initial ssh (client) support using awa-ssh

[hannesm]

ssh.ml is mainly a copy of tcp.ml, some adjustment were needed since in the tcp protocol the client first sends a request, while in ssh a command (git-receive-pack or git-upload-pack) is executed (with the path to the repository as argument) which starts by emitting data.

this means that git-receive/upload-pack needs to be passed while the connection is established (as done with ?cmd here), i.e. before there's a FLOW (or NET as it is called here).

[samoht]

That looks great!

Do you think we can avoid duplicating some much code in tcp.ml/ssh.ml. The protocols are mostly similar (apart the message prefixes if I remember correctly) so I hope there is a way to share more code :-)

[hannesm]

@samoht I'd hope so, yes. I don't expect this PR to be merged as is, but would appreciate if someone could think about the code and come up with the right abstractions. please take also the dual PR for conduit/mirage-conduit mirage/ocaml-conduit#296 into consideration.

[hannesm]

How to use this code in an application?

This PR adds git+ssh protocol support to ocaml-git. In order to get a git+ssh client, you'll need as well:

• conduit (basically to handle these ssh:// urls) with this PR
• awa (the actual ssh implementation) from here

For your convenience, the opam repository https://github.com/roburio/git-ssh-dns-mirage3-repo acts as an overlay for git+ssh support (opam repo add git-ssh https://github.com/roburio/git-ssh-dns-mirage3-repo should be all you need in your switch).

As application, the primary DNS unikernel is an example, TL;DR: in config.ml it has two new command-line parameters "seed" and "authenticator", in unikernel.ml it stitches this together into a "HTTP header" (that's what irmin and git and conduit API allowed) see here and passes this to Store.remote. The seed is a random string (use awa_gen_key to generate your own) used to produce the private key, the authenticator should be the SHA256 fingerprint of the remote host. As remote you can then use ssh://my-server/my-repo.git (or ssh://git@github.com/mirage/ocaml-git.git).

For a non-MirageOS application, it is likely that a similar patch, as done for conduit-mirage, has to be applied to conduit-lwt-unix.

[dinosaure]

Close by #395 which integrate properly SSH.