Policy Extender and Tests (#2461)

Policy application has also been refactored, so that policies return one of `allow`, `deny`, `forceAllow`, `forceDeny`. The result of a set of policies is no longer the first non-null result, but rather the highest priority result (forceDeny > forceAllow > deny > allow, so if a single forceDeny is present, that beats out all other returned results). This removes order in which extensions boot as a factor.

src/Extend/Policy.php

@@ -0,0 +1,69 @@
+<?php
+
+/*
+ * This file is part of Flarum.
+ *
+ * For detailed copyright and license information, please view the
+ * LICENSE file that was distributed with this source code.
+ */
+
+namespace Flarum\Extend;
+
+use Flarum\Extension\Extension;
+use Flarum\User\Access\AbstractPolicy;
+use Illuminate\Contracts\Container\Container;
+
+class Policy implements ExtenderInterface
+{
+    private $globalPolicies = [];
+    private $modelPolicies = [];
+
+    /**
+     * Add a custom policy for when an ability check is ran without a model instance.
+     *
+     * @param string $policy ::class attribute of policy class, which must extend Flarum\User\AbstractPolicy
+     */
+    public function globalPolicy(string $policy)
+    {
+        $this->globalPolicies[] = $policy;
+
+        return $this;
+    }
+
+    /**
+     * Add a custom policy for when an ability check is ran on an instance of a model.
+     *
+     * @param string $modelClass The ::class attribute of the model you are applying policies to.
+     *                           This model should extend from \Flarum\Database\AbstractModel.
+     * @param string $policy ::class attribute of policy class, which must extend Flarum\User\AbstractPolicy
+     */
+    public function modelPolicy(string $modelClass, string $policy)
+    {
+        if (! array_key_exists($modelClass, $this->modelPolicies)) {
+            $this->modelPolicies[$modelClass] = [];
+        }
+
+        $this->modelPolicies[$modelClass][] = $policy;
+
+        return $this;
+    }
+
+    public function extend(Container $container, Extension $extension = null)
+    {
+        $container->extend('flarum.policies', function ($existingPolicies) {
+            foreach ($this->modelPolicies as $modelClass => $addPolicies) {
+                if (! array_key_exists($modelClass, $existingPolicies)) {
+                    $existingPolicies[$modelClass] = [];
+                }
+
+                foreach ($addPolicies as $policy) {
+                    $existingPolicies[$modelClass][] = $policy;
+                }
+            }
+
+            $existingPolicies[AbstractPolicy::GLOBAL] = array_merge($existingPolicies[AbstractPolicy::GLOBAL], $this->globalPolicies);
+
+            return $existingPolicies;
+        });
+    }
+}