Query: Save excessive cache add and sets in WP_Query.

In [53941] database query caching was added to `WP_Query`. However on sites with persistent object caching enabled, this resulted in a high number of unnecessary cache set and adds being run on every request. Caches are not set, if the query cache already exists and is cached. Replace usage of `update_post_caches` with `_prime_post_caches` to ensure that only posts that are not in cache are primed. 

Props spacedmonkey, peterwilsoncc, mukesh27.
See #22176.

git-svn-id: https://develop.svn.wordpress.org/trunk@54352 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/class-wp-query.php

@@ -3099,10 +3099,10 @@ public function get_posts() {
 				$last_changed .= wp_cache_get_last_changed( 'terms' );
 			}
 
-			$cache_key = "wp_query:$key:$last_changed";
-
+			$cache_key   = "wp_query:$key:$last_changed";
+			$cache_found = false;
 			if ( null === $this->posts ) {
-				$cached_results = wp_cache_get( $cache_key, 'posts' );
+				$cached_results = wp_cache_get( $cache_key, 'posts', false, $cache_found );
 
 				if ( $cached_results ) {
 					if ( 'ids' === $q['fields'] ) {
@@ -3256,7 +3256,7 @@ public function get_posts() {
 			$this->posts = array_map( 'get_post', $this->posts );
 		}
 
-		if ( $q['cache_results'] && $id_query_is_cacheable ) {
+		if ( $q['cache_results'] && $id_query_is_cacheable && ! $cache_found ) {
 			$post_ids = wp_list_pluck( $this->posts, 'ID' );
 
 			$cache_value = array(
@@ -3455,7 +3455,8 @@ public function get_posts() {
 			$this->posts = array_map( 'get_post', $this->posts );
 
 			if ( $q['cache_results'] ) {
-				update_post_caches( $this->posts, $post_type, $q['update_post_term_cache'], $q['update_post_meta_cache'] );
+				$post_ids = wp_list_pluck( $this->posts, 'ID' );
+				_prime_post_caches( $post_ids, $q['update_post_term_cache'], $q['update_post_meta_cache'] );
 			}
 
 			/** @var WP_Post */

tests/phpunit/tests/query/cacheResults.php

@@ -984,4 +984,46 @@ public function test_query_cache_should_not_exclude_post_when_excluded_term_is_r
 		$this->assertContains( $post_id, $post_ids_q2, 'Second query does not include the post ID.' );
 		$this->assertNotSame( $num_queries, get_num_queries(), 'Removing term does not invalidate previous cache.' );
 	}
+
+	/**
+	 * @ticket 22176
+	 * @dataProvider data_query_cache_with_empty_result_set
+	 */
+	public function test_query_cache_with_empty_result_set( $fields_q1, $fields_q2 ) {
+		_delete_all_posts();
+
+		$args_q1 = array(
+			'fields' => $fields_q1,
+		);
+
+		$query_1  = new WP_Query();
+		$posts_q1 = $query_1->query( $args_q1 );
+		$this->assertEmpty( $posts_q1, 'First query does not return an empty result set.' );
+
+		$args_q2 = array(
+			'fields' => $fields_q2,
+		);
+
+		$num_queries = get_num_queries();
+		$query_2     = new WP_Query();
+		$posts_q2    = $query_2->query( $args_q2 );
+		$this->assertEmpty( $posts_q2, 'Second query does not return an empty result set.' );
+		$this->assertSame( $num_queries, get_num_queries(), 'Second query is not cached.' );
+	}
+
+	public function data_query_cache_with_empty_result_set() {
+		return array(
+			array( '', '' ),
+			array( '', 'ids' ),
+			array( '', 'id=>parent' ),
+
+			array( 'ids', '' ),
+			array( 'ids', 'ids' ),
+			array( 'ids', 'id=>parent' ),
+
+			array( 'id=>parent', '' ),
+			array( 'id=>parent', 'ids' ),
+			array( 'id=>parent', 'id=>parent' ),
+		);
+	}
 }