feat(ops/terraform): add module for deploying NixOS system closures

This module makes it fairly easy to deploy NixOS system closures using
Terraform, while properly separating the evaluation of a
derivation (to determine whether a deploy is needed) from the building
and copying of the closure itself.

This has been on my stack for a while. It was originally developed for
Resoptima, who agreed to open-sourcing it in depot back when we
completed our work with them. Their contribution has been acknowledged
in the README.

Co-Authored-By: Florian Klink <flokli@flokli.de>
Change-Id: Ica4c170658cd25f1fb7072c9a45735fcc4351474
Reviewed-on: https://cl.tvl.fyi/c/depot/+/7950
Reviewed-by: tazjin <tazjin@tvl.su>
Tested-by: BuildkiteCI

ops/terraform/deploy-nixos/main.tf

@@ -0,0 +1,98 @@
+# This module deploys a NixOS host by building a system closure
+# located at the specified attribute in the current repository.
+#
+# The closure's derivation path is persisted in the Terraform state to
+# determine after Nix evaluation whether the system closure has
+# changed and needs to be built/deployed.
+#
+# The system configuration is then built (or substituted) on the
+# machine that runs `terraform apply`, then copied and activated on
+# the target machine using `nix-copy-closure`.
+
+variable "attrpath" {
+  description = "attribute set path pointing to the NixOS system closure"
+  type        = string
+}
+
+variable "target_name" {
+  description = "unique name of the target machine"
+  type        = string
+}
+
+variable "target_host" {
+  description = "address (IP or hostname) at which the target is reachable"
+  type        = string
+}
+
+variable "target_user" {
+  description = "username on the target machine"
+  type        = string
+}
+
+variable "target_user_ssh_key" {
+  description = "SSH key to use for connecting to the target"
+  type        = string
+  sensitive   = true
+}
+
+# Fetch the derivation hash for the NixOS system.
+data "external" "nixos_system" {
+  program = ["${path.module}/nixos-eval.sh"]
+
+  query = {
+    attrpath = var.attrpath
+  }
+}
+
+# Deploy the NixOS configuration if anything changed.
+resource "null_resource" "nixos_deploy" {
+  connection {
+    type        = "ssh"
+    host        = var.target_host
+    user        = var.target_user
+    private_key = var.target_user_ssh_key
+  }
+
+  # 1. Wait for SSH to become available.
+  provisioner "remote-exec" {
+    inline = ["true"]
+  }
+
+  # 2. Build NixOS system.
+  provisioner "local-exec" {
+    command = "nix-build ${data.external.nixos_system.result.drv} --no-out-link"
+  }
+
+  # 3. Copy closure to the target.
+  provisioner "local-exec" {
+    command = "${path.module}/nixos-copy.sh"
+
+    environment = {
+      SYSTEM_DRV  = data.external.nixos_system.result.drv
+      TARGET_HOST = var.target_host
+      DEPLOY_KEY  = var.target_user_ssh_key
+      TARGET_USER = var.target_user
+    }
+  }
+
+  # 4. Activate closure on the target.
+  provisioner "remote-exec" {
+    inline = [
+      "set -eu",
+      "SYSTEM=$(nix-build ${data.external.nixos_system.result.drv} --no-out-link)",
+      "sudo nix-env --profile /nix/var/nix/profiles/system --set $SYSTEM",
+      "sudo $SYSTEM/bin/switch-to-configuration switch",
+    ]
+  }
+
+  triggers = {
+    nixos_drv   = data.external.nixos_system.result.drv
+    attrpath    = var.attrpath
+    target_host = var.target_host
+    target_name = var.target_name
+  }
+}
+
+output "nixos_drv" {
+  value = data.external.nixos_system.result
+}