Subject: Re: [PATCH v2 2/3] strbuf: set errno to 0 after strbuf_getcwd

From: Patrick Steinhardt <[email-masked]>
Date: Tue, 6 Aug 2024 08:26:16 +0200

On Mon, Aug 05, 2024 at 08:51:50AM -0700, Junio C Hamano wrote:
> Eric Sunshine <[email-masked]> writes:
> 
> > On Fri, Aug 2, 2024 at 5:32 PM Junio C Hamano <[email-masked]> wrote:
> >> > [...]
> >> > Set `errno = 0;` prior to exiting from `strbuf_getcwd` successfully.
> >> > This matches the behavior in functions like `run_transaction_hook`
> >> > (refs.c:2176) and `read_ref_internal` (refs/files-backend.c:564).
> >>
> >> I am still uneasy to see this unconditional clearing, which looks
> >> more like spreading the bad practice from two places you identified
> >> than following good behaviour modelled after these two places.
> >>
> >> But I'll let it pass.
> >>
> >> As long as our programmers understand that across strbuf_getcwd(),
> >> errno will *not* be preserved, even if the function returns success,
> >> it would be OK.  As the usual convention around errno is that a
> >> successful call would leave errno intact, not clear it to 0, it
> >> would make it a bit harder to learn our API for newcomers, though.
> >
> > For what it's worth, I share your misgivings about this change and
> > consider the suggestion[*] to make it save/restore `errno` upon
> > success more sensible. It would also be a welcome change to see the
> > function documentation in strbuf.h updated to mention that it follows
> > the usual convention of leaving `errno` untouched upon success and
> > clobbered upon error.
> >
> > [*]: https://lore.kernel.org/git/[email-masked]/
> 
> Yup, of course save/restore would be safer, and probably easier to
> reason about for many people.

Is it really all that reasonable? We're essentially partitioning our set
of APIs into two sets, where one set knows to keep `errno` intact
whereas another set doesn't. In such a world, you have to be very
careful about which APIs you are calling in a function that wants to
keep `errno` intact, which to me sounds like a maintenance headache.

I'd claim that most callers never care about `errno` at all. For the
callers that do, I feel it is way more fragile to rely on whether or not
a called function leaves `errno` intact or not. For one, it's fragile
because that may easily change due to a bug. Second, it is fragile
because the dependency on `errno` is not explicitly documented via code,
but rather an implicit dependency.

So isn't it more reasonable to rather make the few callers that do
require `errno` to be left intact to save it? It makes the dependency
explicit, avoids splitting our functions into two sets and allows us to
just ignore this issue for the majority of functions that couldn't care
less about `errno`.

Patrick