Re: [PATCH v2 0/5] Git filter protocol

From: Lars Schneider <larsxschneider@gmail.com>
Date: Jul 28, 2016, 07:16 UTC
Message-ID: <BFED7ED8-40DB-46B5-A1B7-4F49624D5A62@gmail.com>
In-Reply-To: <579906C5.1050809@gmail.com>

Show 37 quoted lines

> On 27 Jul 2016, at 21:08, Jakub Narębski <jnareb@gmail.com> wrote:
> 
> W dniu 2016-07-27 o 02:06, larsxschneider@gmail.com pisze:
>> From: Lars Schneider <larsxschneider@gmail.com>
>> 
>> Hi,
>> 
>> thanks a lot for the extensive reviews. I tried to address all mentioned
>> concerns and summarized them below. The most prominent changes since v1 are
>> the following:
>> * Git offers a number of filter capabilities that a filter can request
>>  (right now only "smudge" and "clean" - in the future maybe "cleanFromFile",
>>  "smudgeToFile", and/or "stream")
>> * pipe communication uses a packet format (pkt-line) based protocol
> 
> I wonder if it would make sense to support both whole-file pipe communication,
> and packet format (pkt-line) pipe communication.
> 
> The problem with whole-file pipe communication (original proposal for
> new filter protocol is that it needs file size upfront.  For some types
> of filters it is not a problem:
> - if a filtered file has the same size as original, like for rot13
>   example in the test for the feature
> - if you can calculate the resulting file size from original size,
>   like for most if not all encryption formats (that includes GPG,
>   uudecode, base64, quoted-printable, hex, etc.); same for decryption,
>   and from converting between fixed-width encodings
> - if resulting file size is saved somewhere that is easy to get, like
>   for LFS solutions (I think).
> 
> For other filters it is serious problem.  For example indent, keyword
> expansion, rezipping with zero compression (well, maybe not this one,
> but at least the reverse of it), converting between encodings where
> at least one is variable width (like UTF-8),...
> 
> IMHO writing whole-file persistent filters is easier than using pkt-line.
> On the other hand using pkt-line allow for more detailed progress report.

I initially wanted to support only "while-file" pipe, too.

But Peff ($gmane/299902), Duy, and Eric, seemed to prefer the pkt-line solution (gmane is down - otherwise I would have given you the links).

After I have looked at it I think the pkt-line solution is indeed nicer for the following reasons:

(1) A stream optimized version (read/write in separate threads) of the filter
    protocol can be implemented in the future without changing the protocol
(2) pkt-line is a simple and easy to implement format
(3) Reuse of existing Git communication infrastructure
    -> code and documentation are less surprising to people that know Git
    -> you can use GIT_TRACE_PACKET to easily inspect the
       communication between Git and the filter process
(4) The overheads is neglect able (4 byte header vs 65516 byte content)

>> * a long running filter application is defined with "filter.<driver>.process"
> 
> I hope that won't confuse Git users into trying to use single-shot
> filters with a new protocol...

Yes, that was my intention for this new config.

Thanks, Lars