Fix attempt to read post ID on update_post_thumbnail_cache

src/wp-includes/post-thumbnail-template.php

@@ -112,8 +112,25 @@ function update_post_thumbnail_cache( $wp_query = null ) {
 
 	$thumb_ids = array();
 
+	/*
+	 * $wp_query may contain an array of post objects or post IDs.
+	 *
+	 * This ensures the cache is primed for all post objects to avoid
+	 * `get_post()` calls in `get_the_post_thumbnail()` triggering an
+	 * additional database call for each post.
+	 */
+	$parent_post_ids = array();
+	foreach ( $wp_query->posts as $post ) {
+		if ( $post instanceof WP_Post ) {
+			$parent_post_ids[] = $post->ID;
+		} elseif ( is_int( $post ) ) {
+			$parent_post_ids[] = $post;
+		}
+	}
+	_prime_post_caches( $parent_post_ids, false, true );
+
 	foreach ( $wp_query->posts as $post ) {
-		$id = get_post_thumbnail_id( $post->ID );
+		$id = get_post_thumbnail_id( $post );
 		if ( $id ) {
 			$thumb_ids[] = $id;
 		}

tests/phpunit/tests/post/thumbnails.php

@@ -68,22 +68,59 @@ public function test_get_post_thumbnail_id() {
 		$this->assertSame( self::$attachment_id, get_post_thumbnail_id() );
 	}
 
-	public function test_update_post_thumbnail_cache() {
+	/**
+	 * Ensure `update_post_thumbnail_cache()` works when querying post objects.
+	 *
+	 * @ticket 59521
+	 * @ticket 30017
+	 * @ticket 33968
+	 *
+	 * @covers ::update_post_thumbnail_cache
+	 */
+	public function test_update_post_thumbnail_cache_when_querying_full_post_objects() {
+		set_post_thumbnail( self::$post, self::$attachment_id );
+
+		// Test case where `$query->posts` should return Array of post objects.
+		$query = new WP_Query(
+			array(
+				'post_type' => 'any',
+				'post__in'  => array( self::$post->ID ),
+				'orderby'   => 'post__in',
+			)
+		);
+
+		$this->assertFalse( $query->thumbnails_cached, 'Thumbnails should not be cached prior to calling update_post_thumbnail_cache().' );
+
+		update_post_thumbnail_cache( $query );
+
+		$this->assertTrue( $query->thumbnails_cached, 'Thumbnails should be cached after calling update_post_thumbnail_cache().' );
+	}
+
+	/**
+	 * Ensure `update_post_thumbnail_cache()` works when querying post IDs.
+	 *
+	 * @ticket 59521
+	 *
+	 * @covers ::update_post_thumbnail_cache
+	 */
+	public function test_update_post_thumbnail_cache_when_querying_post_id_field() {
 		set_post_thumbnail( self::$post, self::$attachment_id );
 
+		// Test case where `$query2->posts` should return Array of post IDs.
 		$query = new WP_Query(
 			array(
 				'post_type' => 'any',
 				'post__in'  => array( self::$post->ID ),
 				'orderby'   => 'post__in',
+				'fields'    => 'ids',
 			)
 		);
 
-		$this->assertFalse( $query->thumbnails_cached );
+		$this->assertFalse( $query->thumbnails_cached, 'Thumbnails should not be cached prior to calling update_post_thumbnail_cache().' );
 
 		update_post_thumbnail_cache( $query );
 
-		$this->assertTrue( $query->thumbnails_cached );
+		$this->assertTrue( $query->thumbnails_cached, 'Thumbnails should be cached after calling update_post_thumbnail_cache().' );
 	}
 
 	/**