Create ActivityPub\FetchQueue and ActivityPub\FetchQueueItem classes

- These classes are used to flatten the recursive missing activity fetch that can hit PHP's maximum function nesting limit
- The original caller is responsible for processing the remaining queue once the original activity has been fetched

src/Protocol/ActivityPub/FetchQueue.php

@@ -0,0 +1,57 @@
+<?php
+/**
+ * @copyright Copyright (C) 2010-2022, the Friendica project
+ *
+ * @license GNU AGPL version 3 or any later version
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU Affero General Public License as
+ * published by the Free Software Foundation, either version 3 of the
+ * License, or (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU Affero General Public License for more details.
+ *
+ * You should have received a copy of the GNU Affero General Public License
+ * along with this program.  If not, see <https://www.gnu.org/licenses/>.
+ *
+ */
+
+namespace Friendica\Protocol\ActivityPub;
+
+/**
+ * This class prevents maximum function nesting errors by flattening recursive calls to Processor::fetchMissingActivity
+ */
+class FetchQueue
+{
+	/** @var FetchQueueItem[] */
+	protected $queue = [];
+
+	public function push(FetchQueueItem $item)
+	{
+		array_push($this->queue, $item);
+	}
+
+	/**
+	 * Processes missing activities one by one. It is possible that a processing call will add additional missing
+	 * activities, they will be processed in subsequent iterations of the loop.
+	 *
+	 * Since this process is self-contained, it isn't suitable to retrieve the URI of a single activity.
+	 *
+	 * The simplest way to get the URI of the first activity and ensures all the parents are fetched is this way:
+	 *
+	 * $fetchQueue = new ActivityPub\FetchQueue();
+	 * $fetchedUri = ActivityPub\Processor::fetchMissingActivity($fetchQueue, $activityUri);
+	 * $fetchQueue->process();
+	 */
+	public function process()
+	{
+		while (count($this->queue)) {
+			$fetchQueueItem = array_pop($this->queue);
+
+			call_user_func_array([Processor::class, 'fetchMissingActivity'], array_merge([$this], $fetchQueueItem->toParameters()));
+		}
+	}
+}