Posts, Post Types: Prevent categories from being overwritten when updating a post using `wp_insert_post()`.

This prevents existing category relationships being overridden with the default category when none is provided in the post data.

Props markoheijnen, leewillis77, desrosj

Fixes #19954

Built from https://develop.svn.wordpress.org/trunk@53883


git-svn-id: http://core.svn.wordpress.org/trunk@53442 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/post.php

@@ -4142,6 +4142,8 @@ function wp_insert_post( $postarr, $wp_error = false, $fire_after_hooks = true )
 	if ( ! empty( $postarr['post_category'] ) ) {
 		// Filter out empty terms.
 		$post_category = array_filter( $postarr['post_category'] );
+	} elseif ( $update && ! isset( $postarr['post_category'] ) ) {
+		$post_category = $post_before->post_category;
 	}
 
 	// Make sure we set a valid category.

wp-includes/version.php

@@ -16,7 +16,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '6.1-alpha-53882';
+$wp_version = '6.1-alpha-53883';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.