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

On 29 Jul 2016, at 18:50, Jeff King [off-list ref] wrote:

On Fri, Jul 29, 2016 at 06:20:51PM +0200, Lars Schneider wrote:

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That being said a "fail" response is a very good idea! This allows
the filter to communicate to git that a non required filter process
failed. I will add that to the protocol. Thanks :) 

Maybe just send "ok <size>", "ok -1" (for streaming), or "fail <reason>"
followed by the content? That is similar to other Git protocols, though
I am not sure they are good models for sanity or extensibility. :)

I don't know if you would want to leave room for other "headers" in the
response, but you could also do something more HTTP-like, with a status
code, and arbitrary headers. And presumably git would just ignore
headers it doesn't know about. I think that's what Jakub's example was
leaning towards. I'm just not sure what other headers are really useful,
but it does leave room for extensibility.

Well, "ok <size>" wouldn't make much sense as we already transmitted
the size upfront I think. Right now I have implemented the following options:

Maybe I'm confused about where in the protocol we are. I was imagining:

 git> smudge
 git> <filename>
 git> <size>
 git> ...pkt-lines...
 git> pktline-flush

 git< ok <size>
 git< ...pkt-lines...
 git< flush

That is, we should say "I have something for you" or "I do not" before
sending a size, because in the "I do not" case we have no size to send.

Right now the protocol is like that in the happy case (non-streaming):

git> smudge
git> <filename>
git> <size>
git> ...pkt-lines...
git> pktline-flush

git< <size>
git< ...pkt-lines...
git< flush
git< success

(diff to your version: no "ok" in front of size answer ... plus the
size answer is not present in the streaming case)


Here is the reject case (non-streaming):

git> smudge
git> <filename>
git> <size>
git> ...pkt-lines...
git> pktline-flush

git< 0
git< reject


Do you see a problem with this approach?

A more extensible protocol might look like:

 git> smudge
 git> filename=<filename>
 git> size=<size>
 git> pktline-flush
 git> ...pkt-lines of data...
 git> pktline-flush

 git< ok (or success, or whatever status code you like)
 git< size=<size>
 git< pkt-line-flush
 git< ...pkt-lines of data...
 git< pktline-flush

That leaves room for new "keys" to be added before the first pkt-flush,
without having to change the parsing at all.

Alright. Will be in v3!

quoted

"success\n" --> everything was alright
"reject\n" --> the filter rejected the operation but this is no error 
              if "filter.<driver>.required = false"
<anything else> --> failure that stops/restarts the filter process

I don't think sending any failure reason makes sense because if a failure
happens then we are likely in a bad state already (that's why I restart the
filter process. I think the filter can report trouble on its own via stdout,
no? I think this is what Git-LFS already does.

Git-LFS sends to stderr because there's no other option. I wonder if it
would be nicer to make it Git's responsibility to talk to the user,
because then it could respect things like "--quiet". I guess error
messages are generally printed regardless of verbosity, though, so
printing them unconditionally is OK.

OK!

Thanks,
Lars