Source

File: wp-includes/cron.php

function wp_schedule_single_event( $timestamp, $hook, $args = array() ) {
	// Make sure timestamp is a positive integer.
	if ( ! is_numeric( $timestamp ) || $timestamp <= 0 ) {
		return false;
	}

	$event = (object) array(
		'hook'      => $hook,
		'timestamp' => $timestamp,
		'schedule'  => false,
		'args'      => $args,
	);

	/**
	 * Filter to preflight or hijack scheduling an event.
	 *
	 * Returning a non-null value will short-circuit adding the event to the
	 * cron array, causing the function to return the filtered value instead.
	 *
	 * Both single events and recurring events are passed through this filter;
	 * single events have `$event->schedule` as false, whereas recurring events
	 * have this set to a recurrence from wp_get_schedules(). Recurring
	 * events also have the integer recurrence interval set as `$event->interval`.
	 *
	 * For plugins replacing wp-cron, it is recommended you check for an
	 * identical event within ten minutes and apply the {@see 'schedule_event'}
	 * filter to check if another plugin has disallowed the event before scheduling.
	 *
	 * Return true if the event was scheduled, false if not.
	 *
	 * @since 5.1.0
	 *
	 * @param null|bool $pre   Value to return instead. Default null to continue adding the event.
	 * @param stdClass  $event {
	 *     An object containing an event's data.
	 *
	 *     @type string       $hook      Action hook to execute when the event is run.
	 *     @type int          $timestamp Unix timestamp (UTC) for when to next run the event.
	 *     @type string|false $schedule  How often the event should subsequently recur.
	 *     @type array        $args      Array containing each separate argument to pass to the hook's callback function.
	 *     @type int          $interval  The interval time in seconds for the schedule. Only present for recurring events.
	 * }
	 */
	$pre = apply_filters( 'pre_schedule_event', null, $event );
	if ( null !== $pre ) {
		return $pre;
	}

	/*
	 * Check for a duplicated event.
	 *
	 * Don't schedule an event if there's already an identical event
	 * within 10 minutes.
	 *
	 * When scheduling events within ten minutes of the current time,
	 * all past identical events are considered duplicates.
	 *
	 * When scheduling an event with a past timestamp (ie, before the
	 * current time) all events scheduled within the next ten minutes
	 * are considered duplicates.
	 */
	$crons     = (array) _get_cron_array();
	$key       = md5( serialize( $event->args ) );
	$duplicate = false;

	if ( $event->timestamp < time() + 10 * MINUTE_IN_SECONDS ) {
		$min_timestamp = 0;
	} else {
		$min_timestamp = $event->timestamp - 10 * MINUTE_IN_SECONDS;
	}

	if ( $event->timestamp < time() ) {
		$max_timestamp = time() + 10 * MINUTE_IN_SECONDS;
	} else {
		$max_timestamp = $event->timestamp + 10 * MINUTE_IN_SECONDS;
	}

	foreach ( $crons as $event_timestamp => $cron ) {
		if ( $event_timestamp < $min_timestamp ) {
			continue;
		}
		if ( $event_timestamp > $max_timestamp ) {
			break;
		}
		if ( isset( $cron[ $event->hook ][ $key ] ) ) {
			$duplicate = true;
			break;
		}
	}

	if ( $duplicate ) {
		return false;
	}

	/**
	 * Modify an event before it is scheduled.
	 *
	 * @since 3.1.0
	 *
	 * @param stdClass|false $event {
	 *     An object containing an event's data, or boolean false to prevent the event from being scheduled.
	 *
	 *     @type string       $hook      Action hook to execute when the event is run.
	 *     @type int          $timestamp Unix timestamp (UTC) for when to next run the event.
	 *     @type string|false $schedule  How often the event should subsequently recur.
	 *     @type array        $args      Array containing each separate argument to pass to the hook's callback function.
	 *     @type int          $interval  The interval time in seconds for the schedule. Only present for recurring events.
	 * }
	 */
	$event = apply_filters( 'schedule_event', $event );

	// A plugin disallowed this event.
	if ( ! $event ) {
		return false;
	}

	$crons[ $event->timestamp ][ $event->hook ][ $key ] = array(
		'schedule' => $event->schedule,
		'args'     => $event->args,
	);
	uksort( $crons, 'strnatcasecmp' );
	return _set_cron_array( $crons );
}