zlib: properly clamp to uLong

On platforms where `unsigned long` and `size_t` differ in bit size, we
want to clamp the buffers we pass to zlib to the former's size, as per
d05d666 (git-zlib: handle data streams larger than 4GB, 2026-05-08).

The logic introduced in that commit performs a clamping to the bits,
though, which fails to do what is needed here: If too many bytes are
available in the buffers, we need to clamp to the maximum value of an
`unsigned long`. Otherwise, we ask zlib to use too small buffers, in the
worst case using 0 as the size (think: a value whose 32 lowest bits are
all zero).

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

Author: dscho

git-zlib.c

@@ -38,12 +38,17 @@ static inline uInt zlib_buf_cap(unsigned long len)
 	return (ZLIB_BUF_MAX < len) ? ZLIB_BUF_MAX : len;
 }
 
+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);
 	s->z.avail_in = zlib_buf_cap(s->avail_in);
 	s->z.avail_out = zlib_buf_cap(s->avail_out);
 }
@@ -60,15 +65,15 @@ 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");
 	/*
 	 * zlib does not update total_in when it returns Z_NEED_DICT,
 	 * causing a mismatch here. Skip the sanity check in that case.
 	 */
 	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;