Re: [PATCH v2] object-file: use real paths when adding alternates

[Jeff King <peff@peff.net>]

On Tue, Nov 22, 2022 at 01:56:09AM +0100, Ævar Arnfjörð Bjarmason wrote:

> > @@ -516,12 +517,14 @@ static int link_alt_odb_entry(struct repository *r, const struct strbuf *entry,
> >  	}
> >  	strbuf_addbuf(&pathbuf, entry);
> >  
> > -	if (strbuf_normalize_path(&pathbuf) < 0 && relative_base) {
> > +	if (!strbuf_realpath(&tmp, pathbuf.buf, 0)) {
> >  		error(_("unable to normalize alternate object path: %s"),
> > -		      pathbuf.buf);
> > +			pathbuf.buf);
> 
> This is a mis-indentation, it was OK in the pre-image, not now.
> 
> >  		strbuf_release(&pathbuf);
> 
> Doesn't this leak? I've just skimmed strbuf_realpath_1() but e.g. in the
> "REALPATH_MANY_MISSING" case it'll have allocated the "resolved" (the
> &tmp you pass in here) and then "does a "goto error_out".
> 
> It then *resets* the strbuf, but doesn't release it, assuming that
> you're going to pass it in again. So in that case we'd leak here, no?
> 
> I.e. a NULL return value from strbuf_realpath() doesn't mean that it
> didn't allocate in the scratch area passed to it, so we need to
> strbuf_release(&tmp) here too.

We don't use MANY_MISSING in this code path, but I didn't read
strbuf_realpath_1() carefully enough to see if that is the only case.
But regardless, I think it is a bug in strbuf_realpath(). All of the
strbuf functions generally try to leave a buffer untouched on error.

So IMHO we would want a preparatory patch with s/reset/release/ in that
function, which better matches the intent (we might be freeing an
allocated buffer, but that's OK from the caller perspective). In theory
it ought to just roll back the length for whatever it put into the
buffer, but it looks like the rest of the function is happy to clobber
what's in the buf, even on non-error. That's why we have
strbuf_add_real_path(), but of course it doesn't allow for setting the
die_on_error flag.

-Peff