Posts, Post Types: Use persistent caching in get_adjacent_post function.

The function `get_adjacent_post` cached the results of database query in the cache group `counts`. This is a none persistent group and meant cache would not persist on the next request. Change cache to save to the `posts` cache group. Cache invalidation is done by using get last changed value of the `posts` and `terms` group as a salt for the cache key. 

Props spacedmonkey, peterwilsoncc, johnbillion, boonebgorges, mukesh27, dd32.
Fixes #41131.
Built from https://develop.svn.wordpress.org/trunk@55085


git-svn-id: http://core.svn.wordpress.org/trunk@54618 1a063a9b-81f0-0310-95a4-ce76da25c4cd

Author: VenusPR

wp-includes/link-template.php

@@ -1976,9 +1976,15 @@ function get_adjacent_post( $in_same_term = false, $excluded_terms = '', $previo
 	 */
 	$sort = apply_filters( "get_{$adjacent}_post_sort", "ORDER BY p.post_date $order LIMIT 1", $post, $order );
 
-	$query     = "SELECT p.ID FROM $wpdb->posts AS p $join $where $sort";
-	$query_key = 'adjacent_post_' . md5( $query );
-	$result    = wp_cache_get( $query_key, 'counts' );
+	$query        = "SELECT p.ID FROM $wpdb->posts AS p $join $where $sort";
+	$key          = md5( $query );
+	$last_changed = wp_cache_get_last_changed( 'posts' );
+	if ( $in_same_term || ! empty( $excluded_terms ) ) {
+		$last_changed .= wp_cache_get_last_changed( 'terms' );
+	}
+	$cache_key = "adjacent_post:$key:$last_changed";
+
+	$result = wp_cache_get( $cache_key, 'posts' );
 	if ( false !== $result ) {
 		if ( $result ) {
 			$result = get_post( $result );
@@ -1991,7 +1997,7 @@ function get_adjacent_post( $in_same_term = false, $excluded_terms = '', $previo
 		$result = '';
 	}
 
-	wp_cache_set( $query_key, $result, 'counts' );
+	wp_cache_set( $cache_key, $result, 'posts' );
 
 	if ( $result ) {
 		$result = get_post( $result );

wp-includes/version.php

@@ -16,7 +16,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '6.2-alpha-55084';
+$wp_version = '6.2-alpha-55085';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.