Cron: Fix malformed cron array in `wp_schedule_single_event()` when `_get_cron_array()` returns `false`.

In `wp_schedule_single_event()`, the cron info array is retrieved via a call to `_get_cron_array()` and straight away cast to an array. But as the documentation for that function (correctly) states, the return type of that function is `array|false`, where `false` is returned for a site where no cron jobs have been scheduled (yet).

In the case that `_get_cron_array()` would return `false`, this would now unintentionally create an array with a single entry with key `0` and as the value `false`.

This is a bug. Fixed now by adding validation to the output of `_get_cron_array()` and initializing `$crons` to an empty array if `false` was returned.

Tests added first to prove the bug (a) was introduced in #44818 [44917] and (b) is now fixed.

Follow-up to [44917].

Props jrf, peterwilsoncc.
Fixes #53950.
Built from https://develop.svn.wordpress.org/trunk@51916


git-svn-id: http://core.svn.wordpress.org/trunk@51509 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/cron.php

@@ -118,7 +118,11 @@ function wp_schedule_single_event( $timestamp, $hook, $args = array(), $wp_error
 	 * current time) all events scheduled within the next ten minutes
 	 * are considered duplicates.
 	 */
-	$crons     = (array) _get_cron_array();
+	$crons = _get_cron_array();
+	if ( ! is_array( $crons ) ) {
+		$crons = array();
+	}
+
 	$key       = md5( serialize( $event->args ) );
 	$duplicate = false;
 

wp-includes/version.php

@@ -16,7 +16,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '5.9-alpha-51915';
+$wp_version = '5.9-alpha-51916';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.