[Rest Api Compatibility] Typed endpoints for explain api (#73901)

retrofits the typed endpoints for rest explain api removed in #46926

relates #51816

rest-api-spec/build.gradle

@@ -125,20 +125,14 @@ tasks.named("yamlRestCompatTest").configure {
     // This mean test cases where there is assertion on not finging by type won't work
     'mget/11_default_index_type/Default index/type',
     'mget/16_basic_with_types/Basic multi-get',
-    // 88 - 14 = 74 tests won't be fixed
+    // asserting about type not found won't work as we ignore the type information
+    'explain/40_mix_typeless_typeful/Explain with typeless API on an index that has types',
+    // 89 - 15 = 74 tests won't be fixed
     'cluster.voting_config_exclusions/10_basic/Throw exception when adding voting config exclusion and specifying both node_ids and node_names',
     'cluster.voting_config_exclusions/10_basic/Throw exception when adding voting config exclusion without specifying nodes',
     'count/11_basic_with_types/count body without query element',
     'count/11_basic_with_types/count with body',
     'count/11_basic_with_types/count with empty body',
-    'explain/10_basic/Basic explain',
-    'explain/10_basic/Basic explain with alias',
-    'explain/11_basic_with_types/Basic explain',
-    'explain/11_basic_with_types/Basic explain with alias',
-    'explain/20_source_filtering/Source filtering',
-    'explain/21_source_filtering_with_types/Source filtering',
-    'explain/31_query_string_with_types/explain with query_string parameters',
-    'explain/40_mix_typeless_typeful/Explain with typeless API on an index that has types',
     'field_caps/30_filter/Field caps with index filter',
     'get_source/11_basic_with_types/Basic with types',
     'get_source/16_default_values_with_types/Default values',

server/src/main/java/org/elasticsearch/action/explain/ExplainResponse.java

@@ -12,6 +12,7 @@
 import org.elasticsearch.Version;
 import org.elasticsearch.action.ActionResponse;
 import org.elasticsearch.common.xcontent.ParseField;
+import org.elasticsearch.core.RestApiVersion;
 import org.elasticsearch.common.io.stream.StreamInput;
 import org.elasticsearch.common.io.stream.StreamOutput;
 import org.elasticsearch.common.xcontent.ConstructingObjectParser;
@@ -172,6 +173,10 @@ public static ExplainResponse fromXContent(XContentParser parser, boolean exists
     public XContentBuilder toXContent(XContentBuilder builder, Params params) throws IOException {
         builder.startObject();
         builder.field(_INDEX.getPreferredName(), index);
+        if (builder.getRestApiVersion() == RestApiVersion.V_7) {
+            builder.field(MapperService.TYPE_FIELD_NAME, MapperService.SINGLE_MAPPING_NAME);
+        }
+
         builder.field(_ID.getPreferredName(), id);
         builder.field(MATCHED.getPreferredName(), isMatch());
         if (hasExplanation()) {

server/src/main/java/org/elasticsearch/rest/action/search/RestExplainAction.java

@@ -10,6 +10,7 @@
 
 import org.elasticsearch.action.explain.ExplainRequest;
 import org.elasticsearch.client.node.NodeClient;
+import org.elasticsearch.core.RestApiVersion;
 import org.elasticsearch.common.Strings;
 import org.elasticsearch.index.query.QueryBuilder;
 import org.elasticsearch.rest.BaseRestHandler;
@@ -28,12 +29,20 @@
  * Rest action for computing a score explanation for specific documents.
  */
 public class RestExplainAction extends BaseRestHandler {
+    public static final String TYPES_DEPRECATION_MESSAGE = "[types removal] " +
+        "Specifying a type in explain requests is deprecated.";
 
     @Override
     public List<Route> routes() {
         return List.of(
             new Route(GET, "/{index}/_explain/{id}"),
-            new Route(POST, "/{index}/_explain/{id}"));
+            new Route(POST, "/{index}/_explain/{id}"),
+            Route.builder(GET, "/{index}/{type}/{id}/_explain")
+                .deprecated(TYPES_DEPRECATION_MESSAGE, RestApiVersion.V_7)
+                .build(),
+            Route.builder(POST, "/{index}/{type}/{id}/_explain")
+                .deprecated(TYPES_DEPRECATION_MESSAGE, RestApiVersion.V_7)
+                .build());
     }
 
     @Override
@@ -43,6 +52,9 @@ public String getName() {
 
     @Override
     public RestChannelConsumer prepareRequest(final RestRequest request, final NodeClient client) throws IOException {
+        if(request.getRestApiVersion() == RestApiVersion.V_7 && request.hasParam("type")) {
+            request.param("type");
+        }
         ExplainRequest explainRequest = new ExplainRequest(request.param("index"), request.param("id"));
 
         explainRequest.parent(request.param("parent"));

server/src/test/java/org/elasticsearch/rest/action/search/RestExplainActionTests.java

@@ -0,0 +1,52 @@
+/*
+ * Copyright Elasticsearch B.V. and/or licensed to Elasticsearch B.V. under one
+ * or more contributor license agreements. Licensed under the Elastic License
+ * 2.0 and the Server Side Public License, v 1; you may not use this file except
+ * in compliance with, at your election, the Elastic License 2.0 or the Server
+ * Side Public License, v 1.
+ */
+
+package org.elasticsearch.rest.action.search;
+
+import org.elasticsearch.action.explain.ExplainResponse;
+import org.elasticsearch.core.RestApiVersion;
+import org.elasticsearch.common.xcontent.XContentType;
+import org.elasticsearch.rest.RestRequest;
+import org.elasticsearch.test.rest.FakeRestRequest;
+import org.elasticsearch.test.rest.RestActionTestCase;
+import org.junit.Before;
+import org.mockito.Mockito;
+
+import java.util.Collections;
+import java.util.List;
+import java.util.Map;
+
+public class RestExplainActionTests extends RestActionTestCase {
+    final List<String> contentTypeHeader = Collections.singletonList(compatibleMediaType(XContentType.VND_JSON, RestApiVersion.V_7));
+
+    @Before
+    public void setUpAction() {
+        RestExplainAction action = new RestExplainAction();
+        controller().registerHandler(action);
+        verifyingClient.setExecuteVerifier((actionType, request) -> Mockito.mock(ExplainResponse.class));
+        verifyingClient.setExecuteLocallyVerifier((actionType, request) -> Mockito.mock(ExplainResponse.class));
+    }
+
+    public void testTypeInPath() {
+        RestRequest deprecatedRequest = new FakeRestRequest.Builder(xContentRegistry())
+            .withHeaders(Map.of("Accept", contentTypeHeader))
+            .withMethod(RestRequest.Method.GET)
+            .withPath("/some_index/some_type/some_id/_explain")
+            .build();
+        dispatchRequest(deprecatedRequest);
+        assertWarnings(RestExplainAction.TYPES_DEPRECATION_MESSAGE);
+
+        RestRequest validRequest = new FakeRestRequest.Builder(xContentRegistry())
+            .withHeaders(Map.of("Accept", contentTypeHeader))
+            .withMethod(RestRequest.Method.GET)
+            .withPath("/some_index/_explain/some_id")
+            .build();
+        dispatchRequest(validRequest);
+    }
+
+}