Re: [PATCH v3 12/35] serve: introduce git-serve

From: Jeff King <peff@peff.net>
Date: Feb 22, 2018, 09:33 UTC
Message-ID: <20180222093327.GA12442@sigill.intra.peff.net>
In-Reply-To: <20180207011312.189834-13-bmwill@google.com>

On Tue, Feb 06, 2018 at 05:12:49PM -0800, Brandon Williams wrote:

Show 5 quoted lines

> +In protocol v2 communication is command oriented.  When first contacting a
> +server a list of capabilities will advertised.  Some of these capabilities
> +will be commands which a client can request be executed.  Once a command
> +has completed, a client can reuse the connection and request that other
> +commands be executed.

If I understand this correctly, we'll potentially have a lot more round-trips between the client and server (one per "command"). And for git-over-http, each one will be its own HTTP request?

We've traditionally tried to minimize HTTP requests, but I guess it's not too bad if we can keep the connection open in most cases. Then we just suffer some extra framing bytes, but we don't have to re-establish the TCP connection each time.

I do wonder if the extra round trips will be noticeable in high-latency conditions. E.g., if I'm 200ms away, converting the current ref-advertisement spew to "capabilities, then the client asks for refs, then we spew the refs" is going to cost an extra 200ms, even if the fetch just ends up being a noop. I'm not sure how bad that is in the grand scheme of things (after all, the TCP handshake involves some round-trips, too).

Show 35 quoted lines

> + Capability Advertisement
> +--------------------------
> +
> +A server which decides to communicate (based on a request from a client)
> +using protocol version 2, notifies the client by sending a version string
> +in its initial response followed by an advertisement of its capabilities.
> +Each capability is a key with an optional value.  Clients must ignore all
> +unknown keys.  Semantics of unknown values are left to the definition of
> +each key.  Some capabilities will describe commands which can be requested
> +to be executed by the client.
> +
> +    capability-advertisement = protocol-version
> +			       capability-list
> +			       flush-pkt
> +
> +    protocol-version = PKT-LINE("version 2" LF)
> +    capability-list = *capability
> +    capability = PKT-LINE(key[=value] LF)
> +
> +    key = 1*CHAR
> +    value = 1*CHAR
> +    CHAR = 1*(ALPHA / DIGIT / "-" / "_")
> +
> +A client then responds to select the command it wants with any particular
> +capabilities or arguments.  There is then an optional section where the
> +client can provide any command specific parameters or queries.
> +
> +    command-request = command
> +		      capability-list
> +		      (command-args)
> +		      flush-pkt
> +    command = PKT-LINE("command=" key LF)
> +    command-args = delim-pkt
> +		   *arg
> +    arg = 1*CHAR

For a single stateful TCP connection like git:// or git-over-ssh, the client would get the capabilities once and then issue a series of commands. For git-over-http, how does it work?

The client speaks first in HTTP, so we'd first make a request to get just the capabilities from the server? And then proceed from there with a series of requests, assuming that the capabilities for each server we subsequently contact are the same? That's probably reasonable (and certainly the existing http protocol makes that capabilities assumption).

I don't see any documentation on how this all works with http. But reading patch 34, it looks like we just do the usual service=git-upload-pack request (with the magic request for v2), and then the server would send us capabilities. Which follows my line of thinking in the paragraph above.

-Peff