MDL-20438 Initial support for cron based notifications

If enabled, Moodle sites will check for updates every night sometimes
between 01:00 AM and 06:00 AM local time. The actual offset will be
generated randomly at the beginning and used consistently since then.
This way, the queries against the server are supposed to be spread in
time.

Serious site admins should not try to upgrade their sites later than
01:00 AM as they are too tired already. Neither should they do it before
06:00 AM as they did not have a morning coffee yet and are too tired yet
:-)

lib/pluginlib.php

@@ -694,6 +694,41 @@ class available_update_checker {
         }
     }
 
+    /**
+     * The method being run via cron.php
+     */
+    public function cron() {
+        global $CFG;
+
+        if (!$this->cron_autocheck_enabled()) {
+            $this->cron_mtrace('Automatic check for available updates not enabled, skipping.');
+            return;
+        }
+
+        $now = $this->cron_current_timestamp();
+
+        if ($this->cron_has_fresh_fetch($now)) {
+            $this->cron_mtrace('Recently fetched info about available updates is still fresh enough, skipping.');
+            return;
+        }
+
+        if ($this->cron_has_outdated_fetch($now)) {
+            $this->cron_mtrace('Outdated or missing info about available updates, forced fetching ... ', '');
+            $this->cron_execute();
+            return;
+        }
+
+        $offset = $this->cron_execution_offset();
+        $start = mktime(1, 0, 0, date('n', $now), date('j', $now), date('Y', $now)); // 01:00 AM today local time
+        if ($now > $start + $offset) {
+            $this->cron_mtrace('Regular daily check for available updates ... ', '');
+            $this->cron_execute();
+            return;
+        }
+    }
+
+    /// end of public API //////////////////////////////////////////////////////
+
     /**
      * Makes cURL request to get data from the remote site
      *
@@ -879,6 +914,117 @@ class available_update_checker {
 
         return $params;
     }
+
+    /**
+     * Returns the current timestamp
+     *
+     * @return int the timestamp
+     */
+    protected function cron_current_timestamp() {
+        return time();
+    }
+
+    /**
+     * Output cron debugging info
+     *
+     * @see mtrace()
+     * @param string $msg output message
+     * @param string $eol end of line
+     */
+    protected function cron_mtrace($msg, $eol = PHP_EOL) {
+        mtrace($msg, $eol);
+    }
+
+    /**
+     * Decide if the autocheck feature is disabled in the server setting
+     *
+     * @return bool true if autocheck enabled, false if disabled
+     */
+    protected function cron_autocheck_enabled() {
+        if (empty($CFG->updateautocheck)) {
+            return false;
+        } else {
+            return true;
+        }
+    }
+
+    /**
+     * Decide if the recently fetched data are still fresh enough
+     *
+     * @param int $now current timestamp
+     * @return bool true if no need to re-fetch, false otherwise
+     */
+    protected function cron_has_fresh_fetch($now) {
+        $recent = $this->get_last_timefetched();
+
+        if (empty($recent)) {
+            return false;
+        }
+
+        if ($now < $recent) {
+            $this->cron_mtrace('The most recent fetch is reported to be in the future, this is weird!');
+            return true;
+        }
+
+        if ($now - $recent > HOURSECS) {
+            return false;
+        }
+
+        return true;
+    }
+
+    /**
+     * Decide if the fetch is outadated or even missing
+     *
+     * @param int $now current timestamp
+     * @return bool false if no need to re-fetch, true otherwise
+     */
+    protected function cron_has_outdated_fetch($now) {
+        $recent = $this->get_last_timefetched();
+
+        if (empty($recent)) {
+            return true;
+        }
+
+        if ($now < $recent) {
+            $this->cron_mtrace('The most recent fetch is reported to be in the future, this is weird!');
+            return false;
+        }
+
+        if ($now - $recent > 48 * HOURSECS) {
+            return true;
+        }
+
+        return false;
+    }
+
+    /**
+     * Returns the cron execution offset for this site
+     *
+     * The main {@link self::cron()} is supposed to run every night in some random time
+     * between 01:00 and 06:00 AM (local time). The exact moment is defined by so called
+     * execution offset, that is the amount of time after 01:00 AM. The offset value is
+     * initially generated randomly and then used consistently at the site. This way, the
+     * regular checks against the download.moodle.org server are spread in time.
+     *
+     * @return int the offset number of seconds from range 1 sec to 5 hours
+     */
+    protected function cron_execution_offset() {
+        global $CFG;
+
+        if (empty($CFG->updatecronoffset)) {
+            set_config('updatecronoffset', rand(1, 5 * HOURSECS));
+        }
+
+        return $CFG->updatecronoffset;
+    }
+
+    /**
+     * Fetch available updates info and eventually send notification to site admins
+     */
+    protected function cron_execute() {
+        // todo
+    }
 }