An upgrading class that was planned, but can't work in TSFEM.

upgrade-handler.class.php

<?php
/**
 * @package TSF_Extension_Manager/Bootstrap
 */
namespace TSF_Extension_Manager;

defined( 'TSF_EXTENSION_MANAGER_DB_VERSION' ) or die;

/**
 * The SEO Framework - Extension Manager plugin
 * Copyright (C) 2018 Sybre Waaijer, CyberWire (https://cyberwire.nl/)
 *
 * This program is free software: you can redistribute it and/or modify
 * it under the terms of the GNU General Public License version 3 as published
 * by the Free Software Foundation.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program. If not, see <http://www.gnu.org/licenses/>.
 */

//= This is safe unless we invoke autoloading.
new Upgrade_Handler;
/**
 * Class TSF_Extension_Manager\Upgrade_Handler
 *
 * Upgrades plugin and extensions.
 *
 * @since 1.0.0
 * @access private
 */
final class Upgrade_Handler {

	private $o = 'tsfem_current_db_version';
	protected $previous_db_version;
	protected $current_db_version;
	protected $upgrades;

	private $active_callbacks;

	public function __construct() {

		$this->upgrades = new \stdClass;
		$this->previous_db_version = \get_option( $this->o, '0' );
		$this->current_db_version = $this->previous_db_version;

		\add_action( 'plugins_loaded', [ $this, '_load_hooks' ], 10 );
		\add_action( 'plugins_loaded', [ $this, '_go' ], 11 );
	}

	public function get( $what ) {
		switch ( $what ) :
			case 'previous_db_version' :
				$val = $this->previous_db_version;
				break;

			case 'current_db_version' :
				$val = $this->current_db_version;
				break;

			default :
				$val = '';
		endswitch;

		return $val;
	}

	/**
	 * @param string $version.
	 * @param callable $callback
	 */
	public function _register_upgrade( $version, callable $callback ) {
		$c = &$this->_upgrade_collector();
		isset( $c->{$version} ) or $c->{$version} = [];
		$c->{$version}[] = $callback;
	}

	private function &_upgrade_collector() {
		return $this->upgrades;
	}

	public function _load_hooks() {

		if ( $this->deferred( $this->previous_db_version ) )
			return;

		$ms = \is_multisite();

		if ( \is_admin() ) {
			$this->do_admin_upgrade();
			$ms and $this->do_admin_upgrade();
		}

		$this->do_always_upgrade();
		$ms and $this->do_network_always_upgrade();
	}

	private function do_admin_upgrade() {
		\do_action_ref_array( 'tsfem_prepare_admin_upgrade', [ $this ] );
	}

	private function do_network_admin_upgrade() {
		\do_action_ref_array( 'tsfem_prepare_network_admin_upgrade', [ $this ] );
	}

	private function do_always_upgrade() {
		\do_action_ref_array( 'tsfem_prepare_always_upgrade', [ $this ] );
	}

	private function do_network_always_upgrade() {
		\do_action_ref_array( 'tsfem_prepare_network_always_upgrade', [ $this ] );
	}

	public function _go() {

		$this->upgrades = (array) $this->upgrades;
		ksort( $this->upgrades );

		// $this->upgrades = array_filter( $this->upgrades, [ $this, 'remove_old_upgrades' ] );

		$success = true;
		foreach ( $this->yield_runs( $this->upgrades ) as $version => $_upgraded ) {
			if ( $this->deferred( $version ) ) {
				$success = false;
				break;
			}

			if ( $_upgraded ) {
				$success = \update_option( $this->o, $version );
			}
			if ( $_upgraded && $success ) {
				$this->current_db_version = $version;
			} else {
				// TODO set notice after 3 consecutive fails.
				$success = false;
				break;
			}
		}

		$success and \update_option( $this->o, TSF_EXTENSION_MANAGER_DB_VERSION );
	}

	private function yield_runs( $upgrade ) {
		$this->active_callbacks = [];
		foreach ( $upgrade as $version => $callbacks ) {
			$this->active_callbacks[ $version ] = $callbacks;
			yield [ $version => $this->upgraded() ];
		}
	}

	private function upgraded() {
		foreach ( $this->run_upgrades() as $success ) {
			if ( ! $success )
				return false;
		}

		return true;
	}

	private function &run_upgrades() {

		$version = key( $this->active_callbacks );
		$count = count( $this->active_callbacks[ $version ] );
		$i = 0;

		while ( $i < $count ) {
			if ( call_user_func_array( $this->active_callbacks[ $version ][ $i++ ], [ $version ] ) ) {
				yield true;
			} else {
				yield false;
			}
		}
	}

	/**
	 * @param string $version
	 */
	private function deferred( $version ) {
		/**
		 * Applies filters 'tsf_extension_manager_impede_upgrade'
		 * Allows users to defer upgrades. It's advisable not to use.
		 *
		 * @since 1.5.0
		 * @param bool   $deferred
		 * @param string $version
		 */
		if ( \apply_filters_ref_array( 'tsf_extension_manager_defer_upgrade', [ false, $version ] ) )
			return true;

		return false;
	}
}