nixos/evcc: support passing secrets with envsubst

and apply some newer best practices.

nixos/modules/services/home-automation/evcc.nix

@@ -1,10 +1,19 @@
 {
+  config,
   lib,
   pkgs,
-  config,
+  utils,
   ...
 }:
 let
+  inherit (lib)
+    getExe
+    mkEnableOption
+    mkIf
+    mkOption
+    mkPackageOption
+    ;
+
   cfg = config.services.evcc;
 
   format = pkgs.formats.yaml { };
@@ -17,27 +26,40 @@ in
   meta.maintainers = with lib.maintainers; [ hexa ];
 
   options.services.evcc = with lib.types; {
-    enable = lib.mkEnableOption "EVCC, the extensible EV Charge Controller with PV integration";
+    enable = mkEnableOption "EVCC, the extensible EV Charge Controller and Home Energy Management System";
 
-    extraArgs = lib.mkOption {
+    package = mkPackageOption pkgs "evcc" { };
+
+    extraArgs = mkOption {
       type = listOf str;
       default = [ ];
       description = ''
-        Extra arguments to pass to the evcc executable.
+        Extra arguments to pass to the `evcc` executable.
       '';
     };
 
-    settings = lib.mkOption {
+    environmentFile = mkOption {
+      type = nullOr path;
+      default = null;
+      example = /run/keys/evcc;
+      description = ''
+        File with environment variables to pass into the runtime environment.
+
+        Useful to pass secrets into the configuration, that get applied using `envsubst`.
+      '';
+    };
+
+    settings = mkOption {
       type = format.type;
       description = ''
-        evcc configuration as a Nix attribute set.
+        evcc configuration as a Nix attribute set. Supports substitution of secrets using `envsubst` from the `environmentFile`.
 
         Check for possible options in the sample [evcc.dist.yaml](https://github.com/andig/evcc/blob/${package.version}/evcc.dist.yaml).
       '';
     };
   };
 
-  config = lib.mkIf cfg.enable {
+  config = mkIf cfg.enable {
     systemd.services.evcc = {
       wants = [ "network-online.target" ];
       after = [
@@ -52,7 +74,21 @@ in
         getent
       ];
       serviceConfig = {
-        ExecStart = "${package}/bin/evcc --config ${configFile} ${lib.escapeShellArgs cfg.extraArgs}";
+        EnvironmentFile = lib.optionals (cfg.environmentFile != null) [ cfg.environmentFile ];
+        ExecStartPre = utils.escapeSystemdExecArgs [
+          (getExe pkgs.envsubst)
+          "-i"
+          configFile
+          "-o"
+          "/run/evcc/config.yaml"
+        ];
+        ExecStart = utils.escapeSystemdExecArgs (
+          [
+            (getExe cfg.package)
+            "--config=/run/evcc/config.yaml"
+          ]
+          ++ cfg.extraArgs
+        );
         CapabilityBoundingSet = [ "" ];
         DeviceAllow = [
           "char-ttyUSB"
@@ -61,14 +97,6 @@ in
         DynamicUser = true;
         LockPersonality = true;
         MemoryDenyWriteExecute = true;
-        Restart = "on-failure";
-        RestrictAddressFamilies = [
-          "AF_INET"
-          "AF_INET6"
-          "AF_UNIX"
-        ];
-        RestrictNamespaces = true;
-        RestrictRealtime = true;
         PrivateTmp = true;
         PrivateUsers = true;
         ProcSubset = "pid";
@@ -80,6 +108,15 @@ in
         ProtectKernelModules = true;
         ProtectKernelTunables = true;
         ProtectProc = "invisible";
+        Restart = "on-failure";
+        RestrictAddressFamilies = [
+          "AF_INET"
+          "AF_INET6"
+          "AF_UNIX"
+        ];
+        RestrictNamespaces = true;
+        RestrictRealtime = true;
+        RuntimeDirectory = "evcc";
         StateDirectory = "evcc";
         SystemCallArchitectures = "native";
         SystemCallFilter = [

nixos/tests/evcc.nix

@@ -1,5 +1,8 @@
 { pkgs, lib, ... }:
 
+let
+  port = "1234";
+in
 {
   name = "evcc";
   meta.maintainers = with lib.maintainers; [ hexa ];
@@ -8,11 +11,15 @@
     machine = {
       services.evcc = {
         enable = true;
+        # This is NOT a safe way to deal with secrets in production
+        environmentFile = pkgs.writeText "evcc-secrets" ''
+          PORT=${toString port}
+        '';
         settings = {
           network = {
             schema = "http";
             host = "localhost";
-            port = 7070;
+            port = "$PORT";
           };
 
           log = "info";
@@ -82,14 +89,14 @@
     start_all()
 
     machine.wait_for_unit("evcc.service")
-    machine.wait_for_open_port(7070)
+    machine.wait_for_open_port(${port})
 
     with subtest("Check package version propagates into frontend"):
         machine.fail(
-            "curl --fail http://localhost:7070 | grep '0.0.1-alpha'"
+            "curl --fail http://localhost:${port} | grep '0.0.1-alpha'"
         )
         machine.succeed(
-            "curl --fail http://localhost:7070 | grep '${pkgs.evcc.version}'"
+            "curl --fail http://localhost:${port} | grep '${pkgs.evcc.version}'"
         )
 
     with subtest("Check journal for errors"):