[PATCH v8] posix: Deprecate group_member for Linux

[Paul Eggert]

The change to faccessat.c (quoted below) is problematic, as it could 
entail multiple calls to getegid or to getgid, with inconsistent results 
if some other thread is changing the group id of the current process. 
Instead, I suggest declaring "granted" via a simple "int granted;" and 
then using an if statement to initialize it, so that the logic is not 
duplicated and the multiple calls eliminated.

> --- a/sysdeps/unix/sysv/linux/faccessat.c
> +++ b/sysdeps/unix/sysv/linux/faccessat.c
> @@ -59,11 +59,17 @@ __faccessat (int fd, const char *file, int mode, int flag)
>   		   || (stats.st_mode & (S_IXUSR | S_IXGRP | S_IXOTH))))
>       return 0;
>   
> +  int gm = __group_member2 (stats.st_gid);
> +  if (uid != stats.st_uid &&
> +      (stats.st_gid != ((flag & AT_EACCESS) ? __getegid () : __getgid ())))
> +    if (gm == -1)
> +      return -1;
> +
>     int granted = (uid == stats.st_uid
>   		 ? (unsigned int) (stats.st_mode & (mode << 6)) >> 6
>   		 : (stats.st_gid == ((flag & AT_EACCESS)
>   				     ? __getegid () : __getgid ())
> -		    || __group_member (stats.st_gid))
> +		    || gm)
>   		 ? (unsigned int) (stats.st_mode & (mode << 3)) >> 3
>   		 : (stats.st_mode & mode));
>