[RFC 0/7] transitioning to protocol v2

Another version of Git's wire protocol is a topic that has been discussed and attempted by many in the community over the years. The biggest challenge, as far as I understand, has been coming up with a transition plan to using the new server without breaking existing clients and servers. As such this RFC is really only concerned with solidifying a transition plan. Once it has been decided how we can transition to a new protocol we can get into decided what this new protocol would look like (though it would obviously eliminate the ref advertisement ;).

The best way to preserve functionality with old servers and clients would be to communicate using the same end point but have the client send a bit of extra information with its initial request. This extra information would need to be sent in such a way that old servers ignore it and operate normally (using protocol v1). The client would then need to be able to look at a server's response to determine whether the server understands and is speaking v2 or has ignored the clients request to use a newer protocol and is speaking v1.

Patches 1-5 enable a client to unconditionally send this back-channel information to a server. This is done by sending a version number after a second NUL byte in git://, in the envvar GIT_PROTOCOL in file:// and ssh://, and in an http header in http://, https://. Patches 6-7 teach a client and upload-pack to send and recognize a request to use protocol v2.

The biggest question I'm trying to answer is if these are reasonable ways with which to communicate a request to a server to use a newer protocol, without breaking current servers/clients. As far as I've tested, with patches 1-5 applied I can still communicate with current servers without causing any problems.

Any comments/discussion is welcome!

Brandon Williams (7):
  pkt-line: add packet_write function
  pkt-line: add strbuf_packet_read
  protocol: tell server that the client understands v2
  t: fix ssh tests to cope with using '-o SendEnv=GIT_PROTOCOL'
  http: send Git-Protocol-Version header
  transport: teach client to recognize v2 server response
  upload-pack: ack version 2

 builtin/fetch-pack.c         |   4 +-
 builtin/send-pack.c          |   5 +-
 connect.c                    | 196 +++++++++++++++++++++++++++----------------
 daemon.c                     |  28 ++++++-
 http.c                       |   7 ++
 pkt-line.c                   |  27 ++++++
 pkt-line.h                   |   2 +
 remote-curl.c                |   7 +-
 remote.h                     |  22 ++++-
 t/lib-proto-disable.sh       |   1 +
 t/t5551-http-fetch-smart.sh  |   2 +
 t/t5601-clone.sh             |  10 +--
 t/t5602-clone-remote-exec.sh |   4 +-
 transport.c                  |  60 +++++++++++--
 upload-pack.c                |  11 +++
 15 files changed, 286 insertions(+), 100 deletions(-)