[Toybox] [landley/toybox] Add ln -r (#145)

[enh]

On Fri, Oct 25, 2019 at 8:09 PM Rob Landley <rob at landley.net> wrote:
>
> On 10/25/19 6:29 PM, Rob Landley wrote:
> > I think I need a new lib function to do this, and I should change cp -s to use
> > it too. And I need more cp tests because the above cp -s made a broken symlink...
>
> Well, I made the new lib function, which is just:
>
> // return (malloced) relative path to get from "from" to "to"
> char *relative_path(char *from, char *to)
> {
>   char *s, *ret = 0;
>   int i, j, k;
>
>   if (!(from = xabspath(from, -1))) return 0;
>   if (!(to = xabspath(to, -1))) goto error;
>
>   // skip common directories from root
>   for (i = j = 0; from[i] && from[i] == to[i]; i++) if (to[i] == '/') j = i+1;
>
>   // count remaining destination directories
>   for (i = j, k = 0; from[i]; i++) if (from[i] == '/') k++;
>
>   if (!k) ret = xstrdup(to+j);
>   else {
>     s = ret = xmprintf("%*c%s", 3*k, ' ', to+j);
>     while (k--) memcpy(s+3*k, "../", 3);
>   }
>
> error:
>   free(from);
>   free(to);
>
>   return ret;
> }
>
> But getting cp to use it turns out to be fiddly because the context I have for
> destination path is a relative path under a directory file descriptor (which is
> faster and avoids path depth issues), and I don't actually know how to turn that
> into an absolute path? (Other than reading from /proc which always seems brittle.)
>
> Sigh. In theory I could get the path to top of destination and then copy the
> chunk of dirtree_parent() onto it to get the current directory, which would
> assume we hadn't followed symlinks to get here (when copying into an existing
> path)...
>
> Also, doing two xabspath() per file is kind of expensive for something like cp
> -r, but making a while second codepath to work _upwards_ for this also seems
> overkill...
>
> Rob
>
> P.S. And I need an xabspath(file, -2) variant that resolves everything except
> the last path component, because that's what tar wants for this directory
> replaces symlink thing. I need to resolve before calling unlink() so you can't
> trick tar into deleting random stuff by following a symlink...

(see also the current realpath(1), which doesn't support -s or
--relative-to, which i think will need realpath.c to switch to a
home-grown implementation instead of realpath(3).)

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