              text:
+                type: text
+                fielddata: true
+              class:
+                type: keyword
+
+  - do:
+      index:
+        index: goodbad-2
+        id: 1
+        body: { class: "bad" }
+  - do:
+      index:
+        index: goodbad-2
+        id: 2
+        body: { class: "bad" }
+
+  - do:
+      index:
+        index:  goodbad
+        id:     1
+        body:   { text: "good", class: "good" }
+  - do:
+      index:
+        index:  goodbad
+        id:     2
+        body:   { text: "good", class: "good" }
+  - do:
+      index:
+        index:  goodbad
+        id:     3
+        body:   { text: "bad", class: "bad" }
+  - do:
+      index:
+        index:  goodbad
+        id:     4
+        body:   { text: "bad", class: "bad" }
+
+  - do:
+      indices.refresh:
+        index: [goodbad, goodbad-2]
+
+  - do:
+      search:
+        rest_total_hits_as_int: true
+        index: goodbad*
+
+  - match: {hits.total: 6}
+
+  - do:
+      search:
+        index: goodbad*
+        body: {"aggs": {"sig_terms": {"significant_terms": {"field": "text", "background_filter": {"bool": {"filter": [{"term": {"class": "good" }}]}}}}}}
+  - match: { aggregations.sig_terms.bg_count: 2 }
+---
 "IP test":
   - do:
       indices.create:

server/src/main/java/org/elasticsearch/search/aggregations/bucket/terms/AbstractStringTermsAggregator.java

@@ -36,9 +36,12 @@ protected StringTerms buildEmptyTermsAggregation() {
                 metadata(), format, bucketCountThresholds.getShardSize(), showTermDocCountError, 0, emptyList(), 0L);
     }
 
-    protected SignificantStringTerms buildEmptySignificantTermsAggregation(long subsetSize, SignificanceHeuristic significanceHeuristic) {
+    protected SignificantStringTerms buildEmptySignificantTermsAggregation(
+        long subsetSize,
+        long supersetSize,
+        SignificanceHeuristic significanceHeuristic
+    ) {
         // We need to account for the significance of a miss in our global stats - provide corpus size as context
-        int supersetSize = searcher().getIndexReader().numDocs();
         return new SignificantStringTerms(name, bucketCountThresholds.getRequiredSize(), bucketCountThresholds.getMinDocCount(),
                 metadata(), format, subsetSize, supersetSize, significanceHeuristic, emptyList());
     }

server/src/main/java/org/elasticsearch/search/aggregations/bucket/terms/GlobalOrdinalsStringTermsAggregator.java

@@ -894,12 +894,12 @@ SignificantStringTerms buildResult(long owningBucketOrd, long otherDocCount, Sig
 
         @Override
         SignificantStringTerms buildEmptyResult() {
-            return buildEmptySignificantTermsAggregation(0, significanceHeuristic);
+            return buildEmptySignificantTermsAggregation(0, supersetSize, significanceHeuristic);
         }
 
         @Override
         SignificantStringTerms buildNoValuesResult(long owningBucketOrdinal) {
-            return buildEmptySignificantTermsAggregation(subsetSizes.get(owningBucketOrdinal), significanceHeuristic);
+            return buildEmptySignificantTermsAggregation(subsetSizes.get(owningBucketOrdinal), supersetSize, significanceHeuristic);
         }
 
         @Override

server/src/main/java/org/elasticsearch/search/aggregations/bucket/terms/MapStringTermsAggregator.java

@@ -568,7 +568,7 @@ SignificantStringTerms buildResult(long owningBucketOrd, long otherDocCount, Sig
 
         @Override
         SignificantStringTerms buildEmptyResult() {
-            return buildEmptySignificantTermsAggregation(0, significanceHeuristic);
+            return buildEmptySignificantTermsAggregation(0, supersetSize, significanceHeuristic);
         }
 
         @Override

server/src/main/java/org/elasticsearch/search/aggregations/bucket/terms/NumericTermsAggregator.java

@@ -573,8 +573,6 @@ SignificantLongTerms buildResult(long owningBucketOrd, long otherDocCoun, Signif
 
         @Override
         SignificantLongTerms buildEmptyResult() {
-            // We need to account for the significance of a miss in our global stats - provide corpus size as context
-            int supersetSize = searcher().getIndexReader().numDocs();
             return new SignificantLongTerms(
                 name,
                 bucketCountThresholds.getRequiredSize(),