Fix recursively_filter_blocks sometimes generating blocks that crash

projects/plugins/jetpack/modules/widget-visibility/widget-conditions.php

@@ -715,7 +715,7 @@ public static function filter_widget( $instance ) {
 			// Block-Based widgets: We have gutenberg blocks that could have the 'conditions' attribute
 			// Note: These blocks can be nested, and we would have to apply these conditions at any level.
 			$blocks = parse_blocks( $instance['content'] );
-			$blocks = self::recursively_filter_blocks( $blocks );
+			$blocks = self::recursively_filter_blocks( $blocks )[0];
 			if ( empty( $blocks ) ) {
 				return false;
 			}
@@ -739,17 +739,12 @@ public static function filter_widget( $instance ) {
 	 * Recursively check the visibility conditions in blocks' attr.conditions.rules.
 	 * Any that fail the check will be removed.
 	 *
-	 * Note that this creates blocks that will fail to render in some cases, and should
-	 * be done with a different approach.
-	 * Example: [{"blockName":"core\/columns","attrs":{"conditions":{"action":"show","rules":[],"match_all":"0"}},"innerBlocks":[{"blockName":"core\/column","attrs":{"conditions":{"action":"show","rules":[],"match_all":"0"}},"innerBlocks":[{"blockName":"core\/paragraph","attrs":{"conditions":{"action":"show","rules":[{"major":"page","minor":"front"}],"match_all":"0"}},"innerBlocks":[],"innerHTML":"\n<p>Home Page Only<\/p>\n","innerContent":["\n<p>Home Page Only<\/p>\n"]}],"innerHTML":"\n<div class=\"wp-block-column\"><\/div>\n","innerContent":["\n<div class=\"wp-block-column\">",null,"<\/div>\n"]},{"blockName":"core\/column","attrs":{"conditions":{"action":"show","rules":[],"match_all":"0"}},"innerBlocks":[],"innerHTML":"\n<div class=\"wp-block-column\"><\/div>\n","innerContent":["\n<div class=\"wp-block-column\">",null,"<\/div>\n"]}],"innerHTML":"\n<div class=\"wp-block-columns\">\n\n<\/div>\n","innerContent":["\n<div class=\"wp-block-columns\">",null,"\n\n",null,"<\/div>\n"]}]
-	 * That is two columns, but one of the columns has 0 innerBlocks. However
-	 * it tries to render one anyway and crashes, possibly because of a 'null'
-	 * in the innerContent array (which might mean 'render a child').
-	 *
 	 * @param array $blocks (By reference; will be modified).
-	 * @return array $blocks
+	 * @return array [$blocks, $indexes_removed]
 	 */
 	public static function recursively_filter_blocks( &$blocks ) {
+		$indexes_removed = array();
+
 		foreach ( $blocks as $i => $block ) {
 			$keep = true;
 			if ( ! empty( $block['attrs']['conditions']['rules'] ) ) {
@@ -758,11 +753,46 @@ public static function recursively_filter_blocks( &$blocks ) {
 
 			if ( ! $keep ) {
 				unset( $blocks[ $i ] );
+				array_push( $indexes_removed, $i );
 			} elseif ( is_array( $block['innerBlocks'] ) ) {
-				$blocks[ $i ]['innerBlocks'] = self::recursively_filter_blocks( $blocks[ $i ]['innerBlocks'] );
+				$result                      = self::recursively_filter_blocks( $blocks[ $i ]['innerBlocks'] );
+				$blocks[ $i ]['innerBlocks'] = $result[0];
+
+				// For each block we removed in innerBlocks, we must remove a corresponding 'null' in innerContent
+				// if $sub_indexes_removed = [0, 2], we removed the 0th and 2nd innerBlocks, and need to remove
+				// the 0th and 2nd nulls in innerContent.
+				$sub_indexes_removed = $result[1];
+				$null_locations      = array_keys( $blocks[ $i ]['innerContent'], null, true );
+				$null_count          = count( $null_locations );
+
+				// phpcs:disable Squiz.PHP.CommentedOutCode.Found
+
+				/*
+				Example:
+				null_locations = [ 1, 3, 5 ]    "Nulls exist at j=1, j=3, j=5".
+				null_count = 3.                 "There are 3 nulls".
+				sub_indexes_removed = [ 0, 2 ]. "We want to remove the 0th and 2nd null".
+				We need to remove the 0th null (at j = 1) and the 2nd null (at j = 5).
+				[ 'abc', null, 'def', null, 'ghi', null ].
+				..........^ j = 1, k = 0              ^.
+				.......................^ j = 3, k = 1 ^.
+				......................................^ j = 5, k = 2.
+				*/
+
+				// phpcs:enable Squiz.PHP.CommentedOutCode.Found
+
+				$k = $null_count - 1;
+				// Work backwards so as we delete items, then the other indexes don't change.
+				foreach ( array_reverse( $null_locations ) as $j ) {
+					// We are looking at a null, which is the $j index of the array and the $kth null in total (k is also 0 indexed).
+					if ( in_array( $k, $sub_indexes_removed, true ) ) {
+						array_splice( $blocks[ $i ]['innerContent'], $j, 1 );
+					}
+					$k--;
+				}
 			}
 		}
-		return $blocks;
+		return array( $blocks, $indexes_removed );
 	}
 
 	/**