Feeds: Guard against empty array passed to max() in get_feed_build_date()

src/wp-includes/feed.php

@@ -720,20 +720,46 @@ function get_feed_build_date( $format ) {
 	$utc               = new DateTimeZone( 'UTC' );
 
 	if ( ! empty( $wp_query ) && $wp_query->have_posts() ) {
-		// Extract the post modified times from the posts.
-		$modified_times = wp_list_pluck( $wp_query->posts, 'post_modified_gmt' );
+		/*
+		 * Resolve each entry to a WP_Post so we can read post_modified_gmt.
+		 * Supports queries using fields => 'ids' where $posts contains
+		 * integers, as well as the default WP_Post objects.
+		 */
+		$modified_times = array_filter(
+			array_map(
+				static function ( $post ) {
+					$post_object = get_post( $post );
+					return $post_object instanceof WP_Post ? $post_object->post_modified_gmt : null;
+				},
+				$wp_query->posts
+			)
+		);
 
 		// If this is a comment feed, check those objects too.
 		if ( $wp_query->is_comment_feed() && $wp_query->comment_count ) {
-			// Extract the comment modified times from the comments.
-			$comment_times = wp_list_pluck( $wp_query->comments, 'comment_date_gmt' );
+			/*
+			 * Resolve each entry to a WP_Comment so we can read
+			 * comment_date_gmt. Supports comment queries using
+			 * fields => 'ids' as well as the default WP_Comment objects.
+			 */
+			$comment_times = array_filter(
+				array_map(
+					static function ( $comment ) {
+						$comment_object = get_comment( $comment );
+						return $comment_object instanceof WP_Comment ? $comment_object->comment_date_gmt : null;
+					},
+					$wp_query->comments
+				)
+			);
 
 			// Add the comment times to the post times for comparison.
 			$modified_times = array_merge( $modified_times, $comment_times );
 		}
 
 		// Determine the maximum modified time.
-		$datetime = date_create_immutable_from_format( 'Y-m-d H:i:s', max( $modified_times ), $utc );
+		if ( $modified_times ) {
+			$datetime = date_create_immutable_from_format( 'Y-m-d H:i:s', max( $modified_times ), $utc );
+		}
 	}
 
 	if ( false === $datetime ) {

tests/phpunit/tests/date/getFeedBuildDate.php

@@ -40,6 +40,93 @@ public function test_should_return_correct_feed_build_date() {
 		$this->assertSame( '2018-07-23T03:13:23+00:00', get_feed_build_date( DATE_RFC3339 ) );
 	}
 
+	/**
+	 * Test that get_feed_build_date() does not throw a ValueError
+	 * when $wp_query->posts contains no entries that resolve to a
+	 * WP_Post (e.g. invalid IDs that get_post() returns null for).
+	 *
+	 * @ticket 59956
+	 */
+	public function test_should_not_error_when_modified_times_is_empty() {
+		global $wp_query;
+
+		$datetime     = new DateTimeImmutable( 'now', wp_timezone() );
+		$datetime_utc = $datetime->setTimezone( new DateTimeZone( 'UTC' ) );
+
+		self::factory()->post->create(
+			array(
+				'post_date' => $datetime->format( 'Y-m-d H:i:s' ),
+			)
+		);
+
+		/*
+		 * Build a WP_Query where have_posts() is true but no entry can be
+		 * resolved to a WP_Post. Setting post_count without populating posts
+		 * with valid data exercises the empty $modified_times fallback path.
+		 */
+		$wp_query             = new WP_Query();
+		$wp_query->post_count = 1;
+		$wp_query->posts      = array( PHP_INT_MAX ); // Non-existent post ID.
+
+		$result = get_feed_build_date( DATE_RFC3339 );
+		$this->assertIsString( $result );
+
+		$this->assertEqualsWithDelta(
+			strtotime( $datetime_utc->format( DATE_RFC3339 ) ),
+			strtotime( $result ),
+			2,
+			'Should fall back to last post modified when modified_times is empty.'
+		);
+	}
+
+	/**
+	 * Test that get_feed_build_date() returns the correct modified time
+	 * when $wp_query->posts is an array of post IDs (from fields => 'ids')
+	 * instead of WP_Post objects.
+	 *
+	 * Before this test, the function would fall back to get_lastpostmodified()
+	 * and silently return the site-wide latest modified time instead of the
+	 * latest modified time of the posts actually in the feed.
+	 *
+	 * @ticket 59956
+	 */
+	public function test_should_return_correct_build_date_for_id_only_query() {
+		// Create two posts with different modified times. Post B is newer than Post A.
+		$older_post_id = self::factory()->post->create(
+			array(
+				'post_date'     => '2020-01-01 00:00:00',
+				'post_date_gmt' => '2020-01-01 00:00:00',
+			)
+		);
+
+		self::factory()->post->create(
+			array(
+				'post_date'     => '2024-06-15 12:00:00',
+				'post_date_gmt' => '2024-06-15 12:00:00',
+			)
+		);
+
+		/*
+		 * Query for ONLY the older post using fields => 'ids'. The feed's
+		 * <lastBuildDate> must reflect the modified time of the older post,
+		 * not the site-wide latest (which would be the newer post that is
+		 * not in the feed).
+		 */
+		global $wp_query;
+		$wp_query = new WP_Query(
+			array(
+				'p'      => $older_post_id,
+				'fields' => 'ids',
+			)
+		);
+
+		$this->assertSame(
+			'2020-01-01T00:00:00+00:00',
+			get_feed_build_date( DATE_RFC3339 ),
+			'Build date should match the modified time of the post in the feed, not the site-wide latest.'
+		);
+	}
+
 	/**
 	 * Test that get_feed_build_date() works with invalid post dates.
 	 *