Fix sorting on nested field with unmapped (#42451)

Previously sorting on a missing nested field would fail with an
Exception:
`[nested_field] failed to find nested object under path [nested_path]`
despite `unmapped_type` being set on the query.

Fixes: #33644

(cherry picked from commit 631142d)

Author: matriv

server/src/main/java/org/elasticsearch/search/sort/FieldSortBuilder.java

@@ -311,8 +311,10 @@ public SortFieldAndFormat build(QueryShardContext context) throws IOException {
                 return SORT_DOC;
             }
         } else {
+            boolean isUnmapped = false;
             MappedFieldType fieldType = context.fieldMapper(fieldName);
             if (fieldType == null) {
+                isUnmapped = true;
                 if (unmappedType != null) {
                     fieldType = context.getMapperService().unmappedFieldType(unmappedType);
                 } else {
@@ -330,20 +332,22 @@ public SortFieldAndFormat build(QueryShardContext context) throws IOException {
                 localSortMode = reverse ? MultiValueMode.MAX : MultiValueMode.MIN;
             }
 
-            final Nested nested;
-            if (nestedSort != null) {
-                if (context.indexVersionCreated().before(Version.V_6_5_0) && nestedSort.getMaxChildren() != Integer.MAX_VALUE) {
-                    throw new QueryShardException(context,
-                        "max_children is only supported on v6.5.0 or higher");
-                }
-                if (nestedSort.getNestedSort() != null && nestedSort.getMaxChildren() != Integer.MAX_VALUE)  {
-                    throw new QueryShardException(context,
-                        "max_children is only supported on last level of nested sort");
+            Nested nested = null;
+            if (isUnmapped == false) {
+                if (nestedSort != null) {
+                    if (context.indexVersionCreated().before(Version.V_6_5_0) && nestedSort.getMaxChildren() != Integer.MAX_VALUE) {
+                        throw new QueryShardException(context,
+                            "max_children is only supported on v6.5.0 or higher");
+                    }
+                    if (nestedSort.getNestedSort() != null && nestedSort.getMaxChildren() != Integer.MAX_VALUE) {
+                        throw new QueryShardException(context,
+                            "max_children is only supported on last level of nested sort");
+                    }
+                    // new nested sorts takes priority
+                    nested = resolveNested(context, nestedSort);
+                } else {
+                    nested = resolveNested(context, nestedPath, nestedFilter);
                 }
-                // new nested sorts takes priority
-                nested = resolveNested(context, nestedSort);
-            } else {
-                nested = resolveNested(context, nestedPath, nestedFilter);
             }
 
             IndexFieldData<?> fieldData = context.getForField(fieldType);

server/src/test/java/org/elasticsearch/search/sort/FieldSortIT.java

@@ -901,6 +901,22 @@ public void testIgnoreUnmapped() throws Exception {
                 .addSort(SortBuilders.fieldSort("kkk").unmappedType("keyword"))
                 .get();
         assertNoFailures(searchResponse);
+
+        // nested field
+        searchResponse = client().prepareSearch()
+            .setQuery(matchAllQuery())
+            .addSort(SortBuilders.fieldSort("nested.foo").unmappedType("keyword")
+                .setNestedSort(new NestedSortBuilder("nested").setNestedSort(new NestedSortBuilder("nested.foo"))))
+            .get();
+        assertNoFailures(searchResponse);
+
+        // nestedQuery
+        searchResponse = client().prepareSearch()
+            .setQuery(matchAllQuery())
+            .addSort(SortBuilders.fieldSort("nested.foo").unmappedType("keyword")
+                .setNestedSort(new NestedSortBuilder("nested").setFilter(QueryBuilders.termQuery("nested.foo", "abc"))))
+            .get();
+        assertNoFailures(searchResponse);
     }
 
     public void testSortMVField() throws Exception {