ChangeApi: Implement get/put topic

Change-Id: I8e76f05d52f486b85cb905f7a4bff95966faae79

gerrit-acceptance-tests/src/test/java/com/google/gerrit/acceptance/api/change/ChangeIT.java

@@ -278,4 +278,24 @@ public class ChangeIT extends AbstractDaemonTest {
     revision(r).review(ReviewInput.recommend());
     assertTrue(get(r.getChangeId()).reviewed);
   }
+
+  @Test
+  public void topic() throws Exception {
+    PushOneCommit.Result r = createChange();
+    assertEquals("", gApi.changes()
+        .id(r.getChangeId())
+        .topic());
+    gApi.changes()
+        .id(r.getChangeId())
+        .topic("mytopic");
+    assertEquals("mytopic", gApi.changes()
+        .id(r.getChangeId())
+        .topic());
+    gApi.changes()
+        .id(r.getChangeId())
+        .topic("");
+    assertEquals("", gApi.changes()
+        .id(r.getChangeId())
+        .topic());
+  }
 }

gerrit-extension-api/src/main/java/com/google/gerrit/extensions/api/changes/ChangeApi.java

@@ -37,6 +37,9 @@ public interface ChangeApi {
   ChangeApi revert() throws RestApiException;
   ChangeApi revert(RevertInput in) throws RestApiException;
 
+  String topic() throws RestApiException;
+  void topic(String topic) throws RestApiException;
+
   void addReviewer(AddReviewerInput in) throws RestApiException;
   void addReviewer(String in) throws RestApiException;
 
@@ -102,6 +105,16 @@ public interface ChangeApi {
       throw new NotImplementedException();
     }
 
+    @Override
+    public String topic() throws RestApiException {
+      throw new NotImplementedException();
+    }
+
+    @Override
+    public void topic(String topic) throws RestApiException {
+      throw new NotImplementedException();
+    }
+
     @Override
     public void addReviewer(AddReviewerInput in) throws RestApiException {
       throw new NotImplementedException();

gerrit-server/src/main/java/com/google/gerrit/server/api/changes/ChangeApiImpl.java

@@ -29,7 +29,9 @@ import com.google.gerrit.extensions.restapi.RestApiException;
 import com.google.gerrit.server.change.Abandon;
 import com.google.gerrit.server.change.ChangeJson;
 import com.google.gerrit.server.change.ChangeResource;
+import com.google.gerrit.server.change.GetTopic;
 import com.google.gerrit.server.change.PostReviewers;
+import com.google.gerrit.server.change.PutTopic;
 import com.google.gerrit.server.change.Restore;
 import com.google.gerrit.server.change.Revert;
 import com.google.gerrit.server.change.Revisions;
@@ -53,6 +55,8 @@ class ChangeApiImpl extends ChangeApi.NotImplemented implements ChangeApi {
   private final Abandon abandon;
   private final Revert revert;
   private final Restore restore;
+  private final GetTopic getTopic;
+  private final PutTopic putTopic;
   private final Provider<PostReviewers> postReviewers;
   private final Provider<ChangeJson> changeJson;
 
@@ -63,6 +67,8 @@ class ChangeApiImpl extends ChangeApi.NotImplemented implements ChangeApi {
       Abandon abandon,
       Revert revert,
       Restore restore,
+      GetTopic getTopic,
+      PutTopic putTopic,
       Provider<PostReviewers> postReviewers,
       Provider<ChangeJson> changeJson,
       @Assisted ChangeResource change) {
@@ -72,6 +78,8 @@ class ChangeApiImpl extends ChangeApi.NotImplemented implements ChangeApi {
     this.revisionApi = revisionApi;
     this.abandon = abandon;
     this.restore = restore;
+    this.getTopic = getTopic;
+    this.putTopic = putTopic;
     this.postReviewers = postReviewers;
     this.changeJson = changeJson;
     this.change = change;
@@ -144,6 +152,22 @@ class ChangeApiImpl extends ChangeApi.NotImplemented implements ChangeApi {
     }
   }
 
+  @Override
+  public String topic() throws RestApiException {
+    return getTopic.apply(change);
+  }
+
+  @Override
+  public void topic(String topic) throws RestApiException {
+    PutTopic.Input in = new PutTopic.Input();
+    in.topic = topic;
+    try {
+      putTopic.apply(change, in);
+    } catch (OrmException | IOException e) {
+      throw new RestApiException("Cannot set topic", e);
+    }
+  }
+
   @Override
   public void addReviewer(String reviewer) throws RestApiException {
     AddReviewerInput in = new AddReviewerInput();

gerrit-server/src/main/java/com/google/gerrit/server/change/GetTopic.java

@@ -19,7 +19,7 @@ import com.google.gerrit.extensions.restapi.RestReadView;
 import com.google.inject.Singleton;
 
 @Singleton
-class GetTopic implements RestReadView<ChangeResource> {
+public class GetTopic implements RestReadView<ChangeResource> {
   @Override
   public String apply(ChangeResource rsrc) {
     return Strings.nullToEmpty(rsrc.getChange().getTopic());

gerrit-server/src/main/java/com/google/gerrit/server/change/PutTopic.java

@@ -42,7 +42,7 @@ import com.google.inject.Singleton;
 import java.io.IOException;
 
 @Singleton
-class PutTopic implements RestModifyView<ChangeResource, Input>,
+public class PutTopic implements RestModifyView<ChangeResource, Input>,
     UiAction<ChangeResource> {
   private final Provider<ReviewDb> dbProvider;
   private final ChangeIndexer indexer;
@@ -50,9 +50,9 @@ class PutTopic implements RestModifyView<ChangeResource, Input>,
   private final ChangeUpdate.Factory updateFactory;
   private final ChangeMessagesUtil cmUtil;
 
-  static class Input {
+  public static class Input {
     @DefaultInput
-    String topic;
+    public String topic;
   }
 
   @Inject