MDL-59506 core_lock: use a null lock during install

Use a different lock factory during installation which does not actually
lock at all.

lib/classes/lock/installation_lock_factory.php

@@ -0,0 +1,127 @@
+<?php
+// This file is part of Moodle - http://moodle.org/
+//
+// Moodle is free software: you can redistribute it and/or modify
+// it under the terms of the GNU General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// Moodle 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 General Public License for more details.
+//
+// You should have received a copy of the GNU General Public License
+// along with Moodle.  If not, see <http://www.gnu.org/licenses/>.
+
+/**
+ * Lock factory for use during installation.
+ *
+ * @package    core
+ * @category   lock
+ * @copyright  Andrew Nicols <andrew@nicols.co.uk>
+ * @license    http://www.gnu.org/copyleft/gpl.html GNU GPL v3 or later
+ */
+
+namespace core\lock;
+
+defined('MOODLE_INTERNAL') || die();
+
+/**
+ * Lock factory for use during installation.
+ *
+ * @package   core
+ * @category  lock
+ * @copyright  Andrew Nicols <andrew@nicols.co.uk>
+ * @license   http://www.gnu.org/copyleft/gpl.html GNU GPL v3 or later
+ */
+class installation_lock_factory implements lock_factory {
+
+    /**
+     * Create this lock factory.
+     *
+     * @param string $type - The type, e.g. cron, cache, session
+     */
+    public function __construct($type) {
+    }
+
+    /**
+     * Return information about the blocking behaviour of the lock type on this platform.
+     *
+     * @return boolean - False if attempting to get a lock will block indefinitely.
+     */
+    public function supports_timeout() {
+        return true;
+    }
+
+    /**
+     * This lock type will be automatically released when a process ends.
+     *
+     * @return boolean - True
+     */
+    public function supports_auto_release() {
+        return true;
+    }
+
+    /**
+     * This lock factory is only available during the initial installation.
+     * To use it at any other time would be potentially dangerous.
+     *
+     * @return boolean
+     */
+    public function is_available() {
+        return during_initial_install();
+    }
+
+    /**
+     * Multiple locks for the same resource cannot be held from a single process.
+     *
+     * @return boolean - False
+     */
+    public function supports_recursion() {
+        return false;
+    }
+
+    /**
+     * Get some info that might be useful for debugging.
+     * @return boolean - string
+     */
+    protected function get_debug_info() {
+        return 'host:' . php_uname('n') . ', pid:' . getmypid() . ', time:' . time();
+    }
+
+    /**
+     * Get a lock within the specified timeout or return false.
+     *
+     * @param string $resource - The identifier for the lock. Should use frankenstyle prefix.
+     * @param int $timeout - The number of seconds to wait for a lock before giving up.
+     * @param int $maxlifetime - Unused by this lock type.
+     * @return boolean - true if a lock was obtained.
+     */
+    public function get_lock($resource, $timeout, $maxlifetime = 86400) {
+        return new lock($resource, $this);
+    }
+
+    /**
+     * Release a lock that was previously obtained with @lock.
+     *
+     * @param lock $lock - A lock obtained from this factory.
+     * @return boolean - true if the lock is no longer held (including if it was never held).
+     */
+    public function release_lock(lock $lock) {
+        return true;
+    }
+
+    /**
+     * Extend a lock that was previously obtained with @lock.
+     *
+     * @param lock $lock - not used
+     * @param int $maxlifetime - not used
+     * @return boolean - true if the lock was extended.
+     */
+    public function extend_lock(lock $lock, $maxlifetime = 86400) {
+        // Not supported by this factory.
+        return false;
+    }
+
+}

lib/classes/lock/lock_config.php

@@ -48,7 +48,9 @@ class lock_config {
         global $CFG, $DB;
         $lockfactory = null;
 
-        if (isset($CFG->lock_factory) && $CFG->lock_factory != 'auto') {
+        if (during_initial_install()) {
+            $lockfactory = new \core\lock\installation_lock_factory($type);
+        } else if (isset($CFG->lock_factory) && $CFG->lock_factory != 'auto') {
             if (!class_exists($CFG->lock_factory)) {
                 // In this case I guess it is not safe to continue. Different cluster nodes could end up using different locking
                 // types because of an installation error.