Add a nix-darwin module

This is somewhat but not entirely duplicative with the home manager
support. The difference is primarily for MacOS servers, for which there
is no LaunchAgent support (as launch agents are tied to user sessions).

This PR adds a nix-darwin module (configured similarly to the home
manager module) which is suitable for e.g. other launchd daemons.

flake.nix

@@ -32,6 +32,7 @@
     };
     homeManagerModules.sops = import ./modules/home-manager/sops.nix;
     homeManagerModule = self.homeManagerModules.sops;
+    darwinModules.sops = import ./modules/nix-darwin/sops.nix;
     packages = forAllSystems (system:
       import ./default.nix {
         pkgs = import nixpkgs {inherit system;};

modules/nix-darwin/sops.nix

@@ -0,0 +1,269 @@
+{ config, lib, pkgs, ... }:
+
+let
+  cfg = config.sops;
+  sops-install-secrets = (pkgs.callPackage ../.. {}).sops-install-secrets;
+  secretType = lib.types.submodule ({ config, name, ... }: {
+    options = {
+      name = lib.mkOption {
+        type = lib.types.str;
+        default = name;
+        description = ''
+          Name of the file used in /run/user/*/secrets
+        '';
+      };
+
+      key = lib.mkOption {
+        type = lib.types.str;
+        default = name;
+        description = ''
+          Key used to lookup in the sops file.
+          No tested data structures are supported right now.
+          This option is ignored if format is binary.
+        '';
+      };
+
+      path = lib.mkOption {
+        type = lib.types.str;
+        default = "${cfg.defaultSymlinkPath}/${name}";
+        description = ''
+          Path where secrets are symlinked to.
+          If the default is kept no other symlink is created.
+          `%r` is replaced by $XDG_RUNTIME_DIR on linux or `getconf
+          DARWIN_USER_TEMP_DIR` on darwin.
+        '';
+      };
+
+      format = lib.mkOption {
+        type = lib.types.enum [ "yaml" "json" "binary" "ini" "dotenv" ];
+        default = cfg.defaultSopsFormat;
+        description = ''
+          File format used to decrypt the sops secret.
+          Binary files are written to the target file as is.
+        '';
+      };
+
+      mode = lib.mkOption {
+        type = lib.types.str;
+        default = "0400";
+        description = ''
+          Permissions mode of the in octal.
+        '';
+      };
+
+      sopsFile = lib.mkOption {
+        type = lib.types.path;
+        default = cfg.defaultSopsFile;
+        defaultText = "\${config.sops.defaultSopsFile}";
+        description = ''
+          Sops file the secret is loaded from.
+        '';
+      };
+    };
+  });
+
+  pathNotInStore = lib.mkOptionType {
+    name = "pathNotInStore";
+    description = "path not in the Nix store";
+    descriptionClass = "noun";
+    check = x: !lib.path.hasStorePathPrefix (/. + x);
+    merge = lib.mergeEqualOption;
+  };
+
+  manifestFor = suffix: secrets: pkgs.writeTextFile {
+    name = "manifest${suffix}.json";
+    text = builtins.toJSON {
+      secrets = builtins.attrValues secrets;
+      secretsMountPoint = cfg.defaultSecretsMountPoint;
+      symlinkPath = cfg.defaultSymlinkPath;
+      keepGenerations = cfg.keepGenerations;
+      gnupgHome = cfg.gnupg.home;
+      sshKeyPaths = cfg.gnupg.sshKeyPaths;
+      ageKeyFile = cfg.age.keyFile;
+      ageSshKeyPaths = cfg.age.sshKeyPaths;
+      userMode = true;
+      logging = {
+        keyImport = builtins.elem "keyImport" cfg.log;
+        secretChanges = builtins.elem "secretChanges" cfg.log;
+      };
+    };
+    checkPhase = ''
+      ${sops-install-secrets}/bin/sops-install-secrets -check-mode=${if cfg.validateSopsFiles then "sopsfile" else "manifest"} "$out"
+    '';
+  };
+
+  manifest = manifestFor "" cfg.secrets;
+
+  script = toString (pkgs.writeShellScript "sops-nix-user" ((lib.optionalString (cfg.gnupg.home != null) ''
+    export SOPS_GPG_EXEC=${pkgs.gnupg}/bin/gpg
+  '')
+    + (lib.optionalString cfg.age.generateKey ''
+    if [[ ! -f '${cfg.age.keyFile}' ]]; then
+      echo generating machine-specific age key...
+      ${pkgs.coreutils}/bin/mkdir -p $(${pkgs.coreutils}/bin/dirname ${cfg.age.keyFile})
+      # age-keygen sets 0600 by default, no need to chmod.
+      ${pkgs.age}/bin/age-keygen -o ${cfg.age.keyFile}
+    fi
+  '' + ''
+    ${sops-install-secrets}/bin/sops-install-secrets -ignore-passwd '${manifest}'
+  '')));
+in {
+  options.sops = {
+    logFile = lib.mkOption {
+      type = lib.types.path;
+      default = "/var/log/sops.log";
+      description = "The logfile to use for sops-nix";
+    };
+
+    secrets = lib.mkOption {
+      type = lib.types.attrsOf secretType;
+      default = {};
+      description = ''
+        Secrets to decrypt.
+      '';
+    };
+
+    defaultSopsFile = lib.mkOption {
+      type = lib.types.path;
+      description = ''
+        Default sops file used for all secrets.
+      '';
+    };
+
+    defaultSopsFormat = lib.mkOption {
+      type = lib.types.str;
+      default = "yaml";
+      description = ''
+        Default sops format used for all secrets.
+      '';
+    };
+
+    validateSopsFiles = lib.mkOption {
+      type = lib.types.bool;
+      default = true;
+      description = ''
+        Check all sops files at evaluation time.
+        This requires sops files to be added to the nix store.
+      '';
+    };
+
+    defaultSymlinkPath = lib.mkOption {
+      type = lib.types.str;
+      default = "/var/root/sops-nix/secrets";
+      description = ''
+        Default place where the latest generation of decrypt secrets
+        can be found.
+      '';
+    };
+
+    defaultSecretsMountPoint = lib.mkOption {
+      type = lib.types.str;
+      default = "%r/secrets.d";
+      description = ''
+        Default place where generations of decrypted secrets are stored.
+      '';
+    };
+
+    keepGenerations = lib.mkOption {
+      type = lib.types.ints.unsigned;
+      default = 1;
+      description = ''
+        Number of secrets generations to keep. Setting this to 0 disables pruning.
+      '';
+    };
+
+    log = lib.mkOption {
+      type = lib.types.listOf (lib.types.enum [ "keyImport" "secretChanges" ]);
+      default = [ "keyImport" "secretChanges" ];
+      description = "What to log";
+    };
+
+    age = {
+      keyFile = lib.mkOption {
+        type = lib.types.nullOr pathNotInStore;
+        default = null;
+        example = "/home/someuser/.age-key.txt";
+        description = ''
+          Path to age key file used for sops decryption.
+        '';
+      };
+
+      generateKey = lib.mkOption {
+        type = lib.types.bool;
+        default = false;
+        description = ''
+          Whether or not to generate the age key. If this
+          option is set to false, the key must already be
+          present at the specified location.
+        '';
+      };
+
+      sshKeyPaths = lib.mkOption {
+        type = lib.types.listOf lib.types.path;
+        default = [];
+        description = ''
+          Paths to ssh keys added as age keys during sops description.
+        '';
+      };
+    };
+
+    gnupg = {
+      home = lib.mkOption {
+        type = lib.types.nullOr lib.types.str;
+        default = null;
+        example = "/home/someuser/.gnupg";
+        description = ''
+          Path to gnupg database directory containing the key for decrypting the sops file.
+        '';
+      };
+
+      sshKeyPaths = lib.mkOption {
+        type = lib.types.listOf lib.types.path;
+        default = [];
+        description = ''
+          Path to ssh keys added as GPG keys during sops description.
+          This option must be explicitly unset if <literal>config.sops.gnupg.sshKeyPaths</literal> is set.
+        '';
+      };
+    };
+  };
+
+  config = lib.mkIf (cfg.secrets != {}) {
+    assertions = [{
+      assertion = cfg.gnupg.home != null || cfg.gnupg.sshKeyPaths != [] || cfg.age.keyFile != null || cfg.age.sshKeyPaths != [];
+      message = "No key source configured for sops. Either set services.openssh.enable or set sops.age.keyFile or sops.gnupg.home";
+    } {
+      assertion = !(cfg.gnupg.home != null && cfg.gnupg.sshKeyPaths != []);
+      message = "Exactly one of sops.gnupg.home and sops.gnupg.sshKeyPaths must be set";
+    }] ++ lib.optionals cfg.validateSopsFiles (
+      lib.concatLists (lib.mapAttrsToList (name: secret: [{
+        assertion = builtins.pathExists secret.sopsFile;
+        message = "Cannot find path '${secret.sopsFile}' set in sops.secrets.${lib.strings.escapeNixIdentifier name}.sopsFile";
+      } {
+        assertion =
+          builtins.isPath secret.sopsFile ||
+          (builtins.isString secret.sopsFile && lib.hasPrefix builtins.storeDir secret.sopsFile);
+        message = "'${secret.sopsFile}' is not in the Nix store. Either add it to the Nix store or set sops.validateSopsFiles to false";
+      }]) cfg.secrets)
+    );
+
+    # Darwin: load secrets once on login
+    launchd.daemons.sops-nix = {
+      serviceConfig = {
+        Program = script;
+        KeepAlive = false;
+        RunAtLoad = true;
+        StandardOutPath = cfg.logFile;
+        StandardErrorPath = cfg.logFile;
+      };
+    };
+
+    # [re]load secrets on home-manager activation
+    system.activationScripts.sops-nix.text = let
+        domain-target = "system";
+      in ''
+        /bin/launchctl bootout ${domain-target}/org.nixos.sops-nix && true
+        /bin/launchctl bootstrap ${domain-target} /Library/LaunchAgents/org.nixos.sops-nix.plist
+      '';
+  };
+}