Source

File: wp-admin/includes/class-wp-upgrader.php

public function clear_destination( $remote_destination ) {
		global $wp_filesystem;

		$files = $wp_filesystem->dirlist( $remote_destination, true, true );

		// False indicates that the $remote_destination doesn't exist.
		if ( false === $files ) {
			return true;
		}

		// Flatten the file list to iterate over.
		$files = $this->flatten_dirlist( $files );

		// Check all files are writable before attempting to clear the destination.
		$unwritable_files = array();

		// Check writability.
		foreach ( $files as $filename => $file_details ) {
			if ( ! $wp_filesystem->is_writable( $remote_destination . $filename ) ) {
				// Attempt to alter permissions to allow writes and try again.
				$wp_filesystem->chmod( $remote_destination . $filename, ( 'd' === $file_details['type'] ? FS_CHMOD_DIR : FS_CHMOD_FILE ) );
				if ( ! $wp_filesystem->is_writable( $remote_destination . $filename ) ) {
					$unwritable_files[] = $filename;
				}
			}
		}

		if ( ! empty( $unwritable_files ) ) {
			return new WP_Error( 'files_not_writable', $this->strings['files_not_writable'], implode( ', ', $unwritable_files ) );
		}

		if ( ! $wp_filesystem->delete( $remote_destination, true ) ) {
			return new WP_Error( 'remove_old_failed', $this->strings['remove_old_failed'] );
		}

		return true;
	}