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

+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).

+ 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