Re: [PATCH 2/7] get-tar-commit-id: check write_in_full() return against 0

[Jeff King <peff@peff.net>]

On Wed, Sep 13, 2017 at 02:09:27PM -0700, Jonathan Nieder wrote:

> > We ask to write 41 bytes and make sure that the return value
> > is at least 41. This is the same "dangerous" pattern that
> > was fixed in the prior commit (wherein a negative return
> > value is promoted to unsigned), though it is not dangerous
> > here because our "41" is a constant, not an unsigned
> > variable.
> >
> > But we should convert it anyway to avoid modeling a
> > dangerous construct.
> >
> > Signed-off-by: Jeff King <peff@peff.net>
> > ---
> >  builtin/get-tar-commit-id.c | 3 +--
> >  1 file changed, 1 insertion(+), 2 deletions(-)
> 
> I kind of disagree with calling this dangerous (and I think that is
> what you alluded to above by putting it in quotes), but I like the
> postimage more than the preimage.

Right, this instance is fine, but the pattern of using "<" is not. If
you swapped out "41" for:

  size_t len = 41;

then it would be a bug. Which I think would surprise most people.

> The variable 'n' could be eliminated to simplify this further.  I
> realize that would go against the spirit of this patch, but (1) it's
> on-topic for the patch, since it is another ssize_t vs constant
> comparison and (2) as mentioned elsewhere in this thread, it's a very
> common idiom with read_in_full.  If we want to eliminate it then we
> could introduce a separate helper to distinguish between
> read_this_much_i_mean_it and read_this_much_or_to_eof.

Yes, I noticed that, too, after you brought up read_in_full() as a
potential source of problems. But I would rather deal with
read_in_full() separately on top. Can you do it as a separate patch on
top (possibly with other read_in_full() cleanups, though I think this is
the only "<" case that exists).

-Peff