Re: [PATCH v2] config: avoid "write_in_full(fd, buf, len) < len" pattern

[Jeff King <peff@peff.net>]

On Wed, Sep 13, 2017 at 11:24:31AM -0700, Jonathan Nieder wrote:

> For what it's worth,
> Reviewed-by: Jonathan Nieder <jrnieder@gmail.com>

Thanks, and thank you for questioning the ptrdiff_t bits that didn't
make sense. I _thought_ they felt like nonsense, but couldn't quite
puzzle it out.

> Compilers' signed/unsigned comparison warning can be noisy, but I'm
> starting to feel it's worth the suppression noise to turn it on when
> DEVELOPER=1 anyway.  What do you think?  Is there a way to turn it on
> selectively for certain functions on the LHS (like read() and write()
> style functions)?

Obviously we couldn't turn them on for -Werror at this point. Let me see
how bad "-Wsign-compare -Wno-error=sign-compare" is.

  $ make 2>&1 | grep -c warning:
  1391

Ouch. I'm afraid that's probably not going to be very helpful. Even if I
were introducing a new problem, I'm not likely to see it amidst the mass
of existing complaints.

AFAIK there's no way to turn it on for specific functions, but I'm far
from a gcc-warning guru. Even if you could, though, the error may be far
from the function (e.g., if we store the result in an ssize_t and then
compare that with a size_t).

Interestingly in the write_in_full() case the code was actually
unreachable! But I don't think the compiler is smart enough to know
that, because it would have to realize that write_in_full() can only
return either -1 or the original value (and not a positive value in
between). Coverity _might_ be smart enough, but figuring that out does
take some loop analysis (including the assumption that write() never
returns more bytes than we asked it to write).

-Peff