Feeds: ensure build/update date matches current query.

Displaying the correct build date in feeds is as important today as it was twelve years ago when this ticket was opened.

Fix an issue where all feeds in WordPress showed the same date for their last build date (the datapoint is `lastBuildDate`, `updated` or `dc:date` depending on the feed type). 

Introduce a new `get_last_build_date` filter to adjust the date used for `lastBuildDate`. Developers who previously filtered `get_lastcommentmodified` to alter feed dates should use this filter instead.

* `get_last_build_date` extracts the latest post (or comment) in the current WP_Query object.
* In all feed templates, use `get_last_build_date` vs `get_lastpostmodified( 'GMT' );`.

Props stevenkword, spacedmonkey, ryanshoover, mauteri, nacin, jorbin, MikeNGarrett, Denis-de-Bernardy, peaceablewhale.
Fixes #4575.



git-svn-id: https://develop.svn.wordpress.org/trunk@44948 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/feed.php

@@ -637,6 +637,51 @@ function self_link() {
 	echo esc_url( apply_filters( 'self_link', set_url_scheme( 'http://' . $host['host'] . wp_unslash( $_SERVER['REQUEST_URI'] ) ) ) );
 }
 
+/*
+* Get the timestamp of the most recently modified post from WP_Query.
+*
+* If viewing a comment feed, the date of the most recently modified
+* comment will be returned.
+*
+* @global WP_Query  $wp_query The global WP_Query object.
+*
+* @since 5.2.0
+*
+* @return string The timestamp.
+*/
+function get_last_build_date() {
+	global $wp_query;
+
+	if ( empty( $wp_query ) || ! $wp_query->have_posts() ) {
+		// Fallback to last time any post was modified or published.
+		return get_lastpostmodified( 'GMT' );
+	}
+
+	// Extract the post modified times from the posts.
+	$modified_times = wp_list_pluck( $wp_query->posts, 'post_modified_gmt' );
+
+	// 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' );
+
+		// Add the comment times to the post times for comparison.
+		$modified_times = array_merge( $modified_times, $comment_times );
+	}
+
+	// Determine the maximum modified time.
+	$max_modified_time = max( $modified_times );
+
+	/**
+	 * Filters the date the last post or comment in the query was modified.
+	 *
+	 * @since 5.2.0
+	 *
+	 * @param string $max_modified_times Date the last post or comment was modified in the query.
+	 */
+	return apply_filters( 'get_last_build_date', $max_modified_time );
+}
+
 /**
  * Return the content type for specified feed type.
  *