Re: Can we avoid chdir'ing in resolve_symlinks() ?

On 2022-09-01 Th 19:39, Tom Lane wrote:
> find_my_exec() wants to obtain an absolute, symlink-free path
> to the program's own executable, for what seem to me good
> reasons. However, chasing down symlinks is left to its
> subroutine resolve_symlinks(), which does this:
>
> * To resolve a symlink properly, we have to chdir into its directory and
> * then chdir to where the symlink points; otherwise we may fail to
> * resolve relative links correctly (consider cases involving mount
> * points, for example). After following the final symlink, we use
> * getcwd() to figure out where the heck we're at.
>
> and then afterwards it has to chdir back to the original cwd.
> That last step is a bit of a sore spot, because sometimes
> (especially in sudo situations) we may not have the privileges
> necessary to do that; I think this is the cause of the complaint
> at [1]. Anyway the whole thing seems a bit excessively Rube
> Goldbergian. I'm wondering why we couldn't just read the
> symlink(s), concatenate them together, and use canonicalize_path()
> to clean up any mess.
>
> This code was mine originally (336969e49), but I sure don't
> remember why I wrote it like that. I know we didn't have a
> robust version of canonicalize_path() then, and that may have
> been the main issue, but that offhand comment about mount
> points bothers me. But I can't reconstruct precisely what
> I was worried about there. The only contemporaneous discussion
> thread I can find is [2], which doesn't go into coding details.
>
> Thoughts?
>
> regards, tom lane
>
> [1] https://www.postgresql.org/message-id/flat/CAH8yC8kOj0pmHF1RbK2Gb2t4YCcNG-5h0TwZ7yxk3Hzw6C0Otg%40mail.gmail.com
> [2] https://www.postgresql.org/message-id/flat/4973.1099605411%40sss.pgh.pa.us
>
>

These days there seem to be library functions that do this, realpath(3)
and canonicalize_file_name(3). The latter is what seems to be called by
readlink(1). Should we be using one of those? I don't know how portable
they are. I don't see them here :-(
<https://docs.microsoft.com/en-us/cpp/c-runtime-library/reference/crt-alphabetical-function-reference?view=msvc-170>

cheers

andrew

--
Andrew Dunstan
EDB: https://www.enterprisedb.com