MDL-77186 core: Add a new keepalive setting to cron

admin/cli/cron.php

@@ -43,6 +43,7 @@
         'enable' => false,
         'disable' => false,
         'disable-wait' => false,
+        'keep-alive' => null,
     ],
     [
         'h' => 'help',
@@ -52,6 +53,7 @@
         'e' => 'enable',
         'd' => 'disable',
         'w' => 'disable-wait',
+        'k' => 'keep-alive',
     ]
 );
 
@@ -72,6 +74,9 @@
 -e, --enable             Enable cron
 -d, --disable            Disable cron
 -w, --disable-wait=600   Disable cron and wait until all tasks finished or fail after N seconds (optional param)
+-k, --keep-alive=N       Keep this script alive for N seconds and poll for new tasks
+                         The default value can be set by administrators in:
+                         Site administration > Server > Tasks > Task processing > Keep alive time
 
 Example:
 \$sudo -u www-data /usr/bin/php admin/cli/cron.php
@@ -177,4 +182,6 @@
 
 \core\local\cli\shutdown::script_supports_graceful_exit();
 
-cron_run();
+
+$keepalive = $options['keep-alive'];
+cron_run($keepalive);

admin/cron.php

@@ -77,5 +77,5 @@
 // We do not want html markup in emulated CLI.
 @ini_set('html_errors', 'off');
 
-// Execute the cron.
-cron_run();
+// Execute the cron, disabling keepalive.
+cron_run(0);

admin/settings/server.php

@@ -331,6 +331,27 @@
     $setting->set_updatedcallback('theme_reset_static_caches');
     $temp->add($setting);
 
+    // Note: At the moment cron functions live in lib/cronlib.php which is not guaranteed to be loaded everywhere.
+    // We have no sensible place to put constants.
+    $setting = new admin_setting_configduration(
+        'cron_keepalive',
+        new lang_string('cron_keepalive', 'admin'),
+        new lang_string('cron_keepalive_desc', 'admin'),
+
+        // Use a default value of 3 minutes.
+        // The recommended cron frequency is every minute, and the default adhoc concurrency is 3.
+        // A default value of 3 minutes allows all adhoc tasks to be run concurrently at their default value.
+        3 * MINSECS,
+
+        // The default unit is minutes.
+        MINSECS,
+    );
+
+    // Set an upper limit of 15 minutes.
+    $setting->set_max_duration(15 * MINSEC);
+
+    $temp->add($setting);
+
     $temp->add(
         new admin_setting_configtext(
             'task_scheduled_concurrency_limit',

lang/en/admin.php

@@ -441,6 +441,8 @@
 $string['cron_enabled'] = 'Enable cron';
 $string['cron_enabled_desc'] = 'Cron should normally be enabled, however this setting allows it to be disabled temporarily, for example before a server restart. If disabled, the system is prevented from starting new background tasks. Note that the cron should not be disabled for a long time, as this will prevent important functionality from working.';
 $string['cron_help'] = 'The cron.php script runs a number of tasks at different scheduled intervals, such as sending forum post notification emails. The script should be run regularly - ideally every minute.';
+$string['cron_keepalive'] = 'Keep alive';
+$string['cron_keepalive_desc'] = 'The amount of time to keep polling for additional tasks. This setting is useful for ensuring that cron is always running.<br><br>If you use dedicated task runners then you should set this value to an 0, otherwise we recommend setting this to around a value similar to your adhoc task concurrency limit. Longer values should be avoided and the maximum value is 15 minutes.';
 $string['cron_link'] = 'admin/cron';
 $string['cronclionly'] = 'Cron execution via command line only';
 $string['cronerrorclionly'] = 'Sorry, internet access to this page has been disabled by the administrator.';

lib/cronlib.php

@@ -25,8 +25,10 @@
 
 /**
  * Execute cron tasks
+ *
+ * @param int|null $keepalive The keepalive time for this cron run.
  */
-function cron_run(): void {
+function cron_run(?int $keepalive = null): void {
     global $DB, $CFG;
 
     if (CLI_MAINTENANCE) {
@@ -49,7 +51,6 @@ function cron_run(): void {
     }
 
     core_php_time_limit::raise();
-    $starttime = microtime();
 
     // Increase memory limit.
     raise_memory_limit(MEMORY_EXTRA);
@@ -69,20 +70,66 @@ function cron_run(): void {
         set_config('lastcroninterval', max(1, $timenow - $laststart), 'tool_task');
     }
 
-    // Run all scheduled tasks.
-    cron_run_scheduled_tasks($timenow);
+    // Determine the time when the cron should finish.
+    if ($keepalive === null) {
+        $keepalive = get_config('core', 'cron_keepalive');
+        if ($keepalive === false) {
+            // Use a default value of 3 minutes.
+            // The recommended cron frequency is every minute, and the default adhoc concurrency is 3.
+            // A default value of 3 minutes allows all adhoc tasks to be run concurrently at their default value.
+            $keepalive = 3 * MINSECS;
+        }
+    }
 
-    // Run adhoc tasks.
-    cron_run_adhoc_tasks($timenow);
+    if ($keepalive > 15 * MINSECS) {
+        // Attempt to prevent abnormally long keepalives.
+        mtrace("Cron keepalive time is too long, reducing to 15 minutes.");
+        $keepalive = 15 * MINSECS;
+    }
 
-    mtrace("Cron run completed correctly");
+    // Calculate the finish time based on the start time and keepalive.
+    $finishtime = $timenow + $keepalive;
 
-    gc_collect_cycles();
+    do {
+        $startruntime = microtime();
+        // Run all scheduled tasks.
+        cron_run_scheduled_tasks($timenow);
+
+        // Run adhoc tasks.
+        cron_run_adhoc_tasks($timenow);
+
+        mtrace("Cron run completed correctly");
 
-    $completiontime = date('H:i:s');
-    $difftime = microtime_diff($starttime, microtime());
-    $memoryused = display_size(memory_get_usage());
-    mtrace("Cron completed at {$completiontime} in {$difftime} seconds. Memory used: {$memoryused}.");
+        gc_collect_cycles();
+
+        $completiontime = date('H:i:s');
+        $difftime = microtime_diff($startruntime, microtime());
+        $memoryused = display_size(memory_get_usage());
+
+        $message = "Cron completed at {$completiontime} in {$difftime} seconds. Memory used: {$memoryused}.";
+
+        // Check if we should continue to run.
+        // Only continue to run if:
+        // - The finish time has not been reached; and
+        // - The graceful exit flag has not been set; and
+        // - The static caches have not been cleared since the start of the cron run.
+        $remaining = $finishtime - time();
+        $runagain = $remaining > 0;
+        $runagain = $runagain && !\core\local\cli\shutdown::should_gracefully_exit();
+        $runagain = $runagain && !\core\task\manager::static_caches_cleared_since($timenow);
+
+        if ($runagain) {
+            $message .= " Continuing to check for tasks for {$remaining} more seconds.";
+            mtrace($message);
+            sleep(1);
+
+            // Re-check the graceful exit and cache clear flags after sleeping as these may have changed.
+            $runagain = $runagain && !\core\local\cli\shutdown::should_gracefully_exit();
+            $runagain = $runagain && !\core\task\manager::static_caches_cleared_since($timenow);
+        } else {
+            mtrace($message);
+        }
+    } while ($runagain);
 }
 
 /**

version.php

@@ -29,7 +29,7 @@
 
 defined('MOODLE_INTERNAL') || die();
 
-$version  = 2023031000.00;              // YYYYMMDD      = weekly release date of this DEV branch.
+$version  = 2023031000.01;              // YYYYMMDD      = weekly release date of this DEV branch.
                                         //         RR    = release increments - 00 in DEV branches.
                                         //           .XX = incremental changes.
 $release  = '4.2dev (Build: 20230310)'; // Human-friendly version name