building "guix deploy"

[Thompson, David]

Hello everyone!

Chris Webber and I spent the morning chatting about how we want to
approach making "guix deploy" a reality and then started hacking on it
in the afternoon.  Although we weren't able to complete a working
prototype by the end of the day, we were able to get pretty close.  We
created a 'guix deploy' CLI to build derivations for any number of
remote systems on a local workstation and initiate the transfer to the
remote systems, but we encountered a difficult to debug SSH error that
blocked our progress:

sending 85 store items (0 MiB) to 'test.activitypub.rocks'...
exporting path 
`/gnu/store/ylcdmrj3vf00ixdcjvkl3mbs8f5i9w8l-git-minimal-2.20.1.drv'
;;; [2019/03/09 17:32:48.792589, 0] write_to_channel_port: [GSSH
ERROR] Remote channel is closed: #<input-output: channel (open)
541a220>
Backtrace:
          10 (apply-smob/1 #<catch-closure a26900>)
In ice-9/boot-9.scm:
    705:2  9 (call-with-prompt _ _ #<procedure default-prompt-handle…>)
In ice-9/eval.scm:
    619:8  8 (_ #(#(#<directory (guile-user) ab1140>)))
In guix/ui.scm:
  1654:12  7 (run-guix-command _ . _)
In guix/scripts/deploy.scm:
     72:8  6 (guix-deploy . _)
In srfi/srfi-1.scm:
    640:9  5 (for-each #<procedure 4e5c940 at guix/scripts/deploy.s…> …)
In guix/scripts/deploy.scm:
    74:20  4 (_ _)
In gnu/machine.scm:
    58:22  3 (_ _ _ _)
In guix/ssh.scm:
    313:4  2 (send-files #<store-connection 256.99 48000f0> _ _ # _ # …)
In guix/store.scm:
   1504:7  1 (export-paths #<store-connection 256.99 48000f0> _ #<i…> …)
In unknown file:
           0 (put-bytevector #<input-output: channel (open) 541a220> …)

ERROR: In procedure put-bytevector:
Throw to key `guile-ssh-error' with args `("write_to_channel_port"
"Remote channel is closed" #<input-output: channel (open) 541a220>
#f)'.

If anyone knows what might be going on here and how we could resolve
it, your input would be much appreciated!  We verified via the sshd
logs that we were indeed successfully establishing a connection.

Once we're past this blocking issue and are able to transfer OS
closures to remote systems, we plan to write a modified version of
switch-to-system that uses guile-ssh to switch remote symlinks for the
active system and run the activation script.  We'll save
upgrade-shepherd-services for later, as it is quite a bit more
complex.

There's not a lot of code yet, but you can check it out in the
wip-deploy2 branch.  Currently, the only supported use-case is running
the equivalent of 'guix system reconfigure' on machines already
running GuixSD that have an OpenSSH daemon running, but the basic
design allows for additional use-cases to be supported in the future.

Thanks to Ricardo for his install-berlin.scm script that provided a
basis for most of our code, and to Ludovic for the (guix ssh) module
that I wish I had when I attempted to write 'guix deploy' all those
years ago.

Until next time...

- Dave