Re: [PATCH] pack-write: use hashwrite_be32() instead of double-buffering array

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

Jeff King <peff@peff.net> writes:

> On Sun, Nov 01, 2020 at 09:52:12AM +0100, René Scharfe wrote:
>
>> hashwrite() already buffers writes, so pass the fanout table entries
>> individually via hashwrite_be32(), which also does the endianess
>> conversion for us.  This avoids a memory copy, shortens the code and
>> reduces the number of magic numbers.
>
> Yep, this seems trivially correct. The key observation is that we are
> filling the array in order:
>
>> @@ -101,20 +100,19 @@ const char *write_idx_file(const char *index_name, struct pack_idx_entry **objec
>>  	for (i = 0; i < 256; i++) {
>>  		struct pack_idx_entry **next = list;
>>  		while (next < last) {
>>  			struct pack_idx_entry *obj = *next;
>>  			if (obj->oid.hash[0] != i)
>>  				break;
>>  			next++;
>>  		}
>> -		array[i] = htonl(next - sorted_by_sha);
>> +		hashwrite_be32(f, next - sorted_by_sha);
>>  		list = next;
>>  	}
>> -	hashwrite(f, array, 256 * 4);
>
> Perhaps obvious, but I got bit trying to do another similar conversion
> recently that was filling in the array out-of-order...

Yeah, filling an array out of order and then writing the result in
order would obviously different from writing out individual pieces
in the order the original codeflow used to fill the array.  Worse,
the order the data items are fed to hashwrite() obviously affects
the hash computed at the end.  An example of too much abstraction
biting us? ;-)