AssigneeValidationListener extension point

Listener to provide extensions with validation of assignees.

Change-Id: I61f6558d79b8e4c0d322c293dacdf4f33d882b26


* submodules:
* Update plugins/cookbook-plugin from branch 'master'
  - AssigneeValidator example
    
    Will not allow a user to be assigned to more than 5 changes.
    
    Change-Id: I1887f7007e66bd5ccac207c7789be2712ffbd00a

gerrit-server/src/main/java/com/google/gerrit/server/validators/AssigneeValidationListener.java

@@ -0,0 +1,34 @@
+// Copyright (C) 2016 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.validators;
+
+import com.google.gerrit.extensions.annotations.ExtensionPoint;
+import com.google.gerrit.reviewdb.client.Account;
+import com.google.gerrit.reviewdb.client.Change;
+
+/**
+ * Listener to provide validation of assignees.
+ */
+@ExtensionPoint
+public interface AssigneeValidationListener {
+  /**
+   * Invoked by Gerrit before the assignee of a change is modified.
+   *
+   * @param change the change on which the assignee is changed
+   * @param assignee the new assignee. Null if removed
+   * @throws ValidationException if validation fails
+   */
+  void validateAssignee(Change change, Account assignee) throws ValidationException;
+}