Hello.
I recently discovered that offloading builds to remote machines[1],
hangs when the machines are not available; instead of defaulting to
building locally[2]. This forces the user to use the `--no-offload`
flag.
I saw in the mailing list[2] that someone suggested that, the
`build-machines` field accepts a list of GEXPs instead of plain
`build-machine` record types.
This suggestion is almost correct but it only checks if the host is
known which does not guarantee that it is reachable. Therefore I came up
with this:
--8<---------------cut here---------------start------------->8---
(build-machines
(list
#~(let* ((resolvable? (lambda (machine)
(zero? (system* #$(file-append netcat "/bin/nc")
"-z" "-w1"
(build-machine-name machine) "22")))))
(filter resolvable?
(list (build-machine
(name "my-host")
(systems (list "x86_64-linux" "i686-linux"))
;; NOTE: Located in '/etc/ssh/ssh_host_ed25519_key.pub' on
the machine that does the build.
;; It will be generated by `openssh-service-type'.
(host-key #$(plain-file-content %my-host-host-key))
;; NOTE: User on the build machine that allows SSH access
with the key from `private-key' field.
(user "my-host-user1")
(private-key "/home/user1/.ssh/id_ed25519")))))))
--8<---------------cut here---------------end--------------->8---
Which allows to dynamically detect which machines are reachable.
If the user wanted to never build locally, the `-M 0` flag can be
used. Therefore, I would expect that it would graciously fallback to
building locally instead of getting stuck. If this is the desired
behaviour, I think we should document how to avoid the hanging.
Should we add this snippet to the manual/cookbook?
[1] https://guix.gnu.org/manual/en/html_node/Daemon-Offload-Setup.html
[2] https://lists.gnu.org/archive/html/help-guix/2023-12/msg00114.html
[3] https://lists.gnu.org/archive/html/help-guix/2023-12/msg00120.html
Regards,
Sergio.
