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

> -- >8 --
> Subject: [PATCH] config: avoid "write_in_full(fd, buf, len) < len" pattern
> 
> The return type of write_in_full() is a signed ssize_t,
> because we may return "-1" on failure (even if we succeeded
> in writing some bytes). But "len" itself is may be an
> unsigned type (the function takes a size_t, but of course we
> may have something else in the calling function). So while
> it seems like:
>
>   if (write_in_full(fd, buf, len) < len)
> 	die_errno("write error");
>
> would trigger on error, it won't if "len" is unsigned.  The
> compiler sees a signed/unsigned comparison and promotes the
> signed value, resulting in (size_t)-1, the highest possible
> size_t (or again, whatever type the caller has). This cannot
> possibly be smaller than "len", and so the conditional can
> never trigger.
>
> I scoured the code base for cases of this, but it turns out
> that these two in git_config_set_multivar_in_file_gently()
> are the only ones. Here our "len" is the difference between
> two size_t variables, making the result an unsigned size_t.
> We can fix this by just checking for a negative return value
> directly, as write_in_full() will never return any value
> except -1 or the full count.