WordPress/wp-admin/update.php

<?php
/**
 * Update Plugin/Theme administration panel.
 *
 * @package WordPress
 * @subpackage Administration
 */

/** WordPress Administration Bootstrap */
require_once('admin.php');

if ( ! current_user_can('update_plugins') )
	wp_die(__('You do not have sufficient permissions to update plugins for this blog.'));

/**
 * Plugin upgrade display.
 *
 * @since 2.5
 *
 * @param string $plugin Plugin
 */
function do_plugin_upgrade($plugin) {
	global $wp_filesystem;

	$url = wp_nonce_url("update.php?action=upgrade-plugin&plugin=$plugin", "upgrade-plugin_$plugin");
	if ( false === ($credentials = request_filesystem_credentials($url)) )
		return;

	if ( ! WP_Filesystem($credentials) ) {
		$error = true;
		if ( is_object($wp_filesystem) && $wp_filesystem->errors->get_error_code() )
			$error = $wp_filesystem->errors;
		request_filesystem_credentials($url, '', $error); //Failed to connect, Error and request again
		return;
	}

	echo '<div class="wrap">';
	echo '<h2>' . __('Upgrade Plugin') . '</h2>';
	if ( $wp_filesystem->errors->get_error_code() ) {
		foreach ( $wp_filesystem->errors->get_error_messages() as $message )
			show_message($message);
		echo '</div>';
		return;
	}

	$was_activated = is_plugin_active($plugin); //Check now, It'll be deactivated by the next line if it is

	$result = wp_update_plugin($plugin, 'show_message');

	if ( is_wp_error($result) ) {
		show_message($result);
		show_message( __('Plugin upgrade Failed') );
	} else {
		$plugin_file = $result;
		show_message( __('Plugin upgraded successfully') );
		if( $result && $was_activated ){
			show_message(__('Attempting reactivation of the plugin'));
			echo '<iframe style="border:0;overflow:hidden" width="100%" height="170px" src="' . wp_nonce_url('update.php?action=activate-plugin&plugin=' . $plugin_file, 'activate-plugin_' . $plugin_file) .'"></iframe>';
		}
		$update_actions =  array(
			'activate_plugin' => '<a href="' . wp_nonce_url('plugins.php?action=activate&amp;plugin=' . $plugin_file, 'activate-plugin_' . $plugin_file) . '" title="' . attribute_escape(__('Activate this plugin')) . '" target="_parent">' . __('Activate Plugin') . '</a>',
			'plugins_page' => '<a href="' . admin_url('plugins.php') . '" title="' . attribute_escape(__('Goto plugins page')) . '" target="_parent">' . __('Return to Plugins page') . '</a>'
		);
		if ( $was_activated )
			unset( $update_actions['activate_plugin'] );

		$update_actions = apply_filters('update_plugin_complete_actions', $update_actions, $plugin_file);
		if ( ! empty($update_actions) )
			show_message('<strong>' . __('Actions:') . '</strong> ' . implode(' | ', (array)$update_actions));
	}
	echo '</div>';
}

/**
 * Theme upgrade display.
 *
 * @since 2.5
 *
 * @param string $plugin Plugin
 */
function do_theme_upgrade($theme) {
	global $wp_filesystem;

	$url = wp_nonce_url('update.php?action=upgrade-theme&theme=' . urlencode($theme), 'upgrade-plugin_' . urlencode($theme));
	if ( false === ($credentials = request_filesystem_credentials($url)) )
		return;

	if ( ! WP_Filesystem($credentials) ) {
		$error = true;
		if ( is_object($wp_filesystem) && $wp_filesystem->errors->get_error_code() )
			$error = $wp_filesystem->errors;
		request_filesystem_credentials($url, '', $error); //Failed to connect, Error and request again
		return;
	}

	echo '<div class="wrap">';
	echo '<h2>' . __('Upgrade Theme') . '</h2>';
	if ( $wp_filesystem->errors->get_error_code() ) {
		foreach ( $wp_filesystem->errors->get_error_messages() as $message )
			show_message($message);
		echo '</div>';
		return;
	}

	//TODO: Is theme currently active?
	$was_current = false; //is_plugin_active($plugin); //Check now, It'll be deactivated by the next line if it is

	$result = wp_update_theme($theme, 'show_message');

	if ( is_wp_error($result) ) {
		show_message($result);
		show_message( __('Installation Failed') );
	} else {
		//Result is the new plugin file relative to WP_PLUGIN_DIR
		show_message( __('Theme upgraded successfully') );
		if( $result && $was_current ){
			show_message(__('Setting theme as Current'));
			//TODO: Actually set it as active again.
			//echo '<iframe style="border:0" width="100%" height="170px" src="' . wp_nonce_url('update.php?action=activate-plugin&plugin=' . $result, 'activate-plugin_' . $result) .'"></iframe>';
		}
	}
	echo '</div>';
}

if ( isset($_GET['action']) ) {
	$plugin = isset($_GET['plugin']) ? trim($_GET['plugin']) : '';
	$theme = isset($_REQUEST['theme']) ? urldecode($_REQUEST['theme']) : '';
	$action = isset($_GET['action']) ? $_GET['action'] : '';

	if ( 'upgrade-plugin' == $action ) {
		check_admin_referer('upgrade-plugin_' . $plugin);
		$title = __('Upgrade Plugin');
		$parent_file = 'plugins.php';
		require_once('admin-header.php');
		do_plugin_upgrade($plugin);
		include('admin-footer.php');
	} elseif ('activate-plugin' == $action ) {
		check_admin_referer('activate-plugin_' . $plugin);
		if( ! isset($_GET['failure']) && ! isset($_GET['success']) ) {
			wp_redirect( 'update.php?action=activate-plugin&failure=true&plugin=' . $plugin . '&_wpnonce=' . $_GET['_wpnonce'] );
			activate_plugin($plugin);
			wp_redirect( 'update.php?action=activate-plugin&success=true&plugin=' . $plugin . '&_wpnonce=' . $_GET['_wpnonce'] );
			die();
		}
		iframe_header( __('Plugin Reactivation'), true );
		if( isset($_GET['success']) )
			echo '<p>' . __('Plugin reactivated successfully.') . '</p>';

		if( isset($_GET['failure']) ){
			echo '<p>' . __('Plugin failed to reactivate due to a fatal error.') . '</p>';
			error_reporting( E_ALL ^ E_NOTICE );
			@ini_set('display_errors', true); //Ensure that Fatal errors are displayed.
			include(WP_PLUGIN_DIR . '/' . $plugin);
		}
		iframe_footer();
	} elseif ( 'upgrade-theme' == $action ) {
		check_admin_referer('upgrade-theme_' . $theme);
		$title = __('Upgrade Theme');
		$parent_file = 'themes.php';
		require_once('admin-header.php');
		do_theme_upgrade($theme);
		include('admin-footer.php');
	}
}

?>