Index: wp-admin/includes/file.php
===================================================================
--- wp-admin/includes/file.php	(revision 24580)
+++ wp-admin/includes/file.php	(working copy)
@@ -864,7 +864,21 @@
 		$temp_file_name = $context . 'temp-write-test-' . time();
 		$temp_handle = @fopen($temp_file_name, 'w');
 		if ( $temp_handle ) {
-			if ( getmyuid() == @fileowner($temp_file_name) )
+			// Compare the owning UID of WordPress files to that of the files WordPress/PHP Creates
+			// This ensures that all core files remain owned by the same user and do not "leak" to the PHP processes user during upgrades
+			// This is a step more than simple write access, designed to ensure 100% compatibility rather than 90% compat with certain configurations
+			// Changing this to only require write access is possible, but at the detriment that many FTP configurations & quota configurations used by shared hosts will become inoperable, and/or open up WordPress files to being modified by other accounts PHP scripts in the event of a configuration change.
+			// Any server using a per-user PHP-fpm, suExec, or, suPHP configuration will PASS with the below.
+			// A server with files owned by 'johndoe', and PHP executing as 'apache' will FAIL.
+			// eg:
+			// fail:
+			//  getmyuid() == fileowner(__FILE__) == 501; // johndoe
+			//  posix_getuid() == fileowner( $temp_file_name ) == 500; // apache
+			// pass:
+			//  getmyuid() == fileowner(__FILE__) == posix_getuid() == fileowner( $temp_file_name ) == 501; // johndoe
+			//
+			$uid_of_wordpress_files = @fileowner( __FILE__ ); // getmyuid() also returns this, except it refers to the entry .php rather than this included file
+			if ( $uid_of_wordpress_files && $uid_of_wordpress_files === @fileowner( $temp_file_name ) )
 				$method = 'direct';
 			@fclose($temp_handle);
 			@unlink($temp_file_name);