[LatestPosts] Fixes the excerpt length (#20313)

* adds a filter that sets the excerpt length to what was set in the editor

* adds named function as excerpt filter and removes after use

* adds PHP doc to the new filter callback and global variable

* default to using an anonymous function since it appears that it can be remove and is not affecting theme set length

* revert to named function for excerpt length filter

* proper function names for Guternberg namespace

* lower default priority for the latest posts excerpt length filter

packages/block-library/src/latest-posts/index.php

@@ -5,6 +5,26 @@
  * @package WordPress
  */
 
+/**
+ * The excerpt length set by the Latest Posts core block
+ * set at render time and used by the block itself.
+ *
+ * @var int
+ */
+$block_core_latest_posts_excerpt_length = 0;
+
+/**
+ * Callback for the excerpt_length filter used by
+ * the Latest Posts block at render time.
+ *
+ * @return int Returns the global $block_core_latest_posts_excerpt_length variable
+ *             to allow the excerpt_length filter respect the Latest Block setting.
+ */
+function block_core_latest_posts_get_excerpt_length() {
+	global $block_core_latest_posts_excerpt_length;
+	return $block_core_latest_posts_excerpt_length;
+}
+
 /**
  * Renders the `core/latest-posts` block on server.
  *
@@ -13,6 +33,8 @@
  * @return string Returns the post content with latest posts added.
  */
 function render_block_core_latest_posts( $attributes ) {
+	global $block_core_latest_posts_excerpt_length;
+
 	$args = array(
 		'posts_per_page'   => $attributes['postsToShow'],
 		'post_status'      => 'publish',
@@ -21,6 +43,9 @@ function render_block_core_latest_posts( $attributes ) {
 		'suppress_filters' => false,
 	);
 
+	$block_core_latest_posts_excerpt_length = $attributes['excerptLength'];
+	add_filter( 'excerpt_length', 'block_core_latest_posts_get_excerpt_length', 20 );
+
 	if ( isset( $attributes['categories'] ) ) {
 		$args['category'] = $attributes['categories'];
 	}
@@ -111,6 +136,8 @@ function render_block_core_latest_posts( $attributes ) {
 		$list_items_markup .= "</li>\n";
 	}
 
+	remove_filter( 'excerpt_length', 'block_core_latest_posts_get_excerpt_length', 20 );
+
 	$class = 'wp-block-latest-posts wp-block-latest-posts__list';
 	if ( isset( $attributes['align'] ) ) {
 		$class .= ' align' . $attributes['align'];