Tonight I’m provisioning a new virtual machine at Hetzner and I wanted to share how Consfigurator is helping with that. Hetzner have a Debian “buster” image you can start with, as you’d expect, but it comes with things like cloud-init, preconfiguration to use Hetzner’s apt mirror which doesn’t serve source packages(!), and perhaps other things I haven’t discovered. It’s a fine place to begin, but I want all the configuration for this server to be explicit in my Consfigurator consfig, so it is good to start with pristine upstream Debian. I could boot one of Hetzner’s installation ISOs but that’s slow and manual. Consfigurator can replace the OS in the VM’s root filesystem and reboot for me, and we’re ready to go.

Here’s the configuration:

(defhost (:deploy ((:ssh :user "root") :sbcl))
  (os:debian-stable "buster" :amd64)

  ;; Hetzner's Debian 10 image comes with a three-partition layout and boots
  ;; with traditional BIOS.
    :device-file "/dev/sda" :boots-with '(grub:grub :target "i386-pc")))

  (on-change (installer:cleanly-installed-once
              ;; This is a specification of the OS Hetzner's image has, so
              ;; Consfigurator knows how to install SBCL and debootstrap(8).
              ;; In this case it's the same Debian release as the replacement.
              '(os:debian-stable "buster" :amd64))

    ;; Clear out the old OS's EFI system partition contents, in case we can
    ;; switch to booting with EFI at some point (if we wanted we could specify
    ;; an additional x86_64-efi target above, and grub-install would get run
    ;; to repopulate /boot/efi, but I don't think Hetzner can boot from it yet).
    (file:directory-does-not-exist "/boot/efi/EFI")

    (apt:installed "linux-image-amd64")

       (mounted-ext4-filesystem :mount-point "/")
         :mount-options '("umask=0077") :mount-point "/boot/efi"))))
    (file:lacks-lines "/etc/fstab" "# UNCONFIGURED FSTAB FOR BASE SYSTEM")

    (file:is-copy-of "/etc/resolv.conf" "/old-os/etc/resolv.conf")
    (mount:unmounted-below-and-removed "/old-os"))

  (apt:mirror "")
  (as "root" (ssh:authorized-keys +spwsshkey+))
  (timezone:configured "Etc/UTC")
  (swap:has-swap-file "2G")

  (network:static "enp1s0" "" "" ""))

and to use it you evaluate this at the REPL:

CONSFIG> (deploy ((:ssh :user "root" :hop "") :sbcl)

Here the :HOP parameter specifies the IP address of the new machine, as DNS hasn’t been updated yet. Consfigurator installs SBCL and debootstrap(8), prepares a minimal system, replaces the contents of /, gets to work applying the other properties, and then reboots. This gets us a properly populated fstab:

UUID=...            /           ext4    relatime    0   1
PARTUUID=...        /boot/efi   vfat    umask=0077  0   2
/var/lib/swapfile   swap        swap    defaults    0   0

(slightly doctored for more readable alignment)

There’s ordering logic so that the swapfile will end up after whatever filesystem contains it; a UUID is used for ext4 filesystems, but for fat32 filesystems, to be safe, a PARTUUID is used.

The application of (INSTALLER:BOOTLOADERS-INSTALLED) handles calling both update-grub(8) and grub-install(8), relying on the metadata specified about /dev/sda. Next time we execute Consfigurator against the machine, it’ll ignore all the property applications attached to the application of (INSTALLER:CLEANLY-INSTALLED-ONCE) with ON-CHANGE, and just apply everything following that block.

There are a few things I don’t have good solutions for. When you boot Hetzner’s image the primary network interface is eth0, but then for a freshly debootstrapped Debian you get enp1s0, and I haven’t got a good way of knowing what it’ll be (if you know it’ll have the same name, you can use (NETWORK:PRESERVE-STATIC-ONCE) to create a file in /etc/network/interfaces.d based on the current default route and corresponding interface).

Another tricky thing is SSH host keys. It’s easy to use Consfigurator to add host keys to your laptop’s ~/.ssh/known_hosts, but in this case the host key changes back and forth from whatever the Hetzner image has and the newly generated key you get afterwards. One option might be to copy the old host keys out of /old-os before it gets deleted, like how /etc/resolv.conf is copied.

This work is based on Propellor’s equivalent functionality. I think my approach to handling /etc/fstab and bootloader installation is an improvement on what Joey does.

Posted Sat 10 Jul 2021 04:20:43 UTC Tags:

I realised this week that my recent efforts to improve how Consfigurator makes the fork(2) system call have also created a way to install executables to remote systems which will execute arbitrary Common Lisp code. Distributing precompiled programs using free software implementations of the Common Lisp standard tends to be more of a hassle than with a lot of other high level programming languages. Executables will often be hundreds of megabytes in size even if your codebase is just a few megabytes, because the whole interactive Common Lisp environment gets bundled along with your program’s code. Commercial Common Lisp implementations manage to do better, as I understand it, by knowing how to shake out unused code paths. Consfigurator’s new mechanism uploads only changed source code, which might only be kilobytes in size, and updates the executable on the remote system. So it should be useful for deploying Common Lisp-powered web services, and the like.

Here’s how it works. When you use Consfigurator you define an ASDF system – analagous to a Python package or Perl distribution – called your “consfig”. This defines HOST objects to represent the machines that you’ll use Consfigurator to manage, and any custom properties, functions those properties call, etc.. An ASDF system can depend upon other systems; for example, every consfig depends upon Consfigurator itself. When you execute Consfigurator deployments, Consfigurator uploads the source code of any ASDF systems that have changed since you last deployed this host, starts up Lisp on the remote machine, and loads up all the systems. Now the remote Lisp image is in a similarly clean state to when you’ve just started up Lisp on your laptop and loaded up the libraries you’re going to use. Only then are the actual deployment instructions are sent on stdin.

What I’ve done this week is insert an extra step for the remote Lisp image in between loading up all the ASDF systems and reading the deployment from stdin: the image calls fork(2) and establishes a pipe to communicate with the child process. The child process can be sent Lisp forms to evaluate, but for each Lisp form it receives it will actually fork again, and have its child process evaluate the form. Thus, going into the deployment, the original remote Lisp image has the capability to have arbitrary Lisp forms evaluated in a context in which all that has happened is that a statically defined set of ASDF systems has been loaded – the child processes never see the full deployment instructions sent on stdin. Further, the child process responsible for actually evaluating the Lisp form received from the first process first forks off another child process and sets up its own control pipe, such that it too has the capacbility to have arbitrary Lisp forms evaluated in a cleanly loaded context, no matter what else it might put in its memory in the meantime. (Things are set up such that the child processes responsible for actually evaluating the Lisp forms never see the Lisp forms received for evaluation by other child processes, either.)

So suppose now we have an ASDF system, and there is a function (start-server PORT) which we should call to start listening for connections. Then we can make our consfig depend upon that ASDF system, and do something like this:

CONSFIG> (deploy-these ((:ssh :user "root") :sbcl)
           ;; Set up Apache to proxy requests to our service.
           (apache:https-vhost ...)
           ;; Now apply a property to dump the image.
           (image-dumped "/usr/local/bin/cool-web-service"
                         '(cool-web-service:start-server 1234)))

Consfigurator will: SSH to; upload all the ASDF source for your consfig and its dependencies; compile and load that code into a remote SBCL process; call fork(2) and set up the control pipe; receive the applications of APACHE:HTTPS-VHOST and IMAGE-DUMPED shown above from your laptop, on stdin; apply the APACHE:HTTPS-VHOST property to ensure that Apache is proxying connections to port 1234; send a request into the control pipe to have the child process fork again and dump an executable which, when started, will evaluate the form (cool-web-service:start-server 1234). And that form will get evaluated in a pristine Lisp image, where the only meaningful things that have happened is that some ASDF systems have been loaded and a single fork(2) has taken place. You’d probably need to add some other properties to add some mechanism for actually invoking /usr/local/bin/cool-web-service and restarting it when the executable is updated.

(Background: The primary reason why Consfigurator’s remote Lisp images need to call fork(2) is that they need to do things like setuid from root to other accounts and enter chroots without getting stuck in those contexts. Previously we forked right before entering such contexts, but that meant that Consfigurator deployments could never be multithreaded, because it might later be necessary to fork, and you can’t usually do that once you’ve got more than one thread running. So now we fork before doing anything else, so that the parent can then go multithreaded if desired, but can still execute subdeployments in contexts like chroots by sending Lisp forms to evaluate in those contexts into the control pipe.)

Posted Wed 21 Jul 2021 20:30:09 UTC Tags: