Fix failure for validate API on a terms query

server/src/main/java/org/elasticsearch/action/admin/indices/validate/query/QueryExplanation.java

@@ -75,7 +75,11 @@ public String getExplanation() {
 
     @Override
     public void readFrom(StreamInput in) throws IOException {
-        index = in.readString();
+        if (in.getVersion().onOrAfter(Version.V_7_0_0_alpha1)) {
+            index = in.readOptionalString();
+        } else {
+            index = in.readString();
+        }
         if (in.getVersion().onOrAfter(Version.V_5_4_0)) {
             shard = in.readInt();
         } else {
@@ -88,7 +92,11 @@ public void readFrom(StreamInput in) throws IOException {
 
     @Override
     public void writeTo(StreamOutput out) throws IOException {
-        out.writeString(index);
+        if (out.getVersion().onOrAfter(Version.V_7_0_0_alpha1)) {
+            out.writeOptionalString(index);
+        } else {
+            out.writeString(index);
+        }
         if (out.getVersion().onOrAfter(Version.V_5_4_0)) {
             out.writeInt(shard);
         }

server/src/main/java/org/elasticsearch/action/admin/indices/validate/query/TransportValidateQueryAction.java

@@ -38,8 +38,11 @@
 import org.elasticsearch.common.inject.Inject;
 import org.elasticsearch.common.lease.Releasables;
 import org.elasticsearch.common.settings.Settings;
+import org.elasticsearch.index.IndexNotFoundException;
 import org.elasticsearch.index.query.ParsedQuery;
 import org.elasticsearch.index.query.QueryShardException;
+import org.elasticsearch.index.query.Rewriteable;
+import org.elasticsearch.indices.IndexClosedException;
 import org.elasticsearch.search.SearchService;
 import org.elasticsearch.search.internal.AliasFilter;
 import org.elasticsearch.search.internal.SearchContext;
@@ -54,6 +57,7 @@
 import java.util.Map;
 import java.util.Set;
 import java.util.concurrent.atomic.AtomicReferenceArray;
+import java.util.function.LongSupplier;
 
 public class TransportValidateQueryAction extends TransportBroadcastAction<ValidateQueryRequest, ValidateQueryResponse, ShardValidateQueryRequest, ShardValidateQueryResponse> {
 
@@ -71,7 +75,39 @@ public TransportValidateQueryAction(Settings settings, ThreadPool threadPool, Cl
     @Override
     protected void doExecute(Task task, ValidateQueryRequest request, ActionListener<ValidateQueryResponse> listener) {
         request.nowInMillis = System.currentTimeMillis();
-        super.doExecute(task, request, listener);
+        LongSupplier timeProvider = () -> request.nowInMillis;
+        ActionListener<org.elasticsearch.index.query.QueryBuilder> rewriteListener = ActionListener.wrap(rewrittenQuery -> {
+            request.query(rewrittenQuery);
+            super.doExecute(task, request, listener);
+        },
+            ex -> {
+            if (ex instanceof IndexNotFoundException ||
+                ex instanceof IndexClosedException) {
+                listener.onFailure(ex);
+            }
+            List<QueryExplanation> explanations = new ArrayList<>();
+            explanations.add(new QueryExplanation(null,
+                QueryExplanation.RANDOM_SHARD,
+                false,
+                null,
+                ex.getMessage()));
+            listener.onResponse(
+                new ValidateQueryResponse(
+                    false,
+                    explanations,
+                    // totalShards is documented as "the total shards this request ran against",
+                    // which is 0 since the failure is happening on the coordinating node.
+                    0,
+                    0 ,
+                    0,
+                    null));
+        });
+        if (request.query() == null) {
+            rewriteListener.onResponse(request.query());
+        } else {
+            Rewriteable.rewriteAndFetch(request.query(), searchService.getRewriteContext(timeProvider),
+                rewriteListener);
+        }
     }
 
     @Override

server/src/main/java/org/elasticsearch/index/query/Rewriteable.java

@@ -19,6 +19,7 @@
 package org.elasticsearch.index.query;
 
 import org.elasticsearch.action.ActionListener;
+import org.elasticsearch.common.ParsingException;
 
 import java.io.IOException;
 import java.util.ArrayList;
@@ -111,7 +112,7 @@ static <T extends Rewriteable<T>> void rewriteAndFetch(T original, QueryRewriteC
                 }
             }
             rewriteResponse.onResponse(builder);
-        } catch (IOException ex) {
+        } catch (IOException|IllegalArgumentException|ParsingException ex) {
             rewriteResponse.onFailure(ex);
         }
     }

server/src/test/java/org/elasticsearch/validate/SimpleValidateQueryIT.java

@@ -29,6 +29,8 @@
 import org.elasticsearch.index.query.MoreLikeThisQueryBuilder;
 import org.elasticsearch.index.query.QueryBuilder;
 import org.elasticsearch.index.query.QueryBuilders;
+import org.elasticsearch.index.query.TermsQueryBuilder;
+import org.elasticsearch.indices.TermsLookup;
 import org.elasticsearch.test.ESIntegTestCase;
 import org.elasticsearch.test.ESIntegTestCase.ClusterScope;
 import org.elasticsearch.test.ESIntegTestCase.Scope;
@@ -330,4 +332,21 @@ private static void assertExplanations(QueryBuilder queryBuilder,
             assertThat(response.isValid(), equalTo(true));
         }
     }
+
+    public void testExplainTermsQueryWithLookup() throws Exception {
+        client().admin().indices().prepareCreate("twitter")
+            .addMapping("_doc", "user", "type=integer", "followers", "type=integer")
+            .setSettings(Settings.builder().put(SETTING_NUMBER_OF_SHARDS, 2).put("index.number_of_routing_shards", 2)).get();
+        client().prepareIndex("twitter", "_doc", "1")
+            .setSource("followers", new int[] {1, 2, 3}).get();
+        refresh();
+
+        TermsQueryBuilder termsLookupQuery = QueryBuilders.termsLookupQuery("user", new TermsLookup("twitter", "_doc", "1", "followers"));
+        ValidateQueryResponse response = client().admin().indices().prepareValidateQuery("twitter")
+            .setTypes("_doc")
+            .setQuery(termsLookupQuery)
+            .setExplain(true)
+            .execute().actionGet();
+        assertThat(response.isValid(), is(true));
+    }
 }