Prevent extra db queries in WP_Comment::get_children().

`WP_Comment_Query::fill_descendants()` queries for a comment tree in a way that
minimizes database overhead, and places the located descendants with their
proper parents. However, it doesn't touch leaf nodes - comments with no
children - so future calls to `get_children()` on those comment objects
result in unnecessary database queries. To prevent this, `fill_descendants()`
now sets a `populated_children` flag on all located `WP_Comment` objects.

See #8071.
Built from https://develop.svn.wordpress.org/trunk@34730


git-svn-id: http://core.svn.wordpress.org/trunk@34694 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/class-wp-comment.php

@@ -158,6 +158,15 @@ final class WP_Comment {
 	 */
 	protected $children;
 
+	/**
+	 * Whether children have been populated for this comment object.
+	 *
+	 * @since 4.4.0
+	 * @access protected
+	 * @var bool
+	 */
+	protected $populated_children = false;
+
 	/**
 	 * Post fields.
 	 *
@@ -279,7 +288,11 @@ final class WP_Comment {
 		$_args['parent'] = $this->comment_ID;
 
 		if ( is_null( $this->children ) ) {
-			$this->children = get_comments( $_args );
+			if ( $this->populated_children ) {
+				$this->children = array();
+			} else {
+				$this->children = get_comments( $_args );
+			}
 		}
 
 		if ( 'flat' === $_args['format'] ) {
@@ -330,6 +343,18 @@ final class WP_Comment {
 		return false;
 	}
 
+	/**
+	 * Set the 'populated_children' flag.
+	 *
+	 * This flag is important for ensuring that calling `get_children()` on a childless comment will not trigger
+	 * unneeded database queries.
+	 *
+	 * @since 4.4.0
+	 */
+	public function populated_children( $set ) {
+		$this->populated_children = (bool) $set;
+	}
+
 	/**
 	 * Check whether a non-public property is set.
 	 *