From 0350752a500567b0e932000a4a5484a01589dfc6 Mon Sep 17 00:00:00 2001
From: Madeleine Sydney <lomiskiam@gmail.com>
Date: Thu, 12 Dec 2024 01:38:47 -0700
Subject: [PATCH] Use impermanence

---
 README.org                                  | 12 +++
 hosts/nixos-testbed/erase-home-darlings.clj | 84 ---------------------
 modules/home/impermanence.nix               | 58 ++++++++++++++
 modules/nixos/impermanence.nix              | 66 ++++++++++++++++
 4 files changed, 136 insertions(+), 84 deletions(-)
 delete mode 100644 hosts/nixos-testbed/erase-home-darlings.clj
 create mode 100644 modules/home/impermanence.nix
 create mode 100644 modules/nixos/impermanence.nix

diff --git a/README.org b/README.org
index 4199daa..74e0cea 100644
--- a/README.org
+++ b/README.org
@@ -15,6 +15,8 @@
 
     nixpkgs.url = "github:NixOS/nixpkgs/nixpkgs-unstable";
 
+    impermanence.url = "github:nix-community/impermanence";
+
     home-manager = {
       url = "github:nix-community/home-manager";
       inputs.nixpkgs.follows = "nixpkgs";
@@ -70,6 +72,7 @@ let mkHost = k: v: nixpkgs.lib.nixosSystem {
         inputs.self.nixosModules.default
 
         inputs.disko.nixosModules.disko
+        inputs.impermanence.nixosModules.impermanence
 
         # Directory name should always match host name.
         ({ ... }: { networking.hostName = k; })
@@ -102,6 +105,15 @@ builtins.mapAttrs mkHost (builtins.readDir ./hosts)
   ];
 
   sydnix = {
+    impermanence = {
+      enable = true;
+      directories = [
+        # Warning: Neither /var/lib/nixos nor any of its parents are persisted.
+        # This means all users/groups without specified uids/gids will have them
+        # reassigned on reboot.
+        "/var/lib/nixos"
+      ];
+    };
     users.users = [
       "crumb"
     ];
diff --git a/hosts/nixos-testbed/erase-home-darlings.clj b/hosts/nixos-testbed/erase-home-darlings.clj
deleted file mode 100644
index a146963..0000000
--- a/hosts/nixos-testbed/erase-home-darlings.clj
+++ /dev/null
@@ -1,84 +0,0 @@
-#!/usr/bin/env bb
-
-;;; TODO: option to either move OR copy
-
-(require '[clojure.core.match :refer [match]]
-         '[babashka.cli :as cli]
-         '[babashka.process :refer [$ check] :as p])
-
-(defn get-archive-path []
-  (fs/path
-   "/persist/previous"))
-
-(defn get-files []
-  (let [diff (:out (check ($ {:out :string}
-                             "zfs" "diff" "-HF"
-                             "rpool/local/home@blank"
-                             "rpool/local/home")))]
-    ;; See zfs-diff(8) to understand what we're parsing here.
-    (->> diff
-         str/split-lines
-         (map #(str/split % #"\s"))
-         (filter #(and
-                   ;; We only care to preserve /new/ content.
-                   (contains? #{"+" "M"} (first %))
-                   ;; We only bother with plain old files.  No directories,
-                   ;; symlinks, etc.
-                   (= (second %) "F")))
-         (map #(nth % 2)))))
-
-(defn move-out-of-my-way [file]
-  ;; No TCO. }:<
-  (loop [n 0]
-    (let [file' (format "%s-%d" file n)]
-      (if (fs/exists? file')
-        (recur (inc n))
-        (do (fs/move file file')
-            file')))))
-
-(defn archive-files [archive-path files]
-  (let [new-archive (fs/path archive-path "home/new-archive")]
-    (when (fs/exists? new-archive)
-      (println "Warning: `new-archive' already exists... we'll rename it for you?")
-      (move-out-of-my-way new-archive))
-    (doseq [file files]
-      (let [destination (fs/path
-                         new-archive
-                         (fs/relativize "/home" (fs/parent file)))]
-        (fs/create-dirs destination)
-        (fs/move file destination)))))
-
-(defn cycle-archives [archive-path n]
-  "Delete the oldest archive path, and increment each previous path by one.
-More precisely,
- - Delete the archive path labeled `n` (the oldest allowed).
- - For each remaining path labeled 'i', relabel to 'i + 1'.
- - Lastly, we delete the path labeled `new-archive`, if it exists."
-  (let [gp #(fs/path archive-path "home" (str %))]
-    (fs/delete-if-exists (gp n))
-    (doseq [i (range (dec n) 1 -1)]
-      (when (fs/exists? (gp (dec i)))
-        (fs/move (gp (dec i)) (gp i))))
-    (when (fs/exists? (gp "new-archive"))
-      (fs/move (gp "new-archive") (gp 1)))))
-
-(defn do-rollback []
-  (let [proc (deref ($ "zfs" "rollback" "-r" "rpool/local/home@blank"))]
-    (if (= (:exit proc) 0)
-      (println (str "Successfully rolled back /home.  "
-                    "Enjoy the fresh filesystem smell!  }:D"))
-      (println "Something went wrong rolling back /home... D:{"))))
-
-(defn -main []
-  (let [n (if (< 0 (count *command-line-args*))
-            (parse-long (first *command-line-args*))
-            3)]
-    (binding [p/*defaults*
-              {:pre-start-fn #(println (str "+ " (str/join (:cmd %))))}]
-      (let [archive-path (get-archive-path)
-            files (get-files)]
-        (archive-files archive-path files)
-        (cycle-archives archive-path n)
-        (do-rollback)))))
-
-(-main)
diff --git a/modules/home/impermanence.nix b/modules/home/impermanence.nix
new file mode 100644
index 0000000..65d9212
--- /dev/null
+++ b/modules/home/impermanence.nix
@@ -0,0 +1,58 @@
+{ config, lib, pkgs, ... }:
+
+with lib;
+
+let
+  cfg = config.sydnix.impermanence;
+in {
+  options = {
+    sydnix.impermanence = {
+      enable = mkOption {
+        description = "Enable Impermanence";
+        type = types.bool;
+        default = false;
+      };
+
+      directories = mkOption {
+        description = "";
+        type = with types;
+          listOf (coercedTo str (d: { directory = d; }) userDir);
+        default = [];
+      };
+
+      files = mkOption {
+        description = "";
+        type = with types;
+          listOf (coercedTo str (f: { file = f; }) userFile);
+        default = [];
+      };
+    };
+  };
+
+  config = mkIf cfg.enable {
+    systemd.services.erase-home-darlings =
+      let service = {
+            description = "Rollback home to a blank state on boot";
+            wantedBy = [
+              "multi-user.target"
+            ];
+            after = [
+              "home.mount"
+            ];
+            path = [ pkgs.zfs pkgs.babashka ];
+            # unitConfig.DefaultDependencies = "no";
+            serviceConfig = {
+              Type = "oneshot";
+              RemainAfterExit = true;
+              ExecStart =
+                let script = ./erase-home-darlings.clj;
+                in ''${pkgs.babashka}/bin/bb "${script}" 3'';
+            };
+            stopIfChanged = false;
+            restartIfChanged = false;
+          };
+      in if config.boot.initrd.systemd.enable
+         then service
+         else throw "sydnix.impermanence currently requires config.boot.initrd.systemd.enable'!";
+  };
+}
diff --git a/modules/nixos/impermanence.nix b/modules/nixos/impermanence.nix
new file mode 100644
index 0000000..9d0cb58
--- /dev/null
+++ b/modules/nixos/impermanence.nix
@@ -0,0 +1,66 @@
+# Requires boot.initrd.enable = true and boot.initrd.systemd.enable = true!
+{ config, lib, pkgs, ... }:
+
+with lib;
+
+let
+  cfg = config.sydnix.impermanence;
+in {
+  options = {
+    sydnix.impermanence = {
+      enable = mkOption {
+        description = "Enable Impermanence";
+        type = types.bool;
+        default = false;
+      };
+
+      directories = mkOption {
+        description = "";
+        type = with types; listOf anything;
+        default = [];
+      };
+
+      files = mkOption {
+        description = "";
+
+        type = with types; listOf anything;
+        default = [];
+      };
+    };
+  };
+
+  config = mkIf cfg.enable {
+    boot.initrd.systemd.initrdBin = with pkgs; [
+      zfs
+    ];
+
+    boot.initrd.systemd.services.erase-darlings = {
+      description = "Rollback filesystem to a blank state on boot";
+      wantedBy = [
+        "initrd.target"
+      ];
+      after = [
+        # "zfs-import.service"
+        "zfs-import-rpool.service"
+      ];
+      before = [
+        "sysroot.mount"
+      ];
+      path = [ pkgs.zfs ];
+      unitConfig.DefaultDependencies = "no";
+      serviceConfig = {
+        Type = "oneshot";
+        RemainAfterExit = true;
+      };
+      script = /* bash */ ''
+      zfs rollback -r rpool/local/root@blank \
+        && echo ">> >> rollback complete << <<"
+    '';
+    };
+
+    environment.persistence."/persist" = {
+        directories = cfg.directories;
+        files = cfg.files;
+    };
+  };
+}