Retrieves the current user object.

Will set the current user, if the current user is not set. The current user
will be set to the logged-in person. If no user is logged-in, then it will
set the current user to 0, which is invalid and won’t have any permissions.

This function is used by the pluggable functions wp_get_current_user() and
get_currentuserinfo(), the latter of which is deprecated but used for backward
compatibility.

function _wp_get_current_user() {
	global $current_user;

	if ( ! empty( $current_user ) ) {
		if ( $current_user instanceof WP_User ) {
			return $current_user;
		}

		// Upgrade stdClass to WP_User.
		if ( is_object( $current_user ) && isset( $current_user->ID ) ) {
			$cur_id       = $current_user->ID;
			$current_user = null;
			wp_set_current_user( $cur_id );
			return $current_user;
		}

		// $current_user has a junk value. Force to WP_User with ID 0.
		$current_user = null;
		wp_set_current_user( 0 );
		return $current_user;
	}

	if ( defined( 'XMLRPC_REQUEST' ) && XMLRPC_REQUEST ) {
		wp_set_current_user( 0 );
		return $current_user;
	}

	/**
	 * Filters the current user.
	 *
	 * The default filters use this to determine the current user from the
	 * request's cookies, if available.
	 *
	 * Returning a value of false will effectively short-circuit setting
	 * the current user.
	 *
	 * @since 3.9.0
	 *
	 * @param int|false $user_id User ID if one has been determined, false otherwise.
	 */
	$user_id = apply_filters( 'determine_current_user', false );
	if ( ! $user_id ) {
		wp_set_current_user( 0 );
		return $current_user;
	}

	wp_set_current_user( $user_id );

	return $current_user;
}