Re: Designing the filter process protocol (was: Re: [PATCH v3 10/10] convert: add filter.<driver>.process option)

[Lars Schneider <larsxschneider@gmail.com>]

> On 03 Aug 2016, at 20:30, Jakub Narębski <jnareb@gmail.com> wrote:
> 
> ...
> 
> 
> 2. HANDSHAKE (INITIALIZATION)
> 
> Next, there is deciding on and designing the handshake between Git (between
> Git command) and the filter driver process.  With the `filter.<driver>.process`
> solution the driver needs to tell which operations among (for now) "clean"
> and "smudge" it does support.  Plus it provides a way to extend protocol,
> adding new features, like support for streaming, cleaning from file or
> smudging to file, providing size upfront, perhaps even progress report.
> 
> Current handshake consist of filter driver printing a signature, version
> number and capabilities, in that order.  Git checks that it is well formed
> and matches expectations, and notes which of "clean" and "smudge" operations
> are supported by the filter.
> 
> There is no interaction from the Git side in the handshake, for example to
> set options and expectations common to all files being filtered.  Take
> one possible extension of protocol: supporting streaming.  The filter
> driver needs to know whether it needs to read all the input, or whether
> it can start printing output while input is incoming (e.g. to reduce
> memory consumption)... though we may simply decide it to be next version
> of the protocol.

I would like to change the startup sequence to this:

Git starts the filter when it encounters the first file
that needs to be cleaned or smudged. After the filter started
Git sends a welcome message, a list of supported protocol
version numbers, and a flush packet. Git expects to read the
welcome message and one protocol version number from the
previously sent list. Afterwards Git sends a list of supported
capabilities and a flush packet. Git expects to read a list of
desired capabilities, which must be a subset of the supported
capabilities list, and a flush packet as response:
------------------------
packet:          git> git-filter-client
packet:          git> version=2
packet:          git> version=42
packet:          git> 0000
packet:          git< git-filter-server
packet:          git< version=2
packet:          git> clean=true
packet:          git> smudge=true
packet:          git> not-yet-invented=true
packet:          git> 0000
packet:          git< clean=true
packet:          git< smudge=true
packet:          git< 0000
------------------------

This would allow us to detect the case if a user configures an
existing clean/smudge filter as `filter.<driver>.process`.
Since Git is talking first, it would not "hang" in that case.

Would that be ok with you?

Thanks,
Lars