From 1403486d17ef0fbd698970b90df6c40c22b1d74a Mon Sep 17 00:00:00 2001
From: Adam Stephens <adam@valkor.net>
Date: Thu, 10 Aug 2023 13:58:09 -0400
Subject: [PATCH] nixos/lxd-agent: init module from distrobuilder generator

---
 nixos/modules/module-list.nix              |  1 +
 nixos/modules/virtualisation/lxd-agent.nix | 91 ++++++++++++++++++++++
 2 files changed, 92 insertions(+)
 create mode 100644 nixos/modules/virtualisation/lxd-agent.nix

diff --git a/nixos/modules/module-list.nix b/nixos/modules/module-list.nix
index 5852843b8021..0fff271c8684 100644
--- a/nixos/modules/module-list.nix
+++ b/nixos/modules/module-list.nix
@@ -1464,6 +1464,7 @@
   ./virtualisation/lxc.nix
   ./virtualisation/lxcfs.nix
   ./virtualisation/lxd.nix
+  ./virtualisation/lxd-agent.nix
   ./virtualisation/multipass.nix
   ./virtualisation/nixos-containers.nix
   ./virtualisation/oci-containers.nix
diff --git a/nixos/modules/virtualisation/lxd-agent.nix b/nixos/modules/virtualisation/lxd-agent.nix
new file mode 100644
index 000000000000..5bcc86e3bcbe
--- /dev/null
+++ b/nixos/modules/virtualisation/lxd-agent.nix
@@ -0,0 +1,91 @@
+{ config, lib, pkgs, ... }:
+
+let
+  cfg = config.virtualisation.lxd.agent;
+
+  # the lxd agent is provided by the lxd daemon through a virtiofs or 9p mount
+  # this is a port of the distrobuilder lxd-agent generator
+  # https://github.com/lxc/distrobuilder/blob/f77300bf7d7d5707b08eaf8a434d647d1ba81b5d/generators/lxd-agent.go#L18-L55
+  preStartScript = ''
+    PREFIX="/run/lxd_agent"
+
+    mount_virtiofs() {
+        mount -t virtiofs config "$PREFIX/.mnt" >/dev/null 2>&1
+    }
+
+    mount_9p() {
+        modprobe 9pnet_virtio >/dev/null 2>&1 || true
+        mount -t 9p config "$PREFIX/.mnt" -o access=0,trans=virtio,size=1048576 >/dev/null 2>&1
+    }
+
+    fail() {
+        umount -l "$PREFIX" >/dev/null 2>&1 || true
+        rmdir "$PREFIX" >/dev/null 2>&1 || true
+        echo "$1"
+        exit 1
+    }
+
+    # Setup the mount target.
+    umount -l "$PREFIX" >/dev/null 2>&1 || true
+    mkdir -p "$PREFIX"
+    mount -t tmpfs tmpfs "$PREFIX" -o mode=0700,size=50M
+    mkdir -p "$PREFIX/.mnt"
+
+    # Try virtiofs first.
+    mount_virtiofs || mount_9p || fail "Couldn't mount virtiofs or 9p, failing."
+
+    # Copy the data.
+    cp -Ra "$PREFIX/.mnt/"* "$PREFIX"
+
+    # Unmount the temporary mount.
+    umount "$PREFIX/.mnt"
+    rmdir "$PREFIX/.mnt"
+
+    # Fix up permissions.
+    chown -R root:root "$PREFIX"
+  '';
+in {
+  meta.maintainers = with lib.maintainers; [ adamcstephens ];
+
+  options = {
+    virtualisation.lxd.agent.enable = lib.mkEnableOption (lib.mdDoc "Enable LXD agent");
+  };
+
+  config = lib.mkIf cfg.enable {
+    # https://github.com/lxc/distrobuilder/blob/f77300bf7d7d5707b08eaf8a434d647d1ba81b5d/generators/lxd-agent.go#L108-L125
+    systemd.services.lxd-agent = {
+      enable = true;
+      wantedBy = [ "multi-user.target" ];
+      path = [ pkgs.kmod pkgs.util-linux ];
+
+      preStart = preStartScript;
+
+      # avoid killing nixos-rebuild switch when executed through lxc exec
+      stopIfChanged = false;
+
+      unitConfig = {
+        Description = "LXD - agent";
+        Documentation = "https://documentation.ubuntu.com/lxd/en/latest";
+        ConditionPathExists = "/dev/virtio-ports/org.linuxcontainers.lxd";
+        Before = lib.optionals config.services.cloud-init.enable [ "cloud-init.target" "cloud-init.service" "cloud-init-local.service" ];
+        DefaultDependencies = "no";
+        StartLimitInterval = "60";
+        StartLimitBurst = "10";
+      };
+
+      serviceConfig = {
+        Type = "notify";
+        WorkingDirectory = "-/run/lxd_agent";
+        ExecStart = "/run/lxd_agent/lxd-agent";
+        Restart = "on-failure";
+        RestartSec = "5s";
+      };
+    };
+
+    systemd.paths.lxd-agent = {
+      enable = true;
+      wantedBy = [ "multi-user.target" ];
+      pathConfig.PathExists = "/dev/virtio-ports/org.linuxcontainers.lxd";
+    };
+  };
+}