GetChange: populate ChangeInfo.metaRevId with the NoteDb meta ref SHA1

The NoteDb meta SHA1 is the unique identifier the state of a change,
together with the robot-comment SHA1.

Change-Id: I8b4cf63f9e8255ab9c2d91ddf002820f46a288c8

Documentation/rest-api-changes.txt

@@ -6467,6 +6467,8 @@ patch set to a link:#revision-info[RevisionInfo] entity. +
 Only set if link:#current-revision[the current revision] is requested
 (in which case it will only contain a key for the current revision) or
 if link:#all-revisions[all revisions] are requested.
+|`meta_rev_id`           |optional|
+The SHA1 of the NoteDb meta ref.
 |`tracking_ids`       |optional|
 A list of link:#tracking-id-info[TrackingIdInfo] entities describing
 references to external tracking systems. Only set if

java/com/google/gerrit/extensions/common/ChangeInfo.java

@@ -70,6 +70,7 @@ public class ChangeInfo {
   public String submissionId;
   public Integer cherryPickOfChange;
   public Integer cherryPickOfPatchSet;
+  public String metaRevId;
 
   /**
    * Whether the change contains conflicts.

java/com/google/gerrit/server/change/ChangeJson.java

@@ -54,6 +54,7 @@ import com.google.gerrit.entities.ChangeMessage;
 import com.google.gerrit.entities.PatchSet;
 import com.google.gerrit.entities.PatchSetApproval;
 import com.google.gerrit.entities.Project;
+import com.google.gerrit.entities.RefNames;
 import com.google.gerrit.entities.SubmitRecord;
 import com.google.gerrit.entities.SubmitRecord.Status;
 import com.google.gerrit.entities.SubmitRequirement;
@@ -75,6 +76,7 @@ import com.google.gerrit.extensions.common.RevisionInfo;
 import com.google.gerrit.extensions.common.SubmitRequirementInfo;
 import com.google.gerrit.extensions.common.TrackingIdInfo;
 import com.google.gerrit.extensions.restapi.Url;
+import com.google.gerrit.index.RefState;
 import com.google.gerrit.index.query.QueryResult;
 import com.google.gerrit.metrics.Description;
 import com.google.gerrit.metrics.Description.Units;
@@ -570,6 +572,15 @@ public class ChangeJson {
     out.totalCommentCount = cd.totalCommentCount();
     out.unresolvedCommentCount = cd.unresolvedCommentCount();
 
+    if (cd.getRefStates() != null) {
+      String metaName = RefNames.changeMetaRef(cd.getId());
+      Optional<RefState> metaState =
+          cd.getRefStates().values().stream().filter(r -> r.ref().equals(metaName)).findAny();
+
+      // metaState should always be there, but it doesn't hurt to be extra careful.
+      metaState.ifPresent(rs -> out.metaRevId = rs.id().getName());
+    }
+
     if (user.isIdentifiedUser()) {
       Collection<String> stars = cd.stars(user.getAccountId());
       out.starred = stars.contains(StarredChangesUtil.DEFAULT_LABEL) ? true : null;

javatests/com/google/gerrit/acceptance/rest/change/ChangeMetaIT.java

@@ -0,0 +1,58 @@
+// Copyright (C) 2017 The Android Open Source Project
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package com.google.gerrit.acceptance.rest.change;
+
+import static com.google.common.truth.Truth.assertThat;
+import static com.google.gerrit.entities.RefNames.changeMetaRef;
+
+import com.google.common.collect.Iterables;
+import com.google.gerrit.acceptance.AbstractDaemonTest;
+import com.google.gerrit.acceptance.PushOneCommit;
+import com.google.gerrit.entities.Change;
+import com.google.gerrit.extensions.common.ChangeInfo;
+import org.eclipse.jgit.lib.Repository;
+import org.junit.Test;
+
+/** Test handling of the NoteDb commit hash in the GetChange endpoint */
+public class ChangeMetaIT extends AbstractDaemonTest {
+  @Test
+  public void metaSha1_fromIndex() throws Exception {
+    PushOneCommit.Result result = createChange();
+    String changeId = result.getChangeId();
+
+    try (AutoCloseable ignored = disableNoteDb()) {
+      ChangeInfo change =
+          Iterables.getOnlyElement(gApi.changes().query().withQuery("change:" + changeId).get());
+
+      try (Repository repo = repoManager.openRepository(project)) {
+        assertThat(change.metaRevId)
+            .isEqualTo(
+                repo.exactRef(changeMetaRef(Change.id(change._number))).getObjectId().getName());
+      }
+    }
+  }
+
+  @Test
+  public void metaSha1_fromNoteDb() throws Exception {
+    PushOneCommit.Result result = createChange();
+    String changeId = result.getChangeId();
+    ChangeInfo before = gApi.changes().id(changeId).get();
+    try (Repository repo = repoManager.openRepository(project)) {
+      assertThat(before.metaRevId)
+          .isEqualTo(
+              repo.exactRef(changeMetaRef(Change.id(before._number))).getObjectId().getName());
+    }
+  }
+}