Make sure to rewrite explain query on coordinator (#87013)

Some queries (like terms lookup queries) need to be rewritten on the
coordinator node, usually to fetch some resource. The explain action was
missing this rewrite step, which caused failures later when trying to execute
the query.

Closes #64281.

docs/changelog/87013.yaml

@@ -0,0 +1,6 @@
+pr: 87013
+summary: Make sure to rewrite explain query on coordinator
+area: Search
+type: bug
+issues:
+ - 64281

server/src/internalClusterTest/java/org/elasticsearch/explain/ExplainActionIT.java

@@ -15,7 +15,10 @@
 import org.elasticsearch.common.io.stream.OutputStreamStreamOutput;
 import org.elasticsearch.common.lucene.Lucene;
 import org.elasticsearch.common.settings.Settings;
+import org.elasticsearch.index.mapper.MapperService;
 import org.elasticsearch.index.query.QueryBuilders;
+import org.elasticsearch.index.query.TermsQueryBuilder;
+import org.elasticsearch.indices.TermsLookup;
 import org.elasticsearch.test.ESIntegTestCase;
 
 import java.io.ByteArrayInputStream;
@@ -28,6 +31,7 @@
 import java.util.Set;
 
 import static java.util.Collections.singleton;
+import static org.elasticsearch.cluster.metadata.IndexMetadata.SETTING_NUMBER_OF_SHARDS;
 import static org.elasticsearch.index.query.QueryBuilders.queryStringQuery;
 import static org.elasticsearch.test.hamcrest.ElasticsearchAssertions.assertAcked;
 import static org.elasticsearch.xcontent.XContentFactory.jsonBuilder;
@@ -155,7 +159,7 @@ public void testExplainWithFields() throws Exception {
     }
 
     @SuppressWarnings("unchecked")
-    public void testExplainWitSource() throws Exception {
+    public void testExplainWithSource() throws Exception {
         assertAcked(prepareCreate("test").addAlias(new Alias("alias")));
         ensureGreen("test");
 
@@ -288,4 +292,31 @@ public void testStreamExplain() throws Exception {
         result = Lucene.readExplanation(esBuffer);
         assertThat(exp.toString(), equalTo(result.toString()));
     }
+
+    public void testQueryRewrite() {
+        client().admin()
+            .indices()
+            .prepareCreate("twitter")
+            .addMapping(MapperService.SINGLE_MAPPING_NAME, "user", "type=keyword", "followers", "type=keyword")
+            .setSettings(Settings.builder().put(SETTING_NUMBER_OF_SHARDS, 2))
+            .get();
+        ensureGreen("twitter");
+
+        client().prepareIndex("twitter", MapperService.SINGLE_MAPPING_NAME, "1")
+            .setSource("user", "user1", "followers", new String[] { "user2", "user3" })
+            .get();
+        client().prepareIndex("twitter", MapperService.SINGLE_MAPPING_NAME, "2")
+            .setSource("user", "user2", "followers", new String[] { "user1" })
+            .get();
+        refresh();
+
+        TermsQueryBuilder termsLookupQuery = QueryBuilders.termsLookupQuery("user", new TermsLookup("twitter", "2", "followers"));
+        ExplainResponse response = client().prepareExplain("twitter", MapperService.SINGLE_MAPPING_NAME, "1")
+            .setQuery(termsLookupQuery)
+            .get();
+
+        Explanation explanation = response.getExplanation();
+        assertNotNull(explanation);
+        assertTrue(explanation.isMatch());
+    }
 }

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

@@ -29,6 +29,8 @@
 import org.elasticsearch.index.mapper.IdFieldMapper;
 import org.elasticsearch.index.mapper.MapperService;
 import org.elasticsearch.index.mapper.Uid;
+import org.elasticsearch.index.query.QueryBuilder;
+import org.elasticsearch.index.query.Rewriteable;
 import org.elasticsearch.index.shard.IndexShard;
 import org.elasticsearch.index.shard.ShardId;
 import org.elasticsearch.search.SearchService;
@@ -43,6 +45,7 @@
 
 import java.io.IOException;
 import java.util.Set;
+import java.util.function.LongSupplier;
 
 /**
  * Explain transport action. Computes the explain on the targeted shard.
@@ -77,7 +80,14 @@ public TransportExplainAction(
     @Override
     protected void doExecute(Task task, ExplainRequest request, ActionListener<ExplainResponse> listener) {
         request.nowInMillis = System.currentTimeMillis();
-        super.doExecute(task, request, listener);
+        ActionListener<QueryBuilder> rewriteListener = ActionListener.wrap(rewrittenQuery -> {
+            request.query(rewrittenQuery);
+            super.doExecute(task, request, listener);
+        }, listener::onFailure);
+
+        assert request.query() != null;
+        LongSupplier timeProvider = () -> request.nowInMillis;
+        Rewriteable.rewriteAndFetch(request.query(), searchService.getRewriteContext(timeProvider), rewriteListener);
     }
 
     @Override