Allow plugins to contribute Prolog predicates

A new extension point is defined that allows plugins to provide
additional packages that contain Prolog predicates. The predicates
can e.g. be used in the project submit rules.

Change-Id: I097a9671d6e740755252fb494e75e44dfd3c3d49
Signed-off-by: Edwin Kempin <edwin.kempin@sap.com>

gerrit-server/src/main/java/com/google/gerrit/rules/PredicateProvider.java

@@ -0,0 +1,34 @@
+// 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.rules;
+
+import com.google.common.collect.ImmutableSet;
+import com.google.gerrit.extensions.annotations.ExtensionPoint;
+
+import com.googlecode.prolog_cafe.lang.Predicate;
+
+/**
+ * Provides additional packages that contain Prolog predicates that should be
+ * made available in the Prolog environment. The predicates can e.g. be used in
+ * the project submit rules.
+ *
+ * Each Java class defining a Prolog predicate must be in one of the provided
+ * packages and its name must apply to the 'PRED_[functor]_[arity]' format. In
+ * addition it must extend {@link Predicate}.
+ */
+@ExtensionPoint
+public interface PredicateProvider {
+  /** Return set of packages that contain Prolog predicates */
+  public ImmutableSet<String> getPackages();
+}