Added GroupMembership for group membership checks.

This replaces existing uses of a Set to determine membership and
will make it possible to integrate with group systems that do not
allow efficient enumentation of memberships.

Change-Id: Iba9842ca1a355fc100fb4a02d7954d89032cdba0

gerrit-server/src/main/java/com/google/gerrit/server/account/GroupMembership.java

@@ -0,0 +1,51 @@
+// 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.account;
+
+import com.google.gerrit.reviewdb.client.AccountGroup;
+
+import java.util.Collections;
+import java.util.Set;
+
+/**
+ * Implementations of GroupMembership provide methods to test
+ * the presence of a user in a particular group.
+ */
+public interface GroupMembership {
+
+  public static final GroupMembership EMPTY =
+      new ListGroupMembership(Collections.<AccountGroup.UUID>emptySet());
+
+  /**
+   * Returns {@code true} when the user this object was created for is a member
+   * of the specified group.
+   */
+  boolean contains(AccountGroup.UUID groupId);
+
+  /**
+   * Returns {@code true} when the user this object was created for is a member
+   * of any of the specified group.
+   */
+  boolean containsAnyOf(Iterable<AccountGroup.UUID> groupIds);
+
+  /**
+   * Returns the set of groups that can be determined by the implementation.
+   * This may not return all groups the {@link #contains(AccountGroup.UUID)}
+   * would return {@code true} for, but will at least contain all top level
+   * groups. This restriction stems from the API of some group systems, which
+   * make it expensive to enumate the members of a group.
+   */
+  Set<AccountGroup.UUID> getKnownGroups();
+}