diff -r c7c34916027a -r 177826044cd9 wp/wp-includes/cron.php
--- a/wp/wp-includes/cron.php	Mon Oct 14 18:06:33 2019 +0200
+++ b/wp/wp-includes/cron.php	Mon Oct 14 18:28:13 2019 +0200
@@ -8,120 +8,247 @@
 /**
  * Schedules an event to run only once.
  *
- * Schedules an event which will execute once by the WordPress actions core at
- * a time which you specify. The action will fire off when someone visits your
- * WordPress site, if the schedule time has passed.
+ * Schedules a hook which will be triggered by WordPress at the specified time.
+ * The action will trigger when someone visits your WordPress site if the scheduled
+ * time has passed.
  *
  * Note that scheduling an event to occur within 10 minutes of an existing event
  * with the same action hook will be ignored unless you pass unique `$args` values
  * for each scheduled event.
  *
+ * Use wp_next_scheduled() to prevent duplicate events.
+ *
+ * Use wp_schedule_event() to schedule a recurring event.
+ *
  * @since 2.1.0
+ * @since 5.1.0 Return value modified to boolean indicating success or failure,
+ *              {@see 'pre_schedule_event'} filter added to short-circuit the function.
+ *
  * @link https://codex.wordpress.org/Function_Reference/wp_schedule_single_event
  *
- * @param int $timestamp Unix timestamp (UTC) for when to run the event.
- * @param string $hook Action hook to execute when event is run.
- * @param array $args Optional. Arguments to pass to the hook's callback function.
- * @return false|void False if the event does not get scheduled.
+ * @param int    $timestamp  Unix timestamp (UTC) for when to next run the event.
+ * @param string $hook       Action hook to execute when the event is run.
+ * @param array  $args       Optional. Array containing each separate argument to pass to the hook's callback function.
+ * @return bool True if event successfully scheduled. False for failure.
  */
-function wp_schedule_single_event( $timestamp, $hook, $args = array()) {
+function wp_schedule_single_event( $timestamp, $hook, $args = array() ) {
 	// Make sure timestamp is a positive integer
 	if ( ! is_numeric( $timestamp ) || $timestamp <= 0 ) {
 		return false;
 	}
 
-	// Don't schedule a duplicate if there's already an identical event due within 10 minutes of it
-	$next = wp_next_scheduled($hook, $args);
-	if ( $next && abs( $next - $timestamp ) <= 10 * MINUTE_IN_SECONDS ) {
+	$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;
 	}
 
-	$crons = _get_cron_array();
-	$event = (object) array( 'hook' => $hook, 'timestamp' => $timestamp, 'schedule' => false, 'args' => $args );
 	/**
-	 * Filters a single event before it is scheduled.
+	 * Modify an event before it is scheduled.
 	 *
 	 * @since 3.1.0
 	 *
 	 * @param stdClass $event {
 	 *     An object containing an event's data.
 	 *
-	 *     @type string       $hook      Action hook to execute when event is run.
-	 *     @type int          $timestamp Unix timestamp (UTC) for when to run the event.
-	 *     @type string|false $schedule  How often the event should recur. See `wp_get_schedules()`.
-	 *     @type array        $args      Arguments to pass to the hook's callback function.
+	 *     @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 )
+	if ( ! $event ) {
 		return false;
+	}
 
-	$key = md5(serialize($event->args));
-
-	$crons[$event->timestamp][$event->hook][$key] = array( 'schedule' => $event->schedule, 'args' => $event->args );
-	uksort( $crons, "strnatcasecmp" );
-	_set_cron_array( $crons );
+	$crons[ $event->timestamp ][ $event->hook ][ $key ] = array(
+		'schedule' => $event->schedule,
+		'args'     => $event->args,
+	);
+	uksort( $crons, 'strnatcasecmp' );
+	return _set_cron_array( $crons );
 }
 
 /**
- * Schedule a recurring event.
+ * Schedules a recurring event.
  *
- * Schedules a hook which will be executed by the WordPress actions core on a
- * specific interval, specified by you. The action will trigger when someone
- * visits your WordPress site, if the scheduled time has passed.
+ * Schedules a hook which will be triggered by WordPress at the specified interval.
+ * The action will trigger when someone visits your WordPress site if the scheduled
+ * time has passed.
  *
- * Valid values for the recurrence are hourly, daily, and twicedaily. These can
+ * Valid values for the recurrence are 'hourly', 'daily', and 'twicedaily'. These can
  * be extended using the {@see 'cron_schedules'} filter in wp_get_schedules().
  *
- * Use wp_next_scheduled() to prevent duplicates
+ * Note that scheduling an event to occur within 10 minutes of an existing event
+ * with the same action hook will be ignored unless you pass unique `$args` values
+ * for each scheduled event.
+ *
+ * Use wp_next_scheduled() to prevent duplicate events.
+ *
+ * Use wp_schedule_single_event() to schedule a non-recurring event.
  *
  * @since 2.1.0
+ * @since 5.1.0 Return value modified to boolean indicating success or failure,
+ *              {@see 'pre_schedule_event'} filter added to short-circuit the function.
  *
- * @param int $timestamp Unix timestamp (UTC) for when to run the event.
- * @param string $recurrence How often the event should recur.
- * @param string $hook Action hook to execute when event is run.
- * @param array $args Optional. Arguments to pass to the hook's callback function.
- * @return false|void False if the event does not get scheduled.
+ * @link https://codex.wordpress.org/Function_Reference/wp_schedule_event
+ *
+ * @param int    $timestamp  Unix timestamp (UTC) for when to next run the event.
+ * @param string $recurrence How often the event should subsequently recur. See wp_get_schedules() for accepted values.
+ * @param string $hook       Action hook to execute when the event is run.
+ * @param array  $args       Optional. Array containing each separate argument to pass to the hook's callback function.
+ * @return bool True if event successfully scheduled. False for failure.
  */
-function wp_schedule_event( $timestamp, $recurrence, $hook, $args = array()) {
+function wp_schedule_event( $timestamp, $recurrence, $hook, $args = array() ) {
 	// Make sure timestamp is a positive integer
 	if ( ! is_numeric( $timestamp ) || $timestamp <= 0 ) {
 		return false;
 	}
 
-	$crons = _get_cron_array();
 	$schedules = wp_get_schedules();
 
-	if ( !isset( $schedules[$recurrence] ) )
+	if ( ! isset( $schedules[ $recurrence ] ) ) {
 		return false;
+	}
 
-	$event = (object) array( 'hook' => $hook, 'timestamp' => $timestamp, 'schedule' => $recurrence, 'args' => $args, 'interval' => $schedules[$recurrence]['interval'] );
+	$event = (object) array(
+		'hook'      => $hook,
+		'timestamp' => $timestamp,
+		'schedule'  => $recurrence,
+		'args'      => $args,
+		'interval'  => $schedules[ $recurrence ]['interval'],
+	);
+
+	/** This filter is documented in wp-includes/cron.php */
+	$pre = apply_filters( 'pre_schedule_event', null, $event );
+	if ( null !== $pre ) {
+		return $pre;
+	}
+
 	/** This filter is documented in wp-includes/cron.php */
 	$event = apply_filters( 'schedule_event', $event );
 
 	// A plugin disallowed this event
-	if ( ! $event )
+	if ( ! $event ) {
 		return false;
+	}
+
+	$key = md5( serialize( $event->args ) );
 
-	$key = md5(serialize($event->args));
-
-	$crons[$event->timestamp][$event->hook][$key] = array( 'schedule' => $event->schedule, 'args' => $event->args, 'interval' => $event->interval );
-	uksort( $crons, "strnatcasecmp" );
-	_set_cron_array( $crons );
+	$crons = _get_cron_array();
+	$crons[ $event->timestamp ][ $event->hook ][ $key ] = array(
+		'schedule' => $event->schedule,
+		'args'     => $event->args,
+		'interval' => $event->interval,
+	);
+	uksort( $crons, 'strnatcasecmp' );
+	return _set_cron_array( $crons );
 }
 
 /**
- * Reschedule a recurring event.
+ * Reschedules a recurring event.
+ *
+ * Mainly for internal use, this takes the time stamp of a previously run
+ * recurring event and reschedules it for its next run.
+ *
+ * To change upcoming scheduled events, use wp_schedule_event() to
+ * change the recurrence frequency.
  *
  * @since 2.1.0
+ * @since 5.1.0 Return value modified to boolean indicating success or failure,
+ *              {@see 'pre_reschedule_event'} filter added to short-circuit the function.
  *
- * @param int $timestamp Unix timestamp (UTC) for when to run the event.
- * @param string $recurrence How often the event should recur.
- * @param string $hook Action hook to execute when event is run.
- * @param array $args Optional. Arguments to pass to the hook's callback function.
- * @return false|void False if the event does not get rescheduled.
+ * @param int    $timestamp  Unix timestamp (UTC) for when the event was scheduled.
+ * @param string $recurrence How often the event should subsequently recur. See wp_get_schedules() for accepted values.
+ * @param string $hook       Action hook to execute when the event is run.
+ * @param array  $args       Optional. Array containing each separate argument to pass to the hook's callback function.
+ * @return bool True if event successfully rescheduled. False for failure.
  */
 function wp_reschedule_event( $timestamp, $recurrence, $hook, $args = array() ) {
 	// Make sure timestamp is a positive integer
@@ -129,19 +256,57 @@
 		return false;
 	}
 
-	$crons = _get_cron_array();
 	$schedules = wp_get_schedules();
-	$key = md5( serialize( $args ) );
-	$interval = 0;
+	$interval  = 0;
 
-	// First we try to get it from the schedule
+	// First we try to get the interval from the schedule.
 	if ( isset( $schedules[ $recurrence ] ) ) {
 		$interval = $schedules[ $recurrence ]['interval'];
 	}
-	// Now we try to get it from the saved interval in case the schedule disappears
-	if ( 0 == $interval ) {
-		$interval = $crons[ $timestamp ][ $hook ][ $key ]['interval'];
+
+	// Now we try to get it from the saved interval in case the schedule disappears.
+	if ( 0 === $interval ) {
+		$scheduled_event = wp_get_scheduled_event( $hook, $args, $timestamp );
+		if ( $scheduled_event && isset( $scheduled_event->interval ) ) {
+			$interval = $scheduled_event->interval;
+		}
 	}
+
+	$event = (object) array(
+		'hook'      => $hook,
+		'timestamp' => $timestamp,
+		'schedule'  => $recurrence,
+		'args'      => $args,
+		'interval'  => $interval,
+	);
+
+	/**
+	 * Filter to preflight or hijack rescheduling of events.
+	 *
+	 * Returning a non-null value will short-circuit the normal rescheduling
+	 * process, causing the function to return the filtered value instead.
+	 *
+	 * For plugins replacing wp-cron, return true if the event was successfully
+	 * rescheduled, 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_reschedule_event', null, $event );
+	if ( null !== $pre ) {
+		return $pre;
+	}
+
 	// Now we assume something is wrong and fail to schedule
 	if ( 0 == $interval ) {
 		return false;
@@ -155,7 +320,7 @@
 		$timestamp = $now + ( $interval - ( ( $now - $timestamp ) % $interval ) );
 	}
 
-	wp_schedule_event( $timestamp, $recurrence, $hook, $args );
+	return wp_schedule_event( $timestamp, $recurrence, $hook, $args );
 }
 
 /**
@@ -165,14 +330,15 @@
  * identified.
  *
  * @since 2.1.0
+ * @since 5.1.0 Return value modified to boolean indicating success or failure,
+ *              {@see 'pre_unschedule_event'} filter added to short-circuit the function.
  *
- * @param int $timestamp Unix timestamp (UTC) for when to run the event.
- * @param string $hook Action hook, the execution of which will be unscheduled.
- * @param array $args Arguments to pass to the hook's callback function.
- * Although not passed to a callback function, these arguments are used
- * to uniquely identify the scheduled event, so they should be the same
- * as those used when originally scheduling the event.
- * @return false|void False if the event does not get unscheduled.
+ * @param int    $timestamp Unix timestamp (UTC) of the event.
+ * @param string $hook      Action hook of the event.
+ * @param array  $args      Optional. Array containing each separate argument to pass to the hook's callback function.
+ *                          Although not passed to a callback, these arguments are used to uniquely identify the
+ *                          event, so they should be the same as those used when originally scheduling the event.
+ * @return bool True if event successfully unscheduled. False for failure.
  */
 function wp_unschedule_event( $timestamp, $hook, $args = array() ) {
 	// Make sure timestamp is a positive integer
@@ -180,45 +346,105 @@
 		return false;
 	}
 
+	/**
+	 * Filter to preflight or hijack unscheduling of events.
+	 *
+	 * Returning a non-null value will short-circuit the normal unscheduling
+	 * process, causing the function to return the filtered value instead.
+	 *
+	 * For plugins replacing wp-cron, return true if the event was successfully
+	 * unscheduled, false if not.
+	 *
+	 * @since 5.1.0
+	 *
+	 * @param null|bool $pre       Value to return instead. Default null to continue unscheduling the event.
+	 * @param int       $timestamp Timestamp for when to run the event.
+	 * @param string    $hook      Action hook, the execution of which will be unscheduled.
+	 * @param array     $args      Arguments to pass to the hook's callback function.
+	 */
+	$pre = apply_filters( 'pre_unschedule_event', null, $timestamp, $hook, $args );
+	if ( null !== $pre ) {
+		return $pre;
+	}
+
 	$crons = _get_cron_array();
-	$key = md5(serialize($args));
-	unset( $crons[$timestamp][$hook][$key] );
-	if ( empty($crons[$timestamp][$hook]) )
-		unset( $crons[$timestamp][$hook] );
-	if ( empty($crons[$timestamp]) )
-		unset( $crons[$timestamp] );
-	_set_cron_array( $crons );
+	$key   = md5( serialize( $args ) );
+	unset( $crons[ $timestamp ][ $hook ][ $key ] );
+	if ( empty( $crons[ $timestamp ][ $hook ] ) ) {
+		unset( $crons[ $timestamp ][ $hook ] );
+	}
+	if ( empty( $crons[ $timestamp ] ) ) {
+		unset( $crons[ $timestamp ] );
+	}
+	return _set_cron_array( $crons );
 }
 
 /**
  * Unschedules all events attached to the hook with the specified arguments.
  *
+ * Warning: This function may return Boolean FALSE, but may also return a non-Boolean
+ * value which evaluates to FALSE. For information about casting to booleans see the
+ * {@link https://php.net/manual/en/language.types.boolean.php PHP documentation}. Use
+ * the `===` operator for testing the return value of this function.
+ *
  * @since 2.1.0
+ * @since 5.1.0 Return value modified to indicate success or failure,
+ *              {@see 'pre_clear_scheduled_hook'} filter added to short-circuit the function.
  *
  * @param string $hook Action hook, the execution of which will be unscheduled.
  * @param array $args Optional. Arguments that were to be passed to the hook's callback function.
+ * @return bool|int On success an integer indicating number of events unscheduled (0 indicates no
+ *                  events were registered with the hook and arguments combination), false if
+ *                  unscheduling one or more events fail.
  */
 function wp_clear_scheduled_hook( $hook, $args = array() ) {
 	// Backward compatibility
 	// Previously this function took the arguments as discrete vars rather than an array like the rest of the API
-	if ( !is_array($args) ) {
-		_deprecated_argument( __FUNCTION__, '3.0.0', __('This argument has changed to an array to match the behavior of the other cron functions.') );
+	if ( ! is_array( $args ) ) {
+		_deprecated_argument( __FUNCTION__, '3.0.0', __( 'This argument has changed to an array to match the behavior of the other cron functions.' ) );
 		$args = array_slice( func_get_args(), 1 );
 	}
 
+	/**
+	 * Filter to preflight or hijack clearing a scheduled hook.
+	 *
+	 * Returning a non-null value will short-circuit the normal unscheduling
+	 * process, causing the function to return the filtered value instead.
+	 *
+	 * For plugins replacing wp-cron, return the number of events successfully
+	 * unscheduled (zero if no events were registered with the hook) or false
+	 * if unscheduling one or more events fails.
+	 *
+	 * @since 5.1.0
+	 *
+	 * @param null|array $pre  Value to return instead. Default null to continue unscheduling the event.
+	 * @param string     $hook Action hook, the execution of which will be unscheduled.
+	 * @param array      $args Arguments to pass to the hook's callback function.
+	 */
+	$pre = apply_filters( 'pre_clear_scheduled_hook', null, $hook, $args );
+	if ( null !== $pre ) {
+		return $pre;
+	}
+
 	// This logic duplicates wp_next_scheduled()
 	// It's required due to a scenario where wp_unschedule_event() fails due to update_option() failing,
 	// and, wp_next_scheduled() returns the same schedule in an infinite loop.
 	$crons = _get_cron_array();
-	if ( empty( $crons ) )
-		return;
+	if ( empty( $crons ) ) {
+		return 0;
+	}
 
-	$key = md5( serialize( $args ) );
+	$results = array();
+	$key     = md5( serialize( $args ) );
 	foreach ( $crons as $timestamp => $cron ) {
 		if ( isset( $cron[ $hook ][ $key ] ) ) {
-			wp_unschedule_event( $timestamp, $hook, $args );
+			$results[] = wp_unschedule_event( $timestamp, $hook, $args );
 		}
 	}
+	if ( in_array( false, $results, true ) ) {
+		return false;
+	}
+	return count( $results );
 }
 
 /**
@@ -226,14 +452,49 @@
  *
  * Can be useful for plugins when deactivating to clean up the cron queue.
  *
+ * Warning: This function may return Boolean FALSE, but may also return a non-Boolean
+ * value which evaluates to FALSE. For information about casting to booleans see the
+ * {@link https://php.net/manual/en/language.types.boolean.php PHP documentation}. Use
+ * the `===` operator for testing the return value of this function.
+ *
  * @since 4.9.0
+ * @since 5.1.0 Return value added to indicate success or failure.
  *
  * @param string $hook Action hook, the execution of which will be unscheduled.
+ * @return bool|int On success an integer indicating number of events unscheduled (0 indicates no
+ *                  events were registered on the hook), false if unscheduling fails.
  */
 function wp_unschedule_hook( $hook ) {
-	$crons = _get_cron_array();
+	/**
+	 * Filter to preflight or hijack clearing all events attached to the hook.
+	 *
+	 * Returning a non-null value will short-circuit the normal unscheduling
+	 * process, causing the function to return the filtered value instead.
+	 *
+	 * For plugins replacing wp-cron, return the number of events successfully
+	 * unscheduled (zero if no events were registered with the hook) or false
+	 * if unscheduling one or more events fails.
+	 *
+	 * @since 5.1.0
+	 *
+	 * @param null|array $pre  Value to return instead. Default null to continue unscheduling the hook.
+	 * @param string     $hook Action hook, the execution of which will be unscheduled.
+	 */
+	$pre = apply_filters( 'pre_unschedule_hook', null, $hook );
+	if ( null !== $pre ) {
+		return $pre;
+	}
 
-	foreach( $crons as $timestamp => $args ) {
+	$crons = _get_cron_array();
+	if ( empty( $crons ) ) {
+		return 0;
+	}
+
+	$results = array();
+	foreach ( $crons as $timestamp => $args ) {
+		if ( ! empty( $crons[ $timestamp ][ $hook ] ) ) {
+			$results[] = count( $crons[ $timestamp ][ $hook ] );
+		}
 		unset( $crons[ $timestamp ][ $hook ] );
 
 		if ( empty( $crons[ $timestamp ] ) ) {
@@ -241,7 +502,99 @@
 		}
 	}
 
-	_set_cron_array( $crons );
+	/*
+	 * If the results are empty (zero events to unschedule), no attempt
+	 * to update the cron array is required.
+	 */
+	if ( empty( $results ) ) {
+		return 0;
+	}
+	if ( _set_cron_array( $crons ) ) {
+		return array_sum( $results );
+	}
+	return false;
+}
+
+/**
+ * Retrieve a scheduled event.
+ *
+ * Retrieve the full event object for a given event, if no timestamp is specified the next
+ * scheduled event is returned.
+ *
+ * @since 5.1.0
+ *
+ * @param string   $hook      Action hook of the event.
+ * @param array    $args      Optional. Array containing each separate argument to pass to the hook's callback function.
+ *                            Although not passed to a callback, these arguments are used to uniquely identify the
+ *                            event, so they should be the same as those used when originally scheduling the event.
+ * @param int|null $timestamp Optional. Unix timestamp (UTC) of the event. If not specified, the next scheduled event is returned.
+ * @return bool|object The event object. False if the event does not exist.
+ */
+function wp_get_scheduled_event( $hook, $args = array(), $timestamp = null ) {
+	/**
+	 * Filter to preflight or hijack retrieving a scheduled event.
+	 *
+	 * Returning a non-null value will short-circuit the normal process,
+	 * returning the filtered value instead.
+	 *
+	 * Return false if the event does not exist, otherwise an event object
+	 * should be returned.
+	 *
+	 * @since 5.1.0
+	 *
+	 * @param null|bool $pre       Value to return instead. Default null to continue retrieving the event.
+	 * @param string    $hook      Action hook of the event.
+	 * @param array     $args      Array containing each separate argument to pass to the hook's callback function.
+	 *                             Although not passed to a callback, these arguments are used to uniquely identify the
+	 *                             event.
+	 * @param int|null  $timestamp Unix timestamp (UTC) of the event. Null to retrieve next scheduled event.
+	 */
+	$pre = apply_filters( 'pre_get_scheduled_event', null, $hook, $args, $timestamp );
+	if ( null !== $pre ) {
+		return $pre;
+	}
+
+	if ( null !== $timestamp && ! is_numeric( $timestamp ) ) {
+		return false;
+	}
+
+	$crons = _get_cron_array();
+	if ( empty( $crons ) ) {
+		return false;
+	}
+
+	$key = md5( serialize( $args ) );
+
+	if ( ! $timestamp ) {
+		// Get next event.
+		$next = false;
+		foreach ( $crons as $timestamp => $cron ) {
+			if ( isset( $cron[ $hook ][ $key ] ) ) {
+				$next = $timestamp;
+				break;
+			}
+		}
+		if ( ! $next ) {
+			return false;
+		}
+
+		$timestamp = $next;
+	} elseif ( ! isset( $crons[ $timestamp ][ $hook ][ $key ] ) ) {
+		return false;
+	}
+
+	$event = (object) array(
+		'hook'      => $hook,
+		'timestamp' => $timestamp,
+		'schedule'  => $crons[ $timestamp ][ $hook ][ $key ]['schedule'],
+		'args'      => $args,
+	);
+
+	if ( isset( $crons[ $timestamp ][ $hook ][ $key ]['interval'] ) ) {
+		$event->interval = $crons[ $timestamp ][ $hook ][ $key ]['interval'];
+	}
+
+	return $event;
 }
 
 /**
@@ -249,35 +602,38 @@
  *
  * @since 2.1.0
  *
- * @param string $hook Action hook to execute when event is run.
- * @param array $args Optional. Arguments to pass to the hook's callback function.
- * @return false|int The Unix timestamp of the next time the scheduled event will occur.
+ * @param string $hook Action hook of the event.
+ * @param array  $args Optional. Array containing each separate argument to pass to the hook's callback function.
+ *                     Although not passed to a callback, these arguments are used to uniquely identify the
+ *                     event, so they should be the same as those used when originally scheduling the event.
+ * @return false|int The Unix timestamp of the next time the event will occur. False if the event doesn't exist.
  */
 function wp_next_scheduled( $hook, $args = array() ) {
-	$crons = _get_cron_array();
-	$key = md5(serialize($args));
-	if ( empty($crons) )
+	$next_event = wp_get_scheduled_event( $hook, $args );
+	if ( ! $next_event ) {
 		return false;
-	foreach ( $crons as $timestamp => $cron ) {
-		if ( isset( $cron[$hook][$key] ) )
-			return $timestamp;
 	}
-	return false;
+
+	return $next_event->timestamp;
 }
 
 /**
  * Sends a request to run cron through HTTP request that doesn't halt page loading.
  *
  * @since 2.1.0
+ * @since 5.1.0 Return values added.
  *
  * @param int $gmt_time Optional. Unix timestamp (UTC). Default 0 (current time is used).
+ * @return bool True if spawned, false if no events spawned.
  */
 function spawn_cron( $gmt_time = 0 ) {
-	if ( ! $gmt_time )
+	if ( ! $gmt_time ) {
 		$gmt_time = microtime( true );
+	}
 
-	if ( defined('DOING_CRON') || isset($_GET['doing_wp_cron']) )
-		return;
+	if ( defined( 'DOING_CRON' ) || isset( $_GET['doing_wp_cron'] ) ) {
+		return false;
+	}
 
 	/*
 	 * Get the cron lock, which is a Unix timestamp of when the last cron was spawned
@@ -286,27 +642,31 @@
 	 * Multiple processes on multiple web servers can run this code concurrently,
 	 * this lock attempts to make spawning as atomic as possible.
 	 */
-	$lock = get_transient('doing_cron');
+	$lock = get_transient( 'doing_cron' );
 
-	if ( $lock > $gmt_time + 10 * MINUTE_IN_SECONDS )
+	if ( $lock > $gmt_time + 10 * MINUTE_IN_SECONDS ) {
 		$lock = 0;
+	}
 
 	// don't run if another process is currently running it or more than once every 60 sec.
-	if ( $lock + WP_CRON_LOCK_TIMEOUT > $gmt_time )
-		return;
+	if ( $lock + WP_CRON_LOCK_TIMEOUT > $gmt_time ) {
+		return false;
+	}
 
 	//sanity check
-	$crons = _get_cron_array();
-	if ( !is_array($crons) )
-		return;
+	$crons = wp_get_ready_cron_jobs();
+	if ( empty( $crons ) ) {
+		return false;
+	}
 
 	$keys = array_keys( $crons );
-	if ( isset($keys[0]) && $keys[0] > $gmt_time )
-		return;
+	if ( isset( $keys[0] ) && $keys[0] > $gmt_time ) {
+		return false;
+	}
 
 	if ( defined( 'ALTERNATE_WP_CRON' ) && ALTERNATE_WP_CRON ) {
-		if ( 'GET' !== $_SERVER['REQUEST_METHOD'] || defined( 'DOING_AJAX' ) ||  defined( 'XMLRPC_REQUEST' ) ) {
-			return;
+		if ( 'GET' !== $_SERVER['REQUEST_METHOD'] || defined( 'DOING_AJAX' ) || defined( 'XMLRPC_REQUEST' ) ) {
+			return false;
 		}
 
 		$doing_wp_cron = sprintf( '%.22F', $gmt_time );
@@ -317,11 +677,12 @@
 		echo ' ';
 
 		// flush any buffers and send the headers
-		while ( @ob_end_flush() );
+		while ( @ob_end_flush() ) {
+		}
 		flush();
 
 		WP_DEBUG ? include_once( ABSPATH . 'wp-cron.php' ) : @include_once( ABSPATH . 'wp-cron.php' );
-		return;
+		return true;
 	}
 
 	// Set the cron lock with the current unix timestamp, when the cron is being spawned.
@@ -349,48 +710,75 @@
 	 * }
 	 * @param string $doing_wp_cron The unix timestamp of the cron lock.
 	 */
-	$cron_request = apply_filters( 'cron_request', array(
-		'url'  => add_query_arg( 'doing_wp_cron', $doing_wp_cron, site_url( 'wp-cron.php' ) ),
-		'key'  => $doing_wp_cron,
-		'args' => array(
-			'timeout'   => 0.01,
-			'blocking'  => false,
-			/** This filter is documented in wp-includes/class-wp-http-streams.php */
-			'sslverify' => apply_filters( 'https_local_ssl_verify', false )
-		)
-	), $doing_wp_cron );
+	$cron_request = apply_filters(
+		'cron_request',
+		array(
+			'url'  => add_query_arg( 'doing_wp_cron', $doing_wp_cron, site_url( 'wp-cron.php' ) ),
+			'key'  => $doing_wp_cron,
+			'args' => array(
+				'timeout'   => 0.01,
+				'blocking'  => false,
+				/** This filter is documented in wp-includes/class-wp-http-streams.php */
+				'sslverify' => apply_filters( 'https_local_ssl_verify', false ),
+			),
+		),
+		$doing_wp_cron
+	);
 
-	wp_remote_post( $cron_request['url'], $cron_request['args'] );
+	$result = wp_remote_post( $cron_request['url'], $cron_request['args'] );
+	return ! is_wp_error( $result );
 }
 
 /**
  * Run scheduled callbacks or spawn cron for all scheduled events.
  *
+ * Warning: This function may return Boolean FALSE, but may also return a non-Boolean
+ * value which evaluates to FALSE. For information about casting to booleans see the
+ * {@link https://php.net/manual/en/language.types.boolean.php PHP documentation}. Use
+ * the `===` operator for testing the return value of this function.
+ *
  * @since 2.1.0
+ * @since 5.1.0 Return value added to indicate success or failure.
+ *
+ * @return bool|int On success an integer indicating number of events spawned (0 indicates no
+ *                  events needed to be spawned), false if spawning fails for one or more events.
  */
 function wp_cron() {
 	// Prevent infinite loops caused by lack of wp-cron.php
-	if ( strpos($_SERVER['REQUEST_URI'], '/wp-cron.php') !== false || ( defined('DISABLE_WP_CRON') && DISABLE_WP_CRON ) )
-		return;
+	if ( strpos( $_SERVER['REQUEST_URI'], '/wp-cron.php' ) !== false || ( defined( 'DISABLE_WP_CRON' ) && DISABLE_WP_CRON ) ) {
+		return 0;
+	}
 
-	if ( false === $crons = _get_cron_array() )
-		return;
+	$crons = wp_get_ready_cron_jobs();
+	if ( empty( $crons ) ) {
+		return 0;
+	}
 
 	$gmt_time = microtime( true );
-	$keys = array_keys( $crons );
-	if ( isset($keys[0]) && $keys[0] > $gmt_time )
-		return;
+	$keys     = array_keys( $crons );
+	if ( isset( $keys[0] ) && $keys[0] > $gmt_time ) {
+		return 0;
+	}
 
 	$schedules = wp_get_schedules();
+	$results   = array();
 	foreach ( $crons as $timestamp => $cronhooks ) {
-		if ( $timestamp > $gmt_time ) break;
+		if ( $timestamp > $gmt_time ) {
+			break;
+		}
 		foreach ( (array) $cronhooks as $hook => $args ) {
-			if ( isset($schedules[$hook]['callback']) && !call_user_func( $schedules[$hook]['callback'] ) )
+			if ( isset( $schedules[ $hook ]['callback'] ) && ! call_user_func( $schedules[ $hook ]['callback'] ) ) {
 				continue;
-			spawn_cron( $gmt_time );
+			}
+			$results[] = spawn_cron( $gmt_time );
 			break 2;
 		}
 	}
+
+	if ( in_array( false, $results, true ) ) {
+		return false;
+	}
+	return count( $results );
 }
 
 /**
@@ -415,19 +803,27 @@
  *     // Filter parameter variable name is 'array'.
  *     $array['weekly'] = array(
  *         'interval' => 604800,
- *     	   'display'  => __( 'Once Weekly' )
+ *         'display'  => __( 'Once Weekly' )
  *     );
  *
- *
  * @since 2.1.0
  *
  * @return array
  */
 function wp_get_schedules() {
 	$schedules = array(
-		'hourly'     => array( 'interval' => HOUR_IN_SECONDS,      'display' => __( 'Once Hourly' ) ),
-		'twicedaily' => array( 'interval' => 12 * HOUR_IN_SECONDS, 'display' => __( 'Twice Daily' ) ),
-		'daily'      => array( 'interval' => DAY_IN_SECONDS,       'display' => __( 'Once Daily' ) ),
+		'hourly'     => array(
+			'interval' => HOUR_IN_SECONDS,
+			'display'  => __( 'Once Hourly' ),
+		),
+		'twicedaily' => array(
+			'interval' => 12 * HOUR_IN_SECONDS,
+			'display'  => __( 'Twice Daily' ),
+		),
+		'daily'      => array(
+			'interval' => DAY_IN_SECONDS,
+			'display'  => __( 'Once Daily' ),
+		),
 	);
 	/**
 	 * Filters the non-default cron schedules.
@@ -445,21 +841,80 @@
  * @see wp_get_schedules() for available schedules.
  *
  * @since 2.1.0
+ * @since 5.1.0 {@see 'get_schedule'} filter added.
  *
  * @param string $hook Action hook to identify the event.
  * @param array $args Optional. Arguments passed to the event's callback function.
  * @return string|false False, if no schedule. Schedule name on success.
  */
-function wp_get_schedule($hook, $args = array()) {
+function wp_get_schedule( $hook, $args = array() ) {
+	$schedule = false;
+	$event    = wp_get_scheduled_event( $hook, $args );
+
+	if ( $event ) {
+		$schedule = $event->schedule;
+	}
+
+	/**
+	 * Filter the schedule for a hook.
+	 *
+	 * @since 5.1.0
+	 *
+	 * @param string|bool $schedule Schedule for the hook. False if not found.
+	 * @param string      $hook     Action hook to execute when cron is run.
+	 * @param array       $args     Optional. Arguments to pass to the hook's callback function.
+	 */
+	return apply_filters( 'get_schedule', $schedule, $hook, $args );
+}
+
+/**
+ * Retrieve cron jobs ready to be run.
+ *
+ * Returns the results of _get_cron_array() limited to events ready to be run,
+ * ie, with a timestamp in the past.
+ *
+ * @since 5.1.0
+ *
+ * @return array Cron jobs ready to be run.
+ */
+function wp_get_ready_cron_jobs() {
+	/**
+	 * Filter to preflight or hijack retrieving ready cron jobs.
+	 *
+	 * Returning an array will short-circuit the normal retrieval of ready
+	 * cron jobs, causing the function to return the filtered value instead.
+	 *
+	 * @since 5.1.0
+	 *
+	 * @param null|array $pre Array of ready cron tasks to return instead. Default null
+	 *                        to continue using results from _get_cron_array().
+	 */
+	$pre = apply_filters( 'pre_get_ready_cron_jobs', null );
+	if ( null !== $pre ) {
+		return $pre;
+	}
+
 	$crons = _get_cron_array();
-	$key = md5(serialize($args));
-	if ( empty($crons) )
-		return false;
-	foreach ( $crons as $timestamp => $cron ) {
-		if ( isset( $cron[$hook][$key] ) )
-			return $cron[$hook][$key]['schedule'];
+
+	if ( false === $crons ) {
+		return array();
+	}
+
+	$gmt_time = microtime( true );
+	$keys     = array_keys( $crons );
+	if ( isset( $keys[0] ) && $keys[0] > $gmt_time ) {
+		return array();
 	}
-	return false;
+
+	$results = array();
+	foreach ( $crons as $timestamp => $cronhooks ) {
+		if ( $timestamp > $gmt_time ) {
+			break;
+		}
+		$results[ $timestamp ] = $cronhooks;
+	}
+
+	return $results;
 }
 
 //
@@ -474,15 +929,17 @@
  *
  * @return false|array CRON info array.
  */
-function _get_cron_array()  {
-	$cron = get_option('cron');
-	if ( ! is_array($cron) )
+function _get_cron_array() {
+	$cron = get_option( 'cron' );
+	if ( ! is_array( $cron ) ) {
 		return false;
+	}
 
-	if ( !isset($cron['version']) )
-		$cron = _upgrade_cron_array($cron);
+	if ( ! isset( $cron['version'] ) ) {
+		$cron = _upgrade_cron_array( $cron );
+	}
 
-	unset($cron['version']);
+	unset( $cron['version'] );
 
 	return $cron;
 }
@@ -491,13 +948,16 @@
  * Updates the CRON option with the new CRON array.
  *
  * @since 2.1.0
+ * @since 5.1.0 Return value modified to outcome of update_option().
+ *
  * @access private
  *
  * @param array $cron Cron info array from _get_cron_array().
+ * @return bool True if cron array updated, false on failure.
  */
-function _set_cron_array($cron) {
+function _set_cron_array( $cron ) {
 	$cron['version'] = 2;
-	update_option( 'cron', $cron );
+	return update_option( 'cron', $cron );
 }
 
 /**
@@ -511,16 +971,17 @@
  * @param array $cron Cron info array from _get_cron_array().
  * @return array An upgraded Cron info array.
  */
-function _upgrade_cron_array($cron) {
-	if ( isset($cron['version']) && 2 == $cron['version'])
+function _upgrade_cron_array( $cron ) {
+	if ( isset( $cron['version'] ) && 2 == $cron['version'] ) {
 		return $cron;
+	}
 
 	$new_cron = array();
 
-	foreach ( (array) $cron as $timestamp => $hooks) {
+	foreach ( (array) $cron as $timestamp => $hooks ) {
 		foreach ( (array) $hooks as $hook => $args ) {
-			$key = md5(serialize($args['args']));
-			$new_cron[$timestamp][$hook][$key] = $args;
+			$key                                     = md5( serialize( $args['args'] ) );
+			$new_cron[ $timestamp ][ $hook ][ $key ] = $args;
 		}
 	}