Source

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

public function unpack_package( $package, $delete_package = true ) {
		global $wp_filesystem;

		$this->skin->feedback( 'unpack_package' );

		$upgrade_folder = $wp_filesystem->wp_content_dir() . 'upgrade/';

		// Clean up contents of upgrade directory beforehand.
		$upgrade_files = $wp_filesystem->dirlist( $upgrade_folder );
		if ( ! empty( $upgrade_files ) ) {
			foreach ( $upgrade_files as $file ) {
				$wp_filesystem->delete( $upgrade_folder . $file['name'], true );
			}
		}

		// We need a working directory - strip off any .tmp or .zip suffixes.
		$working_dir = $upgrade_folder . basename( basename( $package, '.tmp' ), '.zip' );

		// Clean up working directory.
		if ( $wp_filesystem->is_dir( $working_dir ) ) {
			$wp_filesystem->delete( $working_dir, true );
		}

		// Unzip package to working directory.
		$result = unzip_file( $package, $working_dir );

		// Once extracted, delete the package if required.
		if ( $delete_package ) {
			unlink( $package );
		}

		if ( is_wp_error( $result ) ) {
			$wp_filesystem->delete( $working_dir, true );
			if ( 'incompatible_archive' === $result->get_error_code() ) {
				return new WP_Error( 'incompatible_archive', $this->strings['incompatible_archive'], $result->get_error_data() );
			}
			return $result;
		}

		return $working_dir;
	}