From 4edede48ebc80089b8d1699e3402298d07cd042e Mon Sep 17 00:00:00 2001
From: peterwilsoncc <peterwilsoncc@602fd350-edb4-49c9-b593-d223f7449a82>
Date: Wed, 9 Nov 2022 00:57:02 +0000
Subject: [PATCH] Query: Prevent ID only queries erroring when starting the
 loop.

Ensure only full post objects are passed to `update_post_author_caches()` when called within `WP_Query::the_post()`. This prevents an error when starting the Loop for Queries initiated with a subset of fields or IDs only.

Props konyoldeath, dd32, lozula, TimothyBlynJacobs, spacedmonkey, mxbclang, peterwilsoncc.
Fixes #56948.



git-svn-id: https://develop.svn.wordpress.org/trunk@54771 602fd350-edb4-49c9-b593-d223f7449a82
---
 src/wp-includes/class-wp-query.php         | 10 +++-
 src/wp-includes/post.php                   |  9 ++++
 tests/phpunit/tests/query/cacheResults.php | 61 ++++++++++++++++++++++
 3 files changed, 79 insertions(+), 1 deletion(-)

diff --git a/src/wp-includes/class-wp-query.php b/src/wp-includes/class-wp-query.php
index 69efdf5e2a..1e695bd052 100644
--- a/src/wp-includes/class-wp-query.php
+++ b/src/wp-includes/class-wp-query.php
@@ -3586,7 +3586,15 @@ class WP_Query {
 		global $post;
 
 		if ( ! $this->in_the_loop ) {
-			update_post_author_caches( $this->posts );
+			// Only prime the post cache for queries limited to the ID field.
+			$post_ids = array_filter( $this->posts, 'is_numeric' );
+			// Exclude any falsey values, such as 0.
+			$post_ids = array_filter( $post_ids );
+			if ( $post_ids ) {
+				_prime_post_caches( $post_ids, $this->query_vars['update_post_term_cache'], $this->query_vars['update_post_meta_cache'] );
+			}
+			$post_objects = array_map( 'get_post', $this->posts );
+			update_post_author_caches( $post_objects );
 		}
 
 		$this->in_the_loop = true;
diff --git a/src/wp-includes/post.php b/src/wp-includes/post.php
index d7e7bfec99..6a7c637831 100644
--- a/src/wp-includes/post.php
+++ b/src/wp-includes/post.php
@@ -7461,6 +7461,15 @@ function update_post_caches( &$posts, $post_type = 'post', $update_term_cache =
  * @param WP_Post[] $posts Array of post objects.
  */
 function update_post_author_caches( $posts ) {
+	/*
+	 * cache_users() is a pluggable function so is not available prior
+	 * to the `plugins_loaded` hook firing. This is to ensure against
+	 * fatal errors when the function is not available.
+	 */
+	if ( ! function_exists( 'cache_users' ) ) {
+		return;
+	}
+
 	$author_ids = wp_list_pluck( $posts, 'post_author' );
 	$author_ids = array_map( 'absint', $author_ids );
 	$author_ids = array_unique( array_filter( $author_ids ) );
diff --git a/tests/phpunit/tests/query/cacheResults.php b/tests/phpunit/tests/query/cacheResults.php
index d133dd862c..6e2dde1f0f 100644
--- a/tests/phpunit/tests/query/cacheResults.php
+++ b/tests/phpunit/tests/query/cacheResults.php
@@ -1210,4 +1210,65 @@ class Test_Query_CacheResults extends WP_UnitTestCase {
 			array( 'id=>parent', 'id=>parent' ),
 		);
 	}
+
+	/**
+	 * Ensure starting the loop warms the author cache.
+	 *
+	 * @since 6.1.1
+	 * @ticket 56948
+	 *
+	 * @covers WP_Query::the_post
+	 *
+	 * @dataProvider data_author_cache_warmed_by_the_loop
+	 *
+	 * @param string $fields Query fields.
+	 */
+	public function test_author_cache_warmed_by_the_loop( $fields ) {
+		// Update post author for the parent post.
+		self::factory()->post->update_object( self::$pages[0], array( 'post_author' => self::$author_id ) );
+
+		self::factory()->post->create(
+			array(
+				'post_author' => self::$author_id,
+				'post_parent' => self::$pages[0],
+				'post_type'   => 'page',
+			)
+		);
+
+		$query_1 = new WP_Query(
+			array(
+				'post_type' => 'page',
+				'fields'    => $fields,
+				'author'    => self::$author_id,
+			)
+		);
+
+		// Start the loop.
+		$start_loop_queries = get_num_queries();
+		$query_1->the_post();
+		$num_loop_queries = get_num_queries() - $start_loop_queries;
+		$this->assertSame( 2, $num_loop_queries, 'Unexpected number of queries while initializing the loop.' );
+
+		$start_author_queries = get_num_queries();
+		get_user_by( 'ID', self::$author_id );
+		$num_author_queries = get_num_queries() - $start_author_queries;
+		$this->assertSame( 0, $num_author_queries, 'Author cache is not warmed by the loop.' );
+	}
+
+	/**
+	 * Data provider for test_author_cache_warmed_by_the_loop
+	 *
+	 * @return array[]
+	 */
+	public function data_author_cache_warmed_by_the_loop() {
+		return array(
+			'fields: empty' => array( '' ),
+			'fields: all'   => array( 'all' ),
+			'fields: ids'   => array( 'ids' ),
+			/*
+			 * `id=>parent` is untested pending the resolution of an existing bug.
+			 * See https://core.trac.wordpress.org/ticket/56992
+			 */
+		);
+	}
 }