Fix docCountError calculation for multiple reduces

benchmarks/src/main/java/org/elasticsearch/benchmark/search/aggregations/TermsReduceBenchmark.java

@@ -154,7 +154,7 @@ private StringTerms newTerms(Random rand, BytesRef[] dict, boolean withNested) {
                 true,
                 0,
                 buckets,
-                0L
+                null
             );
         }
 

benchmarks/src/main/java/org/elasticsearch/benchmark/search/aggregations/bucket/terms/StringTermsSerializationBenchmark.java

@@ -70,7 +70,7 @@ private StringTerms newTerms(boolean withNested) {
             false,
             100000,
             resultBuckets,
-            0L
+            null
         );
     }
 

server/src/internalClusterTest/java/org/elasticsearch/search/aggregations/bucket/TermsDocCountErrorIT.java

@@ -1034,7 +1034,6 @@ public void testFixedDocs() throws Exception {
      * Tests the upper bounds are correct when performing incremental reductions
      * See https://github.com/elastic/elasticsearch/issues/40005 for more details
      */
-    @AwaitsFix(bugUrl = "https://github.com/elastic/elasticsearch/issues/75667")
     public void testIncrementalReduction() {
         SearchResponse response = client().prepareSearch("idx_fixed_docs_3", "idx_fixed_docs_4", "idx_fixed_docs_5")
             .addAggregation(terms("terms")

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

@@ -133,7 +133,7 @@ private long getDocCountError(A terms) {
         if (size == 0 || size < terms.getShardSize() || isKeyOrder(terms.getOrder())) {
             return 0;
         } else if (InternalOrder.isCountDesc(terms.getOrder())) {
-            if (terms.getDocCountError() != null && terms.getDocCountError() > 0) {
+            if (terms.getDocCountError() != null) {
                 // If there is an existing docCountError for this agg then
                 // use this as the error for this aggregation
                 return terms.getDocCountError();
@@ -340,7 +340,7 @@ public InternalAggregation reduce(List<InternalAggregation> aggregations, Intern
 
     protected static XContentBuilder doXContentCommon(XContentBuilder builder,
                                                       Params params,
-                                                      long docCountError,
+                                                      Long docCountError,
                                                       long otherDocCount,
                                                       List<? extends AbstractTermsBucket> buckets) throws IOException {
         builder.field(DOC_COUNT_ERROR_UPPER_BOUND_FIELD_NAME.getPreferredName(), docCountError);

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

@@ -765,7 +765,7 @@ StringTerms buildResult(long owningBucketOrd, long otherDocCount, StringTerms.Bu
             }
             return new StringTerms(name, reduceOrder, order, bucketCountThresholds.getRequiredSize(),
                 bucketCountThresholds.getMinDocCount(), metadata(), format, bucketCountThresholds.getShardSize(), showTermDocCountError,
-                otherDocCount, Arrays.asList(topBuckets), 0L);
+                otherDocCount, Arrays.asList(topBuckets), null);
         }
 
         @Override

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

@@ -53,12 +53,13 @@ protected InternalMappedTerms(String name, BucketOrder reduceOrder, BucketOrder
     protected InternalMappedTerms(StreamInput in, Bucket.Reader<B> bucketReader) throws IOException {
         super(in);
         if (in.getVersion().onOrAfter(Version.V_8_0_0)) { // todo fix after backport
-            docCountError = in.readOptionalLong();
-        } else {
-            docCountError = in.readZLong();
-            if (docCountError == 0) {
+            if (in.readBoolean()) {
+                docCountError = in.readZLong();
+            } else {
                 docCountError = null;
             }
+        } else {
+            docCountError = in.readZLong();
         }
         format = in.readNamedWriteable(DocValueFormat.class);
         shardSize = readSize(in);
@@ -70,7 +71,12 @@ protected InternalMappedTerms(StreamInput in, Bucket.Reader<B> bucketReader) thr
     @Override
     protected final void writeTermTypeInfoTo(StreamOutput out) throws IOException {
         if (out.getVersion().onOrAfter(Version.V_8_0_0)) {  // todo fix after backport
-            out.writeOptionalLong(docCountError);
+            if (docCountError != null) {
+                out.writeBoolean(true);
+                out.writeZLong(docCountError);
+            } else {
+                out.writeBoolean(false);
+            }
         } else {
             out.writeZLong(docCountError == null ? 0 : docCountError);
         }

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

@@ -446,7 +446,7 @@ StringTerms buildResult(long owningBucketOrd, long otherDocCount, StringTerms.Bu
             }
             return new StringTerms(name, reduceOrder, order, bucketCountThresholds.getRequiredSize(),
                 bucketCountThresholds.getMinDocCount(), metadata(), format, bucketCountThresholds.getShardSize(), showTermDocCountError,
-                otherDocCount, Arrays.asList(topBuckets), 0L);
+                otherDocCount, Arrays.asList(topBuckets), null);
         }
 
         @Override

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

@@ -372,7 +372,7 @@ LongTerms buildResult(long owningBucketOrd, long otherDocCount, LongTerms.Bucket
                 showTermDocCountError,
                 otherDocCount,
                 List.of(topBuckets),
-                0L
+                null
             );
         }
 
@@ -454,7 +454,7 @@ DoubleTerms buildResult(long owningBucketOrd, long otherDocCount, DoubleTerms.Bu
                 showTermDocCountError,
                 otherDocCount,
                 List.of(topBuckets),
-                0L
+                null
             );
         }
 

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

@@ -227,7 +227,7 @@ protected boolean lessThan(OrdBucket a, OrdBucket b) {
             showTermDocCountError,
             otherDocsCount,
             buckets,
-            0L
+            null
         );
     }
 

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

@@ -97,7 +97,7 @@ public boolean isMapped() {
 
     @Override
     public final XContentBuilder doXContentBody(XContentBuilder builder, Params params) throws IOException {
-        return doXContentCommon(builder, params, 0, 0, Collections.emptyList());
+        return doXContentCommon(builder, params, 0L, 0, Collections.emptyList());
     }
 
     @Override