Fix regressions around nested hits and disabled _source. (#66572)

This PR fixes two bugs that can arise when _source is disabled and we fetch nested documents:
* Fix exception when highlighting `inner_hits` with disabled _source.
* Fix exception in nested `top_hits` with disabled _source.
* Add more tests for highlighting `inner_hits`.

rest-api-spec/src/main/resources/rest-api-spec/test/search.aggregation/200_top_hits_metric.yml

@@ -56,6 +56,7 @@ setup:
   - do:
       search:
         rest_total_hits_as_int: true
+        index: my-index
         body:
           aggs:
             to-users:
@@ -81,12 +82,62 @@ setup:
   - match: { aggregations.to-users.users.hits.hits.2._nested.field: users }
   - match: { aggregations.to-users.users.hits.hits.2._nested.offset: 1 }
 
+---
+"top_hits aggregation with nested documents and disabled _source":
+  - skip:
+      version: " - 7.99.99"
+      reason: "bug fix is not yet backported"
+  - do:
+      indices.create:
+        index: disabled-source
+        body:
+          settings:
+            number_of_shards: 1
+            number_of_replicas: 0
+          mappings:
+            _source:
+              enabled: false
+            properties:
+              users:
+                type: nested
+
+  - do:
+      index:
+        index:  disabled-source
+        id:     1
+        refresh: true
+        body:
+          users:
+            - first: "John"
+              last:  "Smith"
+
+  - do:
+      search:
+        index: disabled-source
+        rest_total_hits_as_int: true
+        body:
+          aggs:
+            to-users:
+              nested:
+                path: users
+              aggs:
+                users:
+                  top_hits: {}
+
+  - match: { hits.total: 1 }
+  - length: { aggregations.to-users.users.hits.hits: 1 }
+  - match: { aggregations.to-users.users.hits.hits.0._id: "1" }
+  - match: { aggregations.to-users.users.hits.hits.0._index: disabled-source }
+  - match: { aggregations.to-users.users.hits.hits.0._nested.field: users }
+  - match: { aggregations.to-users.users.hits.hits.0._nested.offset: 0 }
+  - is_false: aggregations.to-users.users.hits.hits.0._source
 
 ---
 "top_hits aggregation with sequence numbers":
 
   - do:
       search:
+        index: my-index
         rest_total_hits_as_int: true
         body:
           aggs:

rest-api-spec/src/main/resources/rest-api-spec/test/search.inner_hits/20_highlighting.yml

@@ -0,0 +1,137 @@
+setup:
+  - do:
+      indices.create:
+        index: test
+        body:
+          mappings:
+            _source:
+              excludes: ["nested.stored_only"]
+            properties:
+              nested:
+                type: nested
+                properties:
+                  field:
+                    type: text
+                    fields:
+                      vectors:
+                        type: text
+                        term_vector: "with_positions_offsets"
+                      postings:
+                        type: text
+                        index_options: "offsets"
+                  stored:
+                    type: text
+                    store: true
+                  stored_only:
+                    type: text
+                    store: true
+  - do:
+      index:
+        index: test
+        id:    1
+        refresh: true
+        body:
+          nested:
+            field : "The quick brown fox is brown."
+            stored : "The quick brown fox is brown."
+            stored_only : "The quick brown fox is brown."
+
+---
+"Unified highlighter":
+  - do:
+      search:
+        index: test
+        body:
+          query:
+            nested:
+              path: "nested"
+              query:
+                multi_match:
+                  query: "quick brown fox"
+                  fields: [ "nested.field", "nested.field.vectors", "nested.field.postings" ]
+              inner_hits:
+                highlight:
+                  type: "unified"
+                  fields:
+                    nested.field: {}
+                    nested.field.vectors: {}
+                    nested.field.postings: {}
+
+  - match: { hits.hits.0.inner_hits.nested.hits.hits.0.highlight.nested\.field.0: "The <em>quick</em> <em>brown</em> <em>fox</em> is <em>brown</em>." }
+  - match: { hits.hits.0.inner_hits.nested.hits.hits.0.highlight.nested\.field\.vectors.0: "The <em>quick</em> <em>brown</em> <em>fox</em> is <em>brown</em>." }
+  - match: { hits.hits.0.inner_hits.nested.hits.hits.0.highlight.nested\.field\.postings.0: "The <em>quick</em> <em>brown</em> <em>fox</em> is <em>brown</em>." }
+
+---
+"Unified highlighter with stored fields":
+  - do:
+      search:
+        index: test
+        body:
+          query:
+            nested:
+              path: "nested"
+              query:
+                multi_match:
+                  query: "quick brown fox"
+                  fields: [ "nested.stored", "nested.stored_only" ]
+              inner_hits:
+                highlight:
+                  type: "unified"
+                  fields:
+                    nested.stored: {}
+                    nested.stored_only: {}
+
+  - match: { hits.hits.0.inner_hits.nested.hits.hits.0.highlight.nested\.stored.0: "The <em>quick</em> <em>brown</em> <em>fox</em> is <em>brown</em>." }
+  - match: { hits.hits.0.inner_hits.nested.hits.hits.0.highlight.nested\.stored_only.0: "The <em>quick</em> <em>brown</em> <em>fox</em> is <em>brown</em>." }
+
+---
+"Unified highlighter with stored fields and disabled source":
+  - skip:
+      version: " - 7.99.99"
+      reason: "bug fix is not yet backported"
+  - do:
+      indices.create:
+        index: disabled_source
+        body:
+          mappings:
+            _source:
+              enabled: false
+            properties:
+              nested:
+                type: nested
+                properties:
+                  field:
+                    type: text
+                  stored_only:
+                    type: text
+                    store: true
+  - do:
+      index:
+        index: disabled_source
+        id:    1
+        refresh: true
+        body:
+          nested:
+            field: "The quick brown fox is brown."
+            stored_only: "The quick brown fox is brown."
+
+  - do:
+      search:
+        index: disabled_source
+        body:
+          query:
+            nested:
+              path: "nested"
+              query:
+                multi_match:
+                  query: "quick brown fox"
+                  fields: ["nested.field", "nested.stored_only"]
+              inner_hits:
+                highlight:
+                  type: "unified"
+                  fields:
+                    nested.field: {}
+                    nested.stored_only: {}
+
+  - is_false: hits.hits.0.inner_hits.nested.hits.hits.0.highlight.nested\.field
+  - match: { hits.hits.0.inner_hits.nested.hits.hits.0.highlight.nested\.stored_only.0: "The <em>quick</em> <em>brown</em> <em>fox</em> is <em>brown</em>."}

server/src/main/java/org/elasticsearch/search/fetch/FetchPhase.java

@@ -26,7 +26,6 @@
 import org.apache.lucene.search.TotalHits;
 import org.elasticsearch.common.CheckedBiConsumer;
 import org.elasticsearch.common.Nullable;
-import org.elasticsearch.common.bytes.BytesReference;
 import org.elasticsearch.common.collect.Tuple;
 import org.elasticsearch.common.document.DocumentField;
 import org.elasticsearch.common.lucene.index.SequentialStoredFieldsLeafReader;
@@ -363,10 +362,13 @@ private HitContext prepareNestedHitContext(SearchContext context,
             rootId = rootFieldsVisitor.uid();
 
             if (needSource) {
-                BytesReference rootSource = rootFieldsVisitor.source();
-                Tuple<XContentType, Map<String, Object>> tuple = XContentHelper.convertToMap(rootSource, false);
-                rootSourceAsMap = tuple.v2();
-                rootSourceContentType = tuple.v1();
+                if (rootFieldsVisitor.source() != null) {
+                    Tuple<XContentType, Map<String, Object>> tuple = XContentHelper.convertToMap(rootFieldsVisitor.source(), false);
+                    rootSourceAsMap = tuple.v2();
+                    rootSourceContentType = tuple.v1();
+                } else {
+                    rootSourceAsMap = Collections.emptyMap();
+                }
             }
         }
 
@@ -389,7 +391,7 @@ private HitContext prepareNestedHitContext(SearchContext context,
             nestedIdentity, docFields, metaFields);
         HitContext hitContext = new HitContext(hit, subReaderContext, nestedInfo.doc());
 
-        if (rootSourceAsMap != null) {
+        if (rootSourceAsMap != null && rootSourceAsMap.isEmpty() == false) {
             // Isolate the nested json array object that matches with nested hit and wrap it back into the same json
             // structure with the nested json array object being the actual content. The latter is important, so that
             // features like source filtering and highlighting work consistent regardless of whether the field points