Actually working (probably) root on tmpfs

.gitignore

@@ -1,2 +1,3 @@
 result*
-*.qcow2
+/.vms/*
+!/.vms/.gitkeep

boxes/colony.nix

@@ -1,5 +1,13 @@
 { lib, pkgs, inputs, ... }:
   {
+    fileSystems = {
+      "/persist" = {
+        device = "/dev/disk/by-label/persist";
+        fsType = "ext4";
+        neededForBoot = true;
+      };
+    };
+
     my = {
       server.enable = true;
     };

flake.lock

@@ -60,11 +60,11 @@
     },
     "flake-utils": {
       "locked": {
-        "lastModified": 1642700792,
-        "narHash": "sha256-XqHrk7hFb+zBvRg6Ghl+AZDq03ov6OshJLiSWOoX5es=",
+        "lastModified": 1644229661,
+        "narHash": "sha256-1YdnJAsNy69bpcjuoKdOYQX0YxZBiCYZo4Twxerqv7k=",
         "owner": "numtide",
         "repo": "flake-utils",
-        "rev": "846b2ae0fc4cc943637d3d1def4454213e203cba",
+        "rev": "3cecb5b042f7f209c56ffd8371b2711a290ec797",
         "type": "github"
       },
       "original": {
@@ -80,11 +80,11 @@
         ]
       },
       "locked": {
-        "lastModified": 1643933104,
-        "narHash": "sha256-NZPuFxRsZKN8pjRuHPpzlMyt6JQhcjiduBG8bMghSjE=",
+        "lastModified": 1644534280,
+        "narHash": "sha256-Gzf/Jq/F1vvTp6XkzPU+pBCj3OSAFLiR7f0ptwRseiI=",
         "owner": "nix-community",
         "repo": "home-manager",
-        "rev": "63dccc4e60422c1db2c3929b2fd1541f36b7e664",
+        "rev": "6d9d9294d09b5e88df65f8c6651efb8a4d7d2476",
         "type": "github"
       },
       "original": {
@@ -93,6 +93,22 @@
         "type": "github"
       }
     },
+    "impermanence": {
+      "locked": {
+        "lastModified": 1644541943,
+        "narHash": "sha256-bMiRtPs3xh8S3duX7JiWLZdUZi7RwktpXGhykCdCroE=",
+        "owner": "devplayer0",
+        "repo": "impermanence",
+        "rev": "4bfa7f8b6844be7172264c1f7c11ac0c89027f79",
+        "type": "github"
+      },
+      "original": {
+        "owner": "devplayer0",
+        "ref": "qemu-vm-dirs",
+        "repo": "impermanence",
+        "type": "github"
+      }
+    },
     "lowdown-src": {
       "flake": false,
       "locked": {
@@ -161,11 +177,11 @@
     },
     "nixpkgs-stable": {
       "locked": {
-        "lastModified": 1643788601,
-        "narHash": "sha256-6l5Ax44pC/Oo/Muj5Y/NA27Pd38Wty/7GtGSSmYNug4=",
+        "lastModified": 1644193793,
+        "narHash": "sha256-ggCI5LCXwkUJQwjpkr9qDFbQMiK3JvAfeNR4Uy0ny5Q=",
         "owner": "NixOS",
         "repo": "nixpkgs",
-        "rev": "f6ddd55d5f9d5eca08df138c248008c1ba73ecec",
+        "rev": "521e4d7d13b09bc0a21976b9d19abd197d4e3b1e",
         "type": "github"
       },
       "original": {
@@ -176,11 +192,11 @@
     },
     "nixpkgs-unstable": {
       "locked": {
-        "lastModified": 1643805626,
-        "narHash": "sha256-AXLDVMG+UaAGsGSpOtQHPIKB+IZ0KSd9WS77aanGzgc=",
+        "lastModified": 1644420267,
+        "narHash": "sha256-rFJuctggkjM412OC6OGPdXogFp7czGDW05ueWqpJbj8=",
         "owner": "NixOS",
         "repo": "nixpkgs",
-        "rev": "554d2d8aa25b6e583575459c297ec23750adb6cb",
+        "rev": "98bb5b77c8c6666824a4c13d23befa1e07210ef1",
         "type": "github"
       },
       "original": {
@@ -195,6 +211,7 @@
         "deploy-rs": "deploy-rs",
         "flake-utils": "flake-utils",
         "home-manager": "home-manager",
+        "impermanence": "impermanence",
         "nix": "nix",
         "nixpkgs-stable": "nixpkgs-stable",
         "nixpkgs-unstable": "nixpkgs-unstable"

flake.nix

@@ -8,14 +8,18 @@
     # For extra-stable systems
     nixpkgs-stable.url = "nixpkgs/nixos-21.11";
 
-    nix.url = "nix/latest-release";
+    # Stuff used by the flake for build / deployment
     agenix.url = "github:ryantm/agenix";
     agenix.inputs.nixpkgs.follows = "nixpkgs-unstable";
     deploy-rs.url = "github:serokell/deploy-rs";
     deploy-rs.inputs.nixpkgs.follows = "nixpkgs-unstable";
-
     home-manager.url = "github:nix-community/home-manager";
     home-manager.inputs.nixpkgs.follows = "nixpkgs-unstable";
+
+    # Stuff used by systems
+    nix.url = "nix/latest-release";
+    #impermanence.url = "github:nix-community/impermanence";
+    impermanence.url = "github:devplayer0/impermanence/qemu-vm-dirs";
   };
 
   outputs =
@@ -26,7 +30,6 @@
 
       nixpkgs-unstable, nixpkgs-stable,
 
-      nix,
       agenix,
       deploy-rs,
 
@@ -51,7 +54,7 @@
 
       lib = pkgsFlakes.unstable.lib;
 
-      pkgs' = mapAttrs (_: path: lib.my.mkPkgs path { overlays = [libOverlay]; }) pkgsFlakes;
+      pkgs' = mapAttrs (_: path: lib.my.mkPkgs path { overlays = [ libOverlay ]; }) pkgsFlakes;
     in {
       inherit lib;
 
@@ -60,12 +63,14 @@
           imports = [ (import path') ];
         }) {
         common = "common.nix";
+        build = "build.nix";
         tmproot = "tmproot.nix";
         server = "server.nix";
       };
 
       nixosConfigurations = import ./systems.nix { inherit lib pkgsFlakes inputs; modules = self.nixosModules; };
-      vms = mapAttrs (_: system: system.config.system.build.vm) self.nixosConfigurations;
+      systems = mapAttrs (_: system: system.config.system.build.toplevel) self.nixosConfigurations;
+      vms = mapAttrs (_: system: system.config.my.build.devVM) self.nixosConfigurations;
 
       apps =
         let apps' = {}

modules/build.nix

@@ -0,0 +1,31 @@
+{ lib, extendModules, modulesPath, options, config, ... }:
+  let
+    inherit (lib) mkOption;
+    inherit (lib.my) mkBoolOpt;
+
+    cfg = config.my.build;
+
+    # TODO: This is broken on 21.11 (https://github.com/NixOS/nixpkgs/issues/148343)
+    asDevVM = extendModules {
+      modules = [
+        (import "${modulesPath}/virtualisation/qemu-vm.nix")
+        ({ config, ... }: {
+          my.boot.isDevVM = true;
+        })
+      ];
+    };
+  in {
+    options.my = with lib.types; {
+      boot.isDevVM = mkBoolOpt false;
+      build = options.system.build;
+      asDevVM = mkOption {
+        inherit (asDevVM) type;
+        default = {};
+        visible = "shallow";
+      };
+    };
+
+    config.my.build = {
+      devVM = config.my.asDevVM.system.build.vm;
+    };
+  }

modules/common.nix

@@ -44,6 +44,29 @@
               experimental-features = nix-command flakes ca-derivations
             '';
         };
+        nixpkgs = {
+          config = {
+            allowUnfree = true;
+          };
+        };
+
+        boot = {
+          # Use latest LTS release by default
+          kernelPackages = mkDefault pkgs.linuxKernel.packages.linux_5_15;
+          loader = {
+            efi = {
+              efiSysMountPoint = mkDefault "/boot";
+              canTouchEfiVariables = mkDefault false;
+            };
+            systemd-boot = {
+              enable = mkDefault true;
+              editor = mkDefault true;
+              consoleMode = mkDefault "max";
+              configurationLimit = mkDefault 10;
+              memtest86.enable = mkDefault true;
+            };
+          };
+        };
 
         environment.systemPackages = with pkgs; [
           bash-completion

modules/tmproot.nix

@@ -1,7 +1,7 @@
-{ lib, pkgs, inputs, config, ... }:
+{ lib, pkgs, inputs, config, ... }@args:
   let
-    inherit (lib) concatStringsSep mkIf mkDefault mkAliasDefinitions;
-    inherit (lib.my) mkOpt mkBoolOpt;
+    inherit (lib) any concatStringsSep mkIf mkDefault mkMerge mkVMOverride;
+    inherit (lib.my) mkOpt mkBoolOpt mkVMOverride';
 
     cfg = config.my.tmproot;
 
@@ -45,17 +45,63 @@
 
         recurse(base)
       '';
+
+    rootDef = {
+      device = "yeet";
+      fsType = "tmpfs";
+      options = [ "size=${cfg.size}" ];
+    };
   in {
+    imports = [ inputs.impermanence.nixosModules.impermanence ];
+
     options.my.tmproot = with lib.types; {
       enable = mkBoolOpt true;
+      persistDir = mkOpt str "/persist";
+      size = mkOpt str "2G";
       ignoreUnsaved = mkOpt (listOf str) [
         "/tmp"
       ];
     };
 
-    config = mkIf cfg.enable {
-      environment.systemPackages = [
-        (pkgs.writeScriptBin "tmproot-unsaved" showUnsaved)
-      ];
-    };
+    config = mkMerge [
+      (mkIf cfg.enable {
+        assertions = [
+          {
+            assertion = config.fileSystems ? "${cfg.persistDir}";
+            message = "The 'fileSystems' option does not specify your persistence file system (${cfg.persistDir}).";
+          }
+        ];
+
+        environment.systemPackages = [
+          (pkgs.writeScriptBin "tmproot-unsaved" showUnsaved)
+        ];
+
+        environment.persistence."${cfg.persistDir}" = {
+          hideMounts = mkDefault true;
+          directories = [
+            "/var/log"
+          ];
+          files = [
+            "/etc/machine-id"
+          ];
+        };
+
+        fileSystems."/" = rootDef;
+
+        # If we need to override any VM-specific options that the modules system won't know about this early
+        my.asDevVM.config.virtualisation = {
+          diskImage = "./.vms/${config.system.name}-persist.qcow2";
+        };
+      })
+      (mkIf (cfg.enable && config.my.boot.isDevVM) {
+        fileSystems = mkVMOverride {
+          "/" = mkVMOverride' rootDef;
+          # Hijack the "root" device for persistence in the VM
+          "${cfg.persistDir}" = {
+            device = config.virtualisation.bootDevice;
+            neededForBoot = true;
+          };
+        };
+      })
+    ];
   }

systems.nix

@@ -18,7 +18,7 @@
   in mapAttrs mkSystem {
     colony = {
       system = "x86_64-linux";
-      nixpkgs = "stable";
+      nixpkgs = "unstable";
       config = boxes/colony.nix;
     };
   }

util.nix

@@ -1,6 +1,6 @@
 { lib }:
   let
-    inherit (lib) genAttrs mapAttrs' types mkOption;
+    inherit (lib) genAttrs mapAttrs' types mkOption mkOverride;
     inherit (lib.flake) defaultSystems;
   in {
     addPrefix = prefix: mapAttrs' (n: v: { name = "${prefix}${n}"; value = v; });
@@ -12,4 +12,5 @@
       type = types.bool;
       example = true;
     };
+    mkVMOverride' = mkOverride 9;
   }