bug#31332: touch unnecessarily calls dup2

[Paul Eggert]

John Steele Scott wrote:
> I'd much much obliged if one of you could enlighten me as to why touch needs to 
> treat /dev/stdin as a special case after the file has been opened though. 
> Wouldn't the following work just as well, with one less system call?
>
> --- a/src/touch.c
> +++ b/src/touch.c
> @@ -132,8 +132,8 @@ touch (const char *file)
>     else if (! (no_create || no_dereference))
>       {
>         /* Try to open FILE, creating it if necessary.  */
> -      fd = fd_reopen (STDIN_FILENO, file,
> -                      O_WRONLY | O_CREAT | O_NONBLOCK | O_NOCTTY, MODE_RW_UGO);
> +      fd = open (file,
> +                 O_WRONLY | O_CREAT | O_NONBLOCK | O_NOCTTY, MODE_RW_UGO);
>   
>         /* Don't save a copy of errno if it's EISDIR, since that would lead
>            touch to give a bogus diagnostic for e.g., 'touch /' (assuming
> @@ -166,9 +166,9 @@ touch (const char *file)
>                        (no_dereference && fd == -1) ? AT_SYMLINK_NOFOLLOW : 0)
>           == 0);
>   
> -  if (fd == STDIN_FILENO)
> +  if (fd != STDOUT_FILENO)
>       {
> -      if (close (STDIN_FILENO) != 0)
> +      if (close (fd) != 0)

That patch has trouble if 'open' returns 1. The resulting fd is never closed and 
for some filesystems (NFS, for example) one needs to close the fd to find out 
whether the fdutimensat call actually worked.