Add home-manager module

This is to update and fix the issues I saw in [1] and [2].

Using a service definition instead of an activation script should
resolve the issue about the secrets disappearing after rebooting.

Removed the `user` and `group` option as they do not make sense to me
for a home-manager module, which should target a single user. They can
always be added back if somebody comes screaming.

This is somewhat modeled after sops-nix's own module [3].

[1]: https://github.com/ryantm/agenix/pull/58/
[2]: https://github.com/ryantm/agenix/pull/109
[3]: https://github.com/Mic92/sops-nix/blob/master/modules/home-manager/sops.nix

flake.nix

@@ -23,6 +23,9 @@
     darwinModules.age = import ./modules/age.nix;
     darwinModules.default = self.darwinModules.age;
 
+    homeManagerModules.age = import ./modules/age-home.nix;
+    homeManagerModules.default = self.homeManagerModules.age;
+
     overlays.default = import ./overlay.nix;
 
     formatter.x86_64-darwin = nixpkgs.legacyPackages.x86_64-darwin.alejandra;

modules/age-home.nix

@@ -0,0 +1,234 @@
+{
+  config,
+  options,
+  lib,
+  pkgs,
+  ...
+}:
+with lib; let
+  cfg = config.age;
+
+  ageBin = lib.getExe config.age.package;
+
+  newGeneration = ''
+    _agenix_generation="$(basename "$(readlink "${cfg.secretsDir}")" || echo 0)"
+    (( ++_agenix_generation ))
+    echo "[agenix] creating new generation in ${cfg.secretsMountPoint}/$_agenix_generation"
+    mkdir -p "${cfg.secretsMountPoint}"
+    chmod 0751 "${cfg.secretsMountPoint}"
+    mkdir -p "${cfg.secretsMountPoint}/$_agenix_generation"
+    chmod 0751 "${cfg.secretsMountPoint}/$_agenix_generation"
+  '';
+
+  setTruePath = secretType: ''
+    ${
+      if secretType.symlink
+      then ''
+        _truePath="${cfg.secretsMountPoint}/$_agenix_generation/${secretType.name}"
+      ''
+      else ''
+        _truePath="${secretType.path}"
+      ''
+    }
+  '';
+
+  installSecret = secretType: ''
+    ${setTruePath secretType}
+    echo "decrypting '${secretType.file}' to '$_truePath'..."
+    TMP_FILE="$_truePath.tmp"
+
+    IDENTITIES=()
+    # shellcheck disable=2043
+    for identity in ${toString cfg.identityPaths}; do
+      test -r "$identity" || continue
+      IDENTITIES+=(-i)
+      IDENTITIES+=("$identity")
+    done
+
+    test "''${#IDENTITIES[@]}" -eq 0 && echo "[agenix] WARNING: no readable identities found!"
+
+    mkdir -p "$(dirname "$_truePath")"
+    [ "${secretType.path}" != "${cfg.secretsDir}/${secretType.name}" ] && mkdir -p "$(dirname "${secretType.path}")"
+    (
+      umask u=r,g=,o=
+      test -f "${secretType.file}" || echo '[agenix] WARNING: encrypted file ${secretType.file} does not exist!'
+      test -d "$(dirname "$TMP_FILE")" || echo "[agenix] WARNING: $(dirname "$TMP_FILE") does not exist!"
+      LANG=${config.i18n.defaultLocale or "C"} ${ageBin} --decrypt "''${IDENTITIES[@]}" -o "$TMP_FILE" "${secretType.file}"
+    )
+    chmod ${secretType.mode} "$TMP_FILE"
+    mv -f "$TMP_FILE" "$_truePath"
+
+    ${optionalString secretType.symlink ''
+      [ "${secretType.path}" != "${cfg.secretsDir}/${secretType.name}" ] && ln -sfn "${cfg.secretsDir}/${secretType.name}" "${secretType.path}"
+    ''}
+  '';
+
+  testIdentities =
+    map
+    (path: ''
+      test -f ${path} || echo '[agenix] WARNING: config.age.identityPaths entry ${path} not present!'
+    '')
+    cfg.identityPaths;
+
+  cleanupAndLink = ''
+    _agenix_generation="$(basename "$(readlink "${cfg.secretsDir}")" || echo 0)"
+    (( ++_agenix_generation ))
+    echo "[agenix] symlinking new secrets to ${cfg.secretsDir} (generation $_agenix_generation)..."
+    ln -sfn "${cfg.secretsMountPoint}/$_agenix_generation" "${cfg.secretsDir}"
+
+    (( _agenix_generation > 1 )) && {
+    echo "[agenix] removing old secrets (generation $(( _agenix_generation - 1 )))..."
+    rm -rf "${cfg.secretsMountPoint}/$(( _agenix_generation - 1 ))"
+    }
+  '';
+
+  installSecrets = builtins.concatStringsSep "\n" (
+    ["echo '[agenix] decrypting secrets...'"]
+    ++ testIdentities
+    ++ (map installSecret (builtins.attrValues cfg.secrets))
+    ++ [cleanupAndLink]
+  );
+
+  secretType = types.submodule ({
+    config,
+    name,
+    ...
+  }: {
+    options = {
+      name = mkOption {
+        type = types.str;
+        default = name;
+        description = ''
+          Name of the file used in ''${cfg.secretsDir}
+        '';
+      };
+      file = mkOption {
+        type = types.path;
+        description = ''
+          Age file the secret is loaded from.
+        '';
+      };
+      path = mkOption {
+        type = types.str;
+        default = "${cfg.secretsDir}/${config.name}";
+        description = ''
+          Path where the decrypted secret is installed.
+        '';
+      };
+      mode = mkOption {
+        type = types.str;
+        default = "0400";
+        description = ''
+          Permissions mode of the decrypted secret in a format understood by chmod.
+        '';
+      };
+      symlink = mkEnableOption "symlinking secrets to their destination" // {default = true;};
+    };
+  });
+
+  mountingScript = let
+    app = pkgs.writeShellApplication {
+      name = "agenix-home-manager-mount-secrets";
+      runtimeInputs = with pkgs; [coreutils];
+      text = ''
+        ${newGeneration}
+        ${installSecrets}
+        exit 0
+      '';
+    };
+  in
+    lib.getExe app;
+
+  userDirectory = dir: let
+    inherit (pkgs.stdenv.hostPlatform) isDarwin;
+    baseDir =
+      if isDarwin
+      then "$(getconf DARWIN_USER_TEMP_DIR)"
+      else "$XDG_RUNTIME_DIR";
+  in "${baseDir}/${dir}";
+
+  userDirectoryDescription = dir: ''
+    "$XDG_RUNTIME_DIR"/${dir} on linux or "$(getconf DARWIN_USER_TEMP_DIR)"/${dir} on darwin.
+  '';
+in {
+  options.age = {
+    package = mkPackageOption pkgs "rage" {};
+
+    secrets = mkOption {
+      type = types.attrsOf secretType;
+      default = {};
+      description = ''
+        Attrset of secrets.
+      '';
+    };
+
+    identityPaths = mkOption {
+      type = types.listOf types.path;
+      default = [
+        "${config.home.homeDirectory}/.ssh/id_ed25519"
+        "${config.home.homeDirectory}/.ssh/id_rsa"
+      ];
+      defaultText = litteralExpression ''
+        [
+          "''${config.home.homeDirectory}/.ssh/id_ed25519"
+          "''${config.home.homeDirectory}/.ssh/id_rsa"
+        ]
+      '';
+      description = ''
+        Path to SSH keys to be used as identities in age decryption.
+      '';
+    };
+
+    secretsDir = mkOption {
+      type = types.str;
+      default = userDirectory "agenix";
+      defaultText = userDirectoryDescription "agenix";
+      description = ''
+        Folder where secrets are symlinked to
+      '';
+    };
+
+    secretsMountPoint = mkOption {
+      default = userDirectory "agenix.d";
+      defaultText = userDirectoryDescription "agenix.d";
+      description = ''
+        Where secrets are created before they are symlinked to ''${cfg.secretsDir}
+      '';
+    };
+  };
+
+  config = mkIf (cfg.secrets != {}) {
+    assertions = [
+      {
+        assertion = cfg.identityPaths != [];
+        message = "age.identityPaths must be set.";
+      }
+    ];
+
+    systemd.user.services.agenix = lib.mkIf pkgs.stdenv.hostPlatform.isLinux {
+      Unit = {
+        Description = "agenix activation";
+      };
+      Service = {
+        Type = "oneshot";
+        ExecStart = mountingScript;
+      };
+      Install.WantedBy = ["default.target"];
+    };
+
+    launchd.agents.activate-agenix = {
+      enable = true;
+      config = {
+        ProgramArguments = [mountingScript];
+        KeepAlive = {
+          Crashed = false;
+          SuccessfulExit = false;
+        };
+        RunAtLoad = true;
+        ProcessType = "Background";
+        StandardOutPath = "${config.home.homeDirectory}/Library/Logs/agenix/stdout";
+        StandardErrorPath = "${config.home.homeDirectory}/Library/Logs/agenix/stderr";
+      };
+    };
+  };
+}