Plugin_Upgrader::upgrade()

	public function upgrade( $plugin, $args = array() ) {
		$defaults    = array(
			'clear_update_cache' => true,
		);
		$parsed_args = gc_parse_args( $args, $defaults );

		$this->init();
		$this->upgrade_strings();

		$current = get_site_transient( 'update_plugins' );
		if ( ! isset( $current->response[ $plugin ] ) ) {
			$this->skin->before();
			$this->skin->set_result( false );
			$this->skin->error( 'up_to_date' );
			$this->skin->after();
			return false;
		}

		// Get the URL to the zip file.
		$r = $current->response[ $plugin ];

		add_filter( 'upgrader_pre_install', array( $this, 'deactivate_plugin_before_upgrade' ), 10, 2 );
		add_filter( 'upgrader_pre_install', array( $this, 'active_before' ), 10, 2 );
		add_filter( 'upgrader_clear_destination', array( $this, 'delete_old_plugin' ), 10, 4 );
		add_filter( 'upgrader_post_install', array( $this, 'active_after' ), 10, 2 );
		// There's a Trac ticket to move up the directory for zips which are made a bit differently, useful for non-.org plugins.
		// 'source_selection' => array( $this, 'source_selection' ),
		if ( $parsed_args['clear_update_cache'] ) {
			// Clear cache so gc_update_plugins() knows about the new plugin.
			add_action( 'upgrader_process_complete', 'gc_clean_plugins_cache', 9, 0 );
		}

		$this->run(
			array(
				'package'           => $r->package,
				'destination'       => GC_PLUGIN_DIR,
				'clear_destination' => true,
				'clear_working'     => true,
				'hook_extra'        => array(
					'plugin' => $plugin,
					'type'   => 'plugin',
					'action' => 'update',
				),
			)
		);

		// Cleanup our hooks, in case something else does a upgrade on this connection.
		remove_action( 'upgrader_process_complete', 'gc_clean_plugins_cache', 9 );
		remove_filter( 'upgrader_pre_install', array( $this, 'deactivate_plugin_before_upgrade' ) );
		remove_filter( 'upgrader_pre_install', array( $this, 'active_before' ) );
		remove_filter( 'upgrader_clear_destination', array( $this, 'delete_old_plugin' ) );
		remove_filter( 'upgrader_post_install', array( $this, 'active_after' ) );

		if ( ! $this->result || is_gc_error( $this->result ) ) {
			return $this->result;
		}

		// Force refresh of plugin update information.
		gc_clean_plugins_cache( $parsed_args['clear_update_cache'] );

		// Ensure any future auto-update failures trigger a failure email by removing
		// the last failure notification from the list when plugins update successfully.
		$past_failure_emails = get_option( 'auto_plugin_theme_update_emails', array() );

		if ( isset( $past_failure_emails[ $plugin ] ) ) {
			unset( $past_failure_emails[ $plugin ] );
			update_option( 'auto_plugin_theme_update_emails', $past_failure_emails );
		}

		return true;
	}