Re: [RFC] multi_ack protocol v2

On Thu, 27 Oct 2005 00:13:17 -0700 Junio C Hamano wrote:

[skip]

So let's illustrate the v2 the same way as I understand it.

upload-pack (S) | fetch/clone-pack (C) protocol (v2):

	# Tell the puller what commits we have and what their names are
	S: SHA1 name
	S: ...
	S: SHA1 name
	S: # flush -- it's your turn
	# Tell the pusher what commits we want, and what we have.
        # In addition, we tell the other end that we support protocol
	# extensions, without breaking the old servers.
	C: want SHA1 extended
	C: ..
	C: want SHA1
	C: # flush -- done with "want" lines.

Notice that until we hear from the server, we cannot tell if our
"extended" protocol wish will be granted, and in the original
protocol, "NAK" will come in fixed length, and the only thing we
could tack arbitrary garbage to was "ACK SHA1".  That's why your
"ACK SHA1 continue" works nicely, but at the time you could not
find out if you are talking with updated server until you get at
least one ACK.

Actually, there is another way to pass some data from the server
which would be ignored by older clients - at the first stage,
when upload-pack sends the list of refs to the client:

	packet_write(1, "%s %s%c%s\n", sha1_to_hex(sha1), refname, '\0',
		     server_capabilities);

Old clients will ignore the additional data (functions which
work with the received name will happily stop at the added NUL
character; just some memory will be wasted), but the new
implementation of get_remote_heads() could look for that NUL
inside the received packet and find extended server capabilities
after it.  Then the client could just use new commands at the
"want" stage.

However, at this point, we *could* force the server to reveal
what it supports, by doing an extra flush here, before sending
*ANY* "have" lines yet:

	C: # flush -- this is another one after "I'm done with wants".

Upon receiving this, if we were talking with an old upload-pack,
we would certanly get an NAK.  Note that the server already
knows that we support extended protocol at this point, so our
updated server can send anything here to say it knows what
protocol extensions it supports.  Let's say it says something
like this:

	S: proto v2 v3 v5

to tell the puller it understands protocol v2, v3, and v5, to
which the puller responds:

	C: proto v2

After this exchange, both ends know they understand and would
want to talk at protocol level v2.  This leaves door open for
future protocol extension, but more importantly, I think this
arrangement would make things safer.

This looks cleaner (no NUL bytes in the protocol), but does not
allow to replace the "want" stage with something entirely
different (if we ever want to do that).