Re: [PATCH] zlib: properly clamp to uLong

[Junio C Hamano <gitster@pobox.com>]

The original in js/objects-larger-than-4gb-on-windows says things
like:

+	s->z.total_in = (uLong)(s->total_in & ULONG_MAX_VALUE);
+	s->z.total_out = (uLong)(s->total_out & ULONG_MAX_VALUE);

Your patch ...

> +static inline uLong zlib_uLong_cap(size_t s)
> +{
> +	return s < ULONG_MAX_VALUE ? (uLong)s : ULONG_MAX_VALUE;
> +}
> +
>  static void zlib_pre_call(git_zstream *s)
>  {
>  	s->z.next_in = s->next_in;
>  	s->z.next_out = s->next_out;
> -	s->z.total_in = (uLong)(s->total_in & ULONG_MAX_VALUE);
> -	s->z.total_out = (uLong)(s->total_out & ULONG_MAX_VALUE);
> +	s->z.total_in = zlib_uLong_cap(s->total_in);
> +	s->z.total_out = zlib_uLong_cap(s->total_out);

... is an obvious fix for that.

> @@ -60,7 +65,7 @@ static void zlib_post_call(git_zstream *s, int status)
>  	 * We track our own totals and verify only the low bits match.
>  	 */
>  	if ((s->z.total_out & ULONG_MAX_VALUE) !=
> -	    ((s->total_out + bytes_produced) & ULONG_MAX_VALUE))
> +	    ((zlib_uLong_cap(s->total_out) + bytes_produced) & ULONG_MAX_VALUE))
>  		BUG("total_out mismatch");

Because we now clamp (not "taking lower bits of") s->total_out to a
value between 0..4GB and store it in s->z.total_out in pre-call, let
zlib do its thing that increments s->z.total_out modulo 4GB, and we
clamp the s->total_out (before incrementing) the same way in
post_call here, both sides of "!=" above even out.  But the comment
before this comparison that claims that "we ... verify only the low
bits match" is a bit off the reality, I suspect.

> @@ -68,7 +73,7 @@ static void zlib_post_call(git_zstream *s, int status)
>  	 */
>  	if (status != Z_NEED_DICT &&
>  	    (s->z.total_in & ULONG_MAX_VALUE) !=
> -	    ((s->total_in + bytes_consumed) & ULONG_MAX_VALUE))
> +	    ((zlib_uLong_cap(s->total_in) + bytes_consumed) & ULONG_MAX_VALUE))
>  		BUG("total_in mismatch");
>  
>  	s->total_out += bytes_produced;
>
> base-commit: 7a094d68a27e321a99c8ab6b700909e503904bd9