Clean up how pipeline aggs check for multi-bucket

client/rest-high-level/src/main/java/org/elasticsearch/client/analytics/StringStatsAggregationBuilder.java

@@ -89,6 +89,11 @@ protected void innerWriteTo(StreamOutput out) throws IOException {
         throw new UnsupportedOperationException();
     }
 
+    @Override
+    public BucketCardinality bucketCardinality() {
+        return BucketCardinality.NONE;
+    }
+
     @Override
     protected ValuesSourceAggregatorFactory innerBuild(QueryShardContext queryShardContext, ValuesSourceConfig config,
             AggregatorFactory parent, Builder subFactoriesBuilder) throws IOException {

client/rest-high-level/src/main/java/org/elasticsearch/client/analytics/TopMetricsAggregationBuilder.java

@@ -94,6 +94,11 @@ protected void doWriteTo(StreamOutput out) throws IOException {
         throw new UnsupportedOperationException();
     }
 
+    @Override
+    public BucketCardinality bucketCardinality() {
+        return BucketCardinality.NONE;
+    }
+
     @Override
     protected AggregatorFactory doBuild(QueryShardContext queryShardContext, AggregatorFactory parent, Builder subfactoriesBuilder)
             throws IOException {

modules/aggs-matrix-stats/src/main/java/org/elasticsearch/search/aggregations/matrix/stats/MatrixStatsAggregationBuilder.java

@@ -34,7 +34,7 @@
 import java.util.Map;
 
 public class MatrixStatsAggregationBuilder
-    extends ArrayValuesSourceAggregationBuilder.LeafOnly<MatrixStatsAggregationBuilder> {
+        extends ArrayValuesSourceAggregationBuilder.LeafOnly<MatrixStatsAggregationBuilder> {
     public static final String NAME = "matrix_stats";
 
     private MultiValueMode multiValueMode = MultiValueMode.AVG;

modules/aggs-matrix-stats/src/main/java/org/elasticsearch/search/aggregations/support/ArrayValuesSourceAggregationBuilder.java

@@ -69,6 +69,11 @@ public AB subAggregations(Builder subFactories) {
             throw new AggregationInitializationException("Aggregator [" + name + "] of type [" +
                 getType() + "] cannot accept sub-aggregations");
         }
+
+        @Override
+        public final BucketCardinality bucketCardinality() {
+            return BucketCardinality.NONE;
+        }
     }
 
     private List<String> fields = Collections.emptyList();

modules/parent-join/src/main/java/org/elasticsearch/join/aggregations/ChildrenAggregationBuilder.java

@@ -97,6 +97,10 @@ protected void innerWriteTo(StreamOutput out) throws IOException {
     }
 
     @Override
+    public BucketCardinality bucketCardinality() {
+        return BucketCardinality.ONE;
+    }
+
     protected ValuesSourceAggregatorFactory innerBuild(QueryShardContext queryShardContext,
                                                        ValuesSourceConfig config,
                                                        AggregatorFactory parent,

modules/parent-join/src/main/java/org/elasticsearch/join/aggregations/ParentAggregationBuilder.java

@@ -96,6 +96,11 @@ protected void innerWriteTo(StreamOutput out) throws IOException {
         out.writeString(childType);
     }
 
+    @Override
+    public BucketCardinality bucketCardinality() {
+        return BucketCardinality.ONE;
+    }
+
     @Override
     protected ValuesSourceAggregatorFactory innerBuild(QueryShardContext queryShardContext,
                                                        ValuesSourceConfig config,

rest-api-spec/src/main/resources/rest-api-spec/test/search.aggregation/280_rare_terms.yml

@@ -360,3 +360,64 @@ setup:
   - is_false: aggregations.str_terms.buckets.0.key_as_string
   - match: { aggregations.str_terms.buckets.0.doc_count: 1 }
   - match: { aggregations.str_terms.buckets.0.max_n.value: 3.0 }
+
+---
+"avg_bucket":
+  - skip:
+      version: " - 7.99.99"
+      reason: Fixed in 8.0.0 to be backported to 7.8.0
+  - do:
+      indices.create:
+          index: test
+          body:
+            settings:
+              number_of_replicas: 0
+            mappings:
+              properties:
+                str:
+                   type: keyword
+  - do:
+      bulk:
+        index: test
+        refresh: true
+        body:
+          - '{"index": {}}'
+          - '{"str": "foo", "v": 1}'
+          - '{"index": {}}'
+          - '{"str": "foo", "v": 2}'
+          - '{"index": {}}'
+          - '{"str": "foo", "v": 3}'
+          - '{"index": {}}'
+          - '{"str": "bar", "v": 4}'
+          - '{"index": {}}'
+          - '{"str": "bar", "v": 5}'
+          - '{"index": {}}'
+          - '{"str": "baz", "v": 6}'
+
+  - do:
+      search:
+        index: test
+        body:
+          size: 0
+          aggs:
+            str_terms:
+              rare_terms:
+                field: str
+                max_doc_count: 2
+              aggs:
+                v:
+                  sum:
+                    field: v
+            str_terms_avg_v:
+              avg_bucket:
+                buckets_path: str_terms.v
+
+  - match: { hits.total.value: 6 }
+  - length: { aggregations.str_terms.buckets: 2 }
+  - match: { aggregations.str_terms.buckets.0.key: baz }
+  - match: { aggregations.str_terms.buckets.0.doc_count: 1 }
+  - match: { aggregations.str_terms.buckets.0.v.value: 6 }
+  - match: { aggregations.str_terms.buckets.1.key: bar }
+  - match: { aggregations.str_terms.buckets.1.doc_count: 2 }
+  - match: { aggregations.str_terms.buckets.1.v.value: 9 }
+  - match: { aggregations.str_terms_avg_v.value: 7.5 }

rest-api-spec/src/main/resources/rest-api-spec/test/search.aggregation/330_auto_date_histogram.yml

@@ -0,0 +1,73 @@
+setup:
+  - do:
+      indices.create:
+          index: test
+          body:
+            settings:
+              number_of_replicas: 0
+            mappings:
+              properties:
+                date:
+                  type: date
+  - do:
+      bulk:
+        refresh: true
+        index: test
+        body:
+            - '{"index": {}}'
+            - '{"date": "2020-03-01", "v": 1}'
+            - '{"index": {}}'
+            - '{"date": "2020-03-02", "v": 2}'
+            - '{"index": {}}'
+            - '{"date": "2020-03-08", "v": 3}'
+            - '{"index": {}}'
+            - '{"date": "2020-03-09", "v": 4}'
+
+---
+"basic":
+  - do:
+      search:
+        body:
+          size: 0
+          aggs:
+            histo:
+              auto_date_histogram:
+                field: date
+                buckets: 2
+  - match: { hits.total.value: 4 }
+  - length: { aggregations.histo.buckets: 2 }
+  - match: { aggregations.histo.buckets.0.key_as_string: "2020-03-01T00:00:00.000Z" }
+  - match: { aggregations.histo.buckets.0.doc_count: 2 }
+  - match: { aggregations.histo.buckets.1.key_as_string: "2020-03-08T00:00:00.000Z" }
+  - match: { aggregations.histo.buckets.1.doc_count: 2 }
+
+---
+"avg_bucket":
+  - skip:
+      version: " - 7.99.99"
+      reason: Fixed in 8.0.0 to be backported to 7.8.0
+  - do:
+      search:
+        body:
+          size: 0
+          aggs:
+            histo:
+              auto_date_histogram:
+                field: date
+                buckets: 2
+              aggs:
+                v:
+                  sum:
+                    field: v
+            histo_avg_v:
+              avg_bucket:
+                buckets_path: histo.v
+  - match: { hits.total.value: 4 }
+  - length: { aggregations.histo.buckets: 2 }
+  - match: { aggregations.histo.buckets.0.key_as_string: "2020-03-01T00:00:00.000Z" }
+  - match: { aggregations.histo.buckets.0.doc_count: 2 }
+  - match: { aggregations.histo.buckets.0.v.value: 3 }
+  - match: { aggregations.histo.buckets.1.key_as_string: "2020-03-08T00:00:00.000Z" }
+  - match: { aggregations.histo.buckets.1.doc_count: 2 }
+  - match: { aggregations.histo.buckets.1.v.value: 7 }
+  - match: { aggregations.histo_avg_v.value: 5 }

rest-api-spec/src/main/resources/rest-api-spec/test/search.aggregation/340_geo_distance.yml

@@ -0,0 +1,84 @@
+setup:
+  - do:
+      indices.create:
+        index: test
+        body:
+          mappings:
+            properties:
+              location:
+                type: geo_point
+  - do:
+      bulk:
+        index: test
+        refresh: true
+        body:
+          - '{"index": {}}'
+          - '{"location": {"lat" : 40.7128, "lon" : -74.0060}, "name": "New York", "population": 8623000}'
+          - '{"index": {}}'
+          - '{"location": {"lat" : 34.0522, "lon" : -118.2437}, "name": "Los Angeles", "population": 4000000}'
+          - '{"index": {}}'
+          - '{"location": {"lat" : 41.8781, "lon" : -87.6298}, "name": "Chicago", "population": 2716000}'
+
+---
+"basic":
+  - do:
+      search:
+        body:
+          size: 0
+          aggs:
+            distance:
+              geo_distance:
+                field: location
+                origin: "35.7796, -78.6382"
+                ranges:
+                  - to: 1000000
+                  - from: 1000000
+                    to: 5000000
+                  - from: 5000000
+  - match: { hits.total.value: 3 }
+  - length: { aggregations.distance.buckets: 3 }
+  - match: { aggregations.distance.buckets.0.key: "*-1000000.0" }
+  - match: { aggregations.distance.buckets.0.doc_count: 1 }
+  - match: { aggregations.distance.buckets.1.key: "1000000.0-5000000.0" }
+  - match: { aggregations.distance.buckets.1.doc_count: 2 }
+  - match: { aggregations.distance.buckets.2.key: "5000000.0-*" }
+  - match: { aggregations.distance.buckets.2.doc_count: 0 }
+
+---
+"avg_bucket":
+  - skip:
+      version: " - 7.99.99"
+      reason: Fixed in 8.0.0 to be backported to 7.8.0
+  - do:
+      search:
+        body:
+          size: 0
+          aggs:
+            distance:
+              geo_distance:
+                field: location
+                origin: "35.7796, -78.6382"
+                ranges:
+                  - to: 1000000
+                  - from: 1000000
+                    to: 5000000
+                  - from: 5000000
+              aggs:
+                total_population:
+                  sum:
+                    field: population
+            avg_total_population:
+              avg_bucket:
+                buckets_path: distance.total_population
+  - match: { hits.total.value: 3 }
+  - length: { aggregations.distance.buckets: 3 }
+  - match: { aggregations.distance.buckets.0.key: "*-1000000.0" }
+  - match: { aggregations.distance.buckets.0.doc_count: 1 }
+  - match: { aggregations.distance.buckets.0.total_population.value: 8623000.0 }
+  - match: { aggregations.distance.buckets.1.key: "1000000.0-5000000.0" }
+  - match: { aggregations.distance.buckets.1.doc_count: 2 }
+  - match: { aggregations.distance.buckets.1.total_population.value: 6716000.0 }
+  - match: { aggregations.distance.buckets.2.key: "5000000.0-*" }
+  - match: { aggregations.distance.buckets.2.doc_count: 0 }
+  - match: { aggregations.distance.buckets.2.total_population.value: 0 }
+  - match: { aggregations.avg_total_population.value: 7669500.0 }

rest-api-spec/src/main/resources/rest-api-spec/test/search.aggregation/40_range.yml

@@ -221,6 +221,58 @@ setup:
   - match: { aggregations.ip_range.buckets.1.key: "192.168.0.0-192.169.0.0" }
   - match: { aggregations.ip_range.buckets.2.key: "192.169.0.0-*" }
 
+---
+"IP Range avg_bucket":
+  - skip:
+      version: " - 7.99.99"
+      reason: Fixed in 8.0.0 to be backported to 7.8.0
+  - do:
+      bulk:
+        refresh: true
+        index: test
+        body:
+            - '{"index": {}}'
+            - '{"ip": "::1", "v": 1}'
+            - '{"index": {}}'
+            - '{"ip": "192.168.0.1", "v": 2}'
+            - '{"index": {}}'
+            - '{"ip": "192.168.0.7", "v": 3}'
+
+  - do:
+      search:
+        index: test
+        body:
+          size: 0
+          aggs:
+            range:
+              ip_range:
+                field: ip
+                ranges:
+                  - to: 192.168.0.0
+                  - from: 192.168.0.0
+                    to: 192.169.0.0
+                  - from: 192.169.0.0
+              aggs:
+                v:
+                  sum:
+                    field: v
+            range_avg_v:
+              avg_bucket:
+                buckets_path: range.v
+
+  - match: { hits.total.value: 3 }
+  - length: { aggregations.range.buckets: 3 }
+  - match: { aggregations.range.buckets.0.key: "*-192.168.0.0" }
+  - match: { aggregations.range.buckets.0.doc_count: 1 }
+  - match: { aggregations.range.buckets.0.v.value: 1 }
+  - match: { aggregations.range.buckets.1.key: "192.168.0.0-192.169.0.0" }
+  - match: { aggregations.range.buckets.1.doc_count: 2 }
+  - match: { aggregations.range.buckets.1.v.value: 5 }
+  - match: { aggregations.range.buckets.2.key: "192.169.0.0-*" }
+  - match: { aggregations.range.buckets.2.doc_count: 0 }
+  - match: { aggregations.range.buckets.2.v.value: 0 }
+  - match: { aggregations.range_avg_v.value: 3 }
+
 ---
 "Date range":
   - do:

server/src/main/java/org/elasticsearch/search/aggregations/AggregationBuilder.java

@@ -154,6 +154,19 @@ public PipelineTree buildPipelineTree() {
         return factoriesBuilder.buildPipelineTree();
     }
 
+    /**
+     * Rough measure of how many buckets this aggregation can return. Just
+     * "zero", "one", and "many".
+     */
+    public enum BucketCardinality {
+        NONE, ONE, MANY;
+    }
+    /**
+     * Do aggregations built by this builder contain buckets? If so, do they
+     * contain *always* contain a single bucket?
+     */
+    public abstract BucketCardinality bucketCardinality();
+
     /** Common xcontent fields shared among aggregator builders */
     public static final class CommonFields extends ParseField.CommonFields {
         public static final ParseField VALUE_TYPE = new ParseField("value_type");