do_core_upgrade( bool $reinstall = false )

function do_core_upgrade( $reinstall = false ) {
	global $wp_filesystem;

	include_once( ABSPATH . 'wp-admin/includes/class-wp-upgrader.php' );

	if ( $reinstall )
		$url = 'update-core.php?action=do-core-reinstall';
	else
		$url = 'update-core.php?action=do-core-upgrade';
	$url = wp_nonce_url($url, 'upgrade-core');

	$version = isset( $_POST['version'] )? $_POST['version'] : false;
	$locale = isset( $_POST['locale'] )? $_POST['locale'] : 'en_US';
	$update = find_core_update( $version, $locale );
	if ( !$update )
		return;

	// Allow relaxed file ownership writes for User-initiated upgrades when the API specifies
	// that it's safe to do so. This only happens when there are no new files to create.
	$allow_relaxed_file_ownership = ! $reinstall && isset( $update->new_files ) && ! $update->new_files;

?>
	<div class="wrap">
	<h1><?php _e( 'Update ClassicPress' ); ?></h1>
<?php

	if ( false === ( $credentials = request_filesystem_credentials( $url, '', false, ABSPATH, array( 'version', 'locale' ), $allow_relaxed_file_ownership ) ) ) {
		echo '</div>';
		return;
	}

	if ( ! WP_Filesystem( $credentials, ABSPATH, $allow_relaxed_file_ownership ) ) {
		// Failed to connect, Error and request again
		request_filesystem_credentials( $url, '', true, ABSPATH, array( 'version', 'locale' ), $allow_relaxed_file_ownership );
		echo '</div>';
		return;
	}

	if ( $wp_filesystem->errors->get_error_code() ) {
		foreach ( $wp_filesystem->errors->get_error_messages() as $message )
			show_message($message);
		echo '</div>';
		return;
	}

	if ( $reinstall )
		$update->response = 'reinstall';

	add_filter( 'update_feedback', 'show_message' );

	$upgrader = new Core_Upgrader();
	$result = $upgrader->upgrade( $update, array(
		'allow_relaxed_file_ownership' => $allow_relaxed_file_ownership
	) );

	if ( is_wp_error( $result ) ) {
		show_message( $result );
		switch ( $result->get_error_code() ) {
			case 'up_to_date':
				// ClassicPress is already up to date, no need to show a different message
				break;

			case 'locked':
				// Show a bit more info for this fairly common error
				show_message( __(
					'It\'s possible that an update started, but the server encountered a temporary issue and could not continue.'
				) );
				show_message( __(
					'Or, you may have clicked the update button multiple times.'
				) );
				show_message( __(
					'Please wait <strong>15 minutes</strong> and try again.'
				) );
				show_message( sprintf(
					/* translators: URL to support forum */
					__( 'If you see this message after waiting 15 minutes and trying the update again, please make a post on our <a href="%s">support forum</a>.' ),
					'https://forums.classicpress.net/c/support/'
				) );
				break;

			default:
				// Show a generic failure message
				show_message( __( 'Installation Failed' ) );
				break;
		}
		echo '</div>';
		return;
	}

	show_message( __( 'ClassicPress updated successfully' ) );
	show_message( '<span class="hide-if-no-js">' . sprintf( __( 'Welcome to ClassicPress %1$s. You will be redirected to the About ClassicPress screen. If not, click <a href="%2$s">here</a>.' ), $result, esc_url( self_admin_url( 'about.php?updated' ) ) ) . '</span>' );
	show_message( '<span class="hide-if-js">' . sprintf( __( 'Welcome to ClassicPress %1$s. <a href="%2$s">Learn more</a>.' ), $result, esc_url( self_admin_url( 'about.php?updated' ) ) ) . '</span>' );
	?>
	</div>
	<script type="text/javascript">
	window.location = '<?php echo self_admin_url( 'about.php?updated' ); ?>';
	</script>
	<?php
}