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From: Lars Schneider <larsxschneider@gmail.com>
To: Jeff King <peff@peff.net>
Cc: Git Mailing List <git@vger.kernel.org>,
	gitster@pobox.com, jnareb@gmail.com, tboegi@web.de,
	mlbright@gmail.com, remi.galan-alfonso@ensimag.grenoble-inp.fr,
	pclouds@gmail.com, e@80x24.org, ramsay@ramsayjones.plus.com
Subject: Re: [PATCH v2 5/5] convert: add filter.<driver>.process option
Date: Thu, 28 Jul 2016 14:10:12 +0200	[thread overview]
Message-ID: <9AB58AFB-7533-4897-8497-187C6D1239C8@gmail.com> (raw)
In-Reply-To: <20160727181148.GC32219@sigill.intra.peff.net>


> On 27 Jul 2016, at 20:11, Jeff King <peff@peff.net> wrote:
> 
> On Wed, Jul 27, 2016 at 07:31:26PM +0200, Lars Schneider wrote:
> 
>>>> +	strbuf_grow(sb, size + 1);	// we need one extra byte for the packet flush
>>> 
>>> What happens if size is the maximum for size_t here (i.e., 4GB-1 on a
>>> 32-bit system)?
>> 
>> Would that be an acceptable solution?
>> 
>> if (size + 1 > SIZE_MAX)
>> 	return die("unrepresentable length for filter buffer");
> 
> No, because by definition "size" will wrap to 0. :)
> 
> You have to do:
> 
>  if (size > SIZE_MAX - 1)
> 	die("whoops");
> 
>> Can you point me to an example in the Git source how this kind of thing should
>> be handled?
> 
> The strbuf code itself checks for overflows. So you could do:
> 
>  strbuf_grow(sb, size);
>  ... fill up with size bytes ...
>  strbuf_addch(sb, ...); /* extra byte for whatever */
> 
> That does mean _possibly_ making a second allocation just to add the
> extra byte, but in practice it's not likely (unless the input exactly
> matches the strbuf's growth pattern).
> 
> If you want to do it yourself, I think:
> 
>  strbuf_grow(sb, st_add(size, 1));

I like that solution! Thanks!


> would work.
> 
>>>> +	while (
>>>> +		bytes_read > 0 && 					// the last packet was no flush
>>>> +		sb->len - total_bytes_read - 1 > 0 	// we still have space left in the buffer
>>>> +	);
>>> 
>>> And I'm not sure if you need to distinguish between "0" and "-1" when
>>> checking byte_read here.
>> 
>> We want to finish reading in both cases, no?
> 
> If we get "-1", that's from an unexpected EOF during the packet_read(),
> because you set GENTLE_ON_EOF. So there's nothing left to read, and we
> should break and return an error.

Right.


> I guess "0" would come from a flush packet? Why would the filter send
> back a flush packet (unless you were using them to signal end-of-input,
> but then why does the filter have to send back the number of bytes ahead
> of time?).

Sending the bytes ahead of time (if available) might be nice for efficient
buffer allocation. I am changing the code so that both cases can be handled
(size ahead of time and no size ahead of time).


>>> Why 8K? The pkt-line format naturally restricts us to just under 64K, so
>>> why not take advantage of that and minimize the framing overhead for
>>> large data?
>> 
>> I took inspiration from here for 8K MAX_IO_SIZE:
>> https://github.com/git/git/blob/master/copy.c#L6
>> 
>> Is this read limit correct? Should I read 8 times to fill a pkt-line?
> 
> MAX_IO_SIZE is generally 8 _megabytes_, not 8K. The loop in copy.c just
> haad to pick an arbitrary size for doing its read/write proxying.  I
> think in practice you are not likely to get much benefit from going
> beyond 8K or so there, just because operating systems tend to do things
> in page-sizes anyway, which are usually 4K.
> 
> But since you are formatting the result into a form that has framing
> overhead, anything up to LARGE_PACKET_MAX will see benefits (though
> admittedly even 4 bytes per 8K is not much).
> 
> I don't think it's worth the complexity of reading 8 times, but just
> using a buffer size of LARGE_PACKET_MAX-4 would be the most efficient.
> 
> I doubt it matters _that much_ in practice, but any time I see a magic
> number I have to wonder at the "why". At least basing it on
> LARGE_PACKET_MAX has some basis, whereas 8K is largely just made-up. :)

Sounds good. I will use LARGE_PACKET_MAX-4 !

> 
>>> We do sometimes do "ret |= something()" but that is in cases where
>>> "ret" is zero for success, and non-zero (usually -1) otherwise. Perhaps
>>> your function's error-reporting is inverted from our usual style?
>> 
>> I thought it makes the code easier to read and the filter doesn't care
>> at what point the error happens anyways. The filter either succeeds
>> or fails. What style would you suggest?
> 
> I think that's orthogonal. I just mean that using zero for success puts
> you in our usual style, and then accumulating errors can be done with
> "|=".

Ah. I guess I was misguided by the way errors are currently handled
in `apply_filter` (success = 1; failure = 0):
https://github.com/git/git/blob/8c6d1f9807c67532e7fb545a944b064faff0f70b/convert.c#L437-L479

I wouldn't like if the different filter protocols would use different
error exit codes. Would it be OK to adjust the existing `apply_filter`
function in a cleanup patch?


> I didn't look carefully at whether the accumulating style you're using
> makes sense or not. But something like:
> 
>>>> +		ret &= write_in_full(out, &header, sizeof(header)) == sizeof(header);
>>>> +		ret &= write_in_full(out, src, bytes_to_write) == bytes_to_write;
> 
> does mean that we call the second write() even if the first one failed.
> That's a waste of time (albeit a minor one), but it also means you could
> potentially cover up the value of "errno" from the first one (though in
> practice I'd expect the second one to fail for the same reason).

Oh. You're right. For some reason I thought the second operator would
never be evaluated if the first operator is 0. Apparently that is not
the case for bit-wise & ... only for logical & ... thanks for the lesson!

- Lars

  reply	other threads:[~2016-07-28 12:10 UTC|newest]

Thread overview: 77+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2016-07-22 15:48 [PATCH v1 0/3] Git filter protocol larsxschneider
2016-07-22 15:48 ` [PATCH v1 1/3] convert: quote filter names in error messages larsxschneider
2016-07-22 15:48 ` [PATCH v1 2/3] convert: modernize tests larsxschneider
2016-07-26 15:18   ` Remi Galan Alfonso
2016-07-26 20:40     ` Junio C Hamano
2016-07-22 15:49 ` [PATCH v1 3/3] convert: add filter.<driver>.useProtocol option larsxschneider
2016-07-22 22:32   ` Torsten Bögershausen
2016-07-24 12:09     ` Lars Schneider
2016-07-22 23:19   ` Ramsay Jones
2016-07-22 23:28     ` Ramsay Jones
2016-07-24 17:16     ` Lars Schneider
2016-07-24 22:36       ` Ramsay Jones
2016-07-24 23:22         ` Jakub Narębski
2016-07-25 20:32           ` Lars Schneider
2016-07-26 10:58             ` Jakub Narębski
2016-07-25 20:24         ` Lars Schneider
2016-07-23  0:11   ` Jakub Narębski
2016-07-23  7:27     ` Eric Wong
2016-07-26 20:00       ` Jeff King
2016-07-24 18:36     ` Lars Schneider
2016-07-24 20:14       ` Jakub Narębski
2016-07-24 21:30         ` Jakub Narębski
2016-07-25 20:16           ` Lars Schneider
2016-07-26 12:24             ` Jakub Narębski
2016-07-25 20:09         ` Lars Schneider
2016-07-26 14:18           ` Jakub Narębski
2016-07-23  8:14   ` Eric Wong
2016-07-24 19:11     ` Lars Schneider
2016-07-25  7:27       ` Eric Wong
2016-07-25 15:48       ` Duy Nguyen
2016-07-22 21:39 ` [PATCH v1 0/3] Git filter protocol Junio C Hamano
2016-07-24 11:24   ` Lars Schneider
2016-07-26 20:11     ` Jeff King
2016-07-27  0:06 ` [PATCH v2 0/5] " larsxschneider
2016-07-27  0:06   ` [PATCH v2 1/5] convert: quote filter names in error messages larsxschneider
2016-07-27 20:01     ` Jakub Narębski
2016-07-28  8:23       ` Lars Schneider
2016-07-27  0:06   ` [PATCH v2 2/5] convert: modernize tests larsxschneider
2016-07-27  0:06   ` [PATCH v2 3/5] pkt-line: extract and use `set_packet_header` function larsxschneider
2016-07-27  0:20     ` Junio C Hamano
2016-07-27  9:13       ` Lars Schneider
2016-07-27 16:31         ` Junio C Hamano
2016-07-27  0:06   ` [PATCH v2 4/5] convert: generate large test files only once larsxschneider
2016-07-27  2:35     ` Torsten Bögershausen
2016-07-27 13:32       ` Jeff King
2016-07-27 16:50         ` Lars Schneider
2016-07-27  0:06   ` [PATCH v2 5/5] convert: add filter.<driver>.process option larsxschneider
2016-07-27  1:32     ` Jeff King
2016-07-27 17:31       ` Lars Schneider
2016-07-27 18:11         ` Jeff King
2016-07-28 12:10           ` Lars Schneider [this message]
2016-07-28 13:35             ` Jeff King
2016-07-27  9:41     ` Eric Wong
2016-07-29 10:38       ` Lars Schneider
2016-07-29 11:24         ` Jakub Narębski
2016-07-29 11:31           ` Lars Schneider
2016-08-05 18:55         ` Eric Wong
2016-08-05 23:26           ` Lars Schneider
2016-08-05 23:38             ` Eric Wong
2016-07-27 23:31     ` Jakub Narębski
2016-07-29  8:04       ` Lars Schneider
2016-07-29 17:35         ` Junio C Hamano
2016-07-29 23:11           ` Jakub Narębski
2016-07-29 23:44             ` Lars Schneider
2016-07-30  9:32               ` Jakub Narębski
2016-07-28 10:32     ` Torsten Bögershausen
2016-07-27 19:08   ` [PATCH v2 0/5] Git filter protocol Jakub Narębski
2016-07-28  7:16     ` Lars Schneider
2016-07-28 10:42       ` Jakub Narębski
2016-07-28 13:29       ` Jeff King
2016-07-29  7:40         ` Jakub Narębski
2016-07-29  8:14           ` Lars Schneider
2016-07-29 15:57             ` Jeff King
2016-07-29 16:20               ` Lars Schneider
2016-07-29 16:50                 ` Jeff King
2016-07-29 17:43                   ` Lars Schneider
2016-07-29 18:27                     ` Jeff King

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