Support GET, PUT, DELETE on /changes/{id}/topic

The topic is now modified by REST API calls against /topic. The web UI
uses PUT to alter the topic with a JSON payload, but the server
accepts quite a few different REST forms that clients can easily send.

Change-Id: Ia5edbb232bc288acdad8b145956ad275d170629a

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

@@ -0,0 +1,120 @@
+// Copyright (C) 2012 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.server.change;
+
+import com.google.common.base.Strings;
+import com.google.gerrit.extensions.restapi.AuthException;
+import com.google.gerrit.extensions.restapi.BadRequestException;
+import com.google.gerrit.extensions.restapi.DefaultInput;
+import com.google.gerrit.extensions.restapi.ResourceConflictException;
+import com.google.gerrit.extensions.restapi.RestModifyView;
+import com.google.gerrit.reviewdb.client.Change;
+import com.google.gerrit.reviewdb.client.ChangeMessage;
+import com.google.gerrit.reviewdb.server.ReviewDb;
+import com.google.gerrit.server.ChangeUtil;
+import com.google.gerrit.server.IdentifiedUser;
+import com.google.gerrit.server.change.PutTopic.Input;
+import com.google.gerrit.server.project.ChangeControl;
+import com.google.gwtorm.server.AtomicUpdate;
+import com.google.inject.Inject;
+import com.google.inject.Provider;
+
+import java.util.Collections;
+
+class PutTopic implements RestModifyView<ChangeResource, Input> {
+  private final Provider<ReviewDb> dbProvider;
+
+  static class Input {
+    @DefaultInput
+    String topic;
+    String message;
+  }
+
+  @Inject
+  PutTopic(Provider<ReviewDb> dbProvider) {
+    this.dbProvider = dbProvider;
+  }
+
+  @Override
+  public Class<Input> inputType() {
+    return Input.class;
+  }
+
+  @Override
+  public Object apply(ChangeResource req, Input input)
+      throws BadRequestException, AuthException,
+      ResourceConflictException, Exception {
+    if (input == null) {
+      input = new Input();
+    }
+
+    ChangeControl control = req.getControl();
+    Change change = req.getChange();
+    if (!control.canEditTopicName()) {
+      throw new AuthException("changing topic not permitted");
+    } else if (!change.getStatus().isOpen()) {
+      throw new ResourceConflictException("change is " + status(change));
+    }
+
+    ReviewDb db = dbProvider.get();
+    final String newTopicName = Strings.nullToEmpty(input.topic);
+    String oldTopicName = Strings.nullToEmpty(change.getTopic());
+    if (!oldTopicName.equals(newTopicName)) {
+      String summary;
+      if (oldTopicName.isEmpty()) {
+        summary = "Topic set to \"" + newTopicName + "\".";
+      } else if (newTopicName.isEmpty()) {
+        summary = "Topic \"" + oldTopicName + "\" removed.";
+      } else {
+        summary = String.format(
+            "Topic updated from \"%s\" to \"%s\".",
+            oldTopicName, newTopicName);
+      }
+
+      ChangeMessage cmsg = new ChangeMessage(
+          new ChangeMessage.Key(change.getId(), ChangeUtil.messageUUID(db)),
+          ((IdentifiedUser) control.getCurrentUser()).getAccountId(),
+          change.currentPatchSetId());
+      StringBuilder msgBuf = new StringBuilder(summary);
+      if (!Strings.isNullOrEmpty(input.message)) {
+        msgBuf.append("\n\n");
+        msgBuf.append(input.message);
+      }
+      cmsg.setMessage(msgBuf.toString());
+
+      Change updatedChange = db.changes().atomicUpdate(change.getId(),
+        new AtomicUpdate<Change>() {
+          @Override
+          public Change update(Change change) {
+            if (change.getStatus().isOpen()) {
+              change.setTopic(Strings.emptyToNull(newTopicName));
+              return change;
+            }
+            return null;
+          }
+        });
+      if (updatedChange == null) {
+        change = db.changes().get(change.getId());
+        throw new ResourceConflictException("change is " + status(change));
+      }
+      db.changeMessages().insert(Collections.singleton(cmsg));
+    }
+    return Strings.nullToEmpty(newTopicName);
+  }
+
+  private static String status(Change change) {
+    return change != null ? change.getStatus().name().toLowerCase() : "deleted";
+  }
+}