Re: [PATCH v8 07/11] pkt-line: add functions to read/write flush terminated packet streams

[Lars Schneider <larsxschneider@gmail.com>]

> On 27 Sep 2016, at 11:00, Jeff King <peff@peff.net> wrote:
> 
> On Tue, Sep 27, 2016 at 10:14:16AM +0200, Lars Schneider wrote:
> 
>>>>> +		strbuf_grow(sb_out, PKTLINE_DATA_MAXLEN+1);
>>>>> +		paket_len = packet_read(fd_in, NULL, NULL,
>>>>> +			sb_out->buf + sb_out->len, PKTLINE_DATA_MAXLEN+1, options);
>> [...]
>> After looking at it with fresh eyes I think the existing code is probably correct,
>> but maybe a bit confusing.
>> 
>> packet_read() adds a '\0' at the end of the destination buffer:
>> https://github.com/git/git/blob/21f862b498925194f8f1ebe8203b7a7df756555b/pkt-line.c#L206
>> 
>> That is why the destination buffer needs to be one byte larger than the expected content.
>> 
>> However, in this particular case that wouldn't be necessary because the destination
>> buffer is a 'strbuf' that allocates an extra byte for '\0' at the end. But we are not
>> supposed to write to this extra byte:
>> https://github.com/git/git/blob/21f862b498925194f8f1ebe8203b7a7df756555b/strbuf.h#L25-L31
> 
> Right. The allocation happens as part of strbuf_grow(), but whatever
> fills the buffer is expected to write the actual NUL (either manually,
> or by calling strbuf_setlen().
> 
> I see the bit you quoted warns not to touch the extra byte yourself,
> though I wonder if that is a bit heavy-handed (I guess it would matter
> if we moved the extra 1-byte growth into strbuf_setlen(), but I find
> that a rather unlikely change).
> 
> That being said, why don't you just use LARGE_PACKET_MAX here? It is
> already the accepted size for feeding to packet_read(), and we know it
> has enough space to hold a NUL terminator. Yes, we may over-allocate by
> 4 bytes, but that isn't really relevant. Strbufs over-allocate anyway.

TBH in that case I would prefer the "PKTLINE_DATA_MAXLEN+1" solution with
an additional comment explaining "+1".

Would that be OK for you?

I am not worried about the extra 4 bytes. I am worried that we make it harder
to see what is going on if we use LARGE_PACKET_MAX.


> So just:
> 
>  for (;;) {
>        strbuf_grow(sb_out, LARGE_PACKET_MAX);
>        packet_len = packet_read(fd_in, NULL, NULL,
> 	                         sb_out->buf + sb_out->len, LARGE_PACKET_MAX,
> 				 options);
>        if (packet_len <= 0)
>                break;
>        /*
>         * no need for strbuf_setlen() here; packet_read always adds a
>         * NUL terminator.
>         */
>        sb_out->len += packet_len;
>  }
> 
> You _could_ make the final line of the loop use strbuf_setlen(); it
> would just overwrite something we already know is a NUL (and we know
> that no extra allocation is necessary).
> 
> Also, using LARGE_PACKET_MAX fixes the fact that this patch is using
> PKTLINE_DATA_MAXLEN before it is actually defined. :)

Yeah, I noticed that too and fixed it in v9 :-) Thanks for the reminder!


> You might want to occasionally run:
> 
>  git rebase -x make
> 
> to make sure all of your incremental steps are valid (or even "make
> test" if you are extremely patient; I often do that once after a big
> round of complex interactive-rebase reordering).

That is a good suggestion. I'll add that to my "tool-belt" :-)


Thanks,
Lars