Re: [PATCH v4 11/12] convert: add filter.<driver>.process option

On 06 Aug 2016, at 14:14, Jeff King [off-list ref] wrote:

On Sat, Aug 06, 2016 at 01:55:23PM +0200, Lars Schneider wrote:

quoted

quoted

And I expect it makes the lives of the client
easier to get a code up front, before it starts taking steps to handle
what it _thinks_ is probably a valid response.

I am not sure I can follow you here. Which actor are you referring to when
you write "client" -- Git, right? If the response is rejected right away
then Git just needs to read a single flush. If the response experiences
an error only later, then the filter wouldn't know about the error when
it starts sending. Therefore I don't see how an error code up front could
make it easier for Git.

Yes, I mean git (I see it as the "client" side of the connection in that
it is making requests of the filter, which will then provide responses).

What I mean is that the git code could look something like:

 status == send_filter_request();
 if (status == OK) {
	prepare_storage();
	read_response_into_storage();
 } else {
	complain();
 }

But if there's no status up front, then you probably have:

 send_filter_request();
 prepare_storage();
 status = read_response_into_storage();
 if (status != OK) {
	rollback_storage();
	complain();
 }

In the first case, we could easily avoid preparing the storage if our
request wasn't going to be filled, whereas in the second we have to do
it unconditionally. That's not a big deal if preparing the storage is
initializing a strbuf. It's more so if you're opening a temporary object
file to stream into.

You _do_ still have to deal with rollback in the first one (for the case
that the stream ends prematurely for whatever reason). So it's really a
question of where and how often we expect the failures to come, and
whether it is worth git knowing up front that the request is not going
to be fulfilled.

I dunno. It's not _that_ big a deal to code around. I was just surprised
not to see an up-front status when responding to a request. It seems
like the normal thing in just about every protocol I've ever used.

Alright. The fact that it "surprised" you is a bad sign. 
How about this:

Happy answer:
------------------------
packet:          git< status=accept\n
packet:          git< SMUDGED_CONTENT
packet:          git< 0000
packet:          git< status=success\n
------------------------

Happy answer with no content:
------------------------
packet:          git< status=success\n
------------------------

Rejected content:
------------------------
packet:          git< status=reject\n
------------------------

Error during content response:
------------------------
packet:          git< status=accept\n
packet:          git< HALF_WRITTEN_ERRONEOUS_CONTENT
packet:          git< 0000
packet:          git< status=error\n
------------------------

Cheers,
Lars