krebs.exim: validate config file syntax

krebs/3modules/exim.nix

@@ -37,7 +37,7 @@ in {
   };
   config = lib.mkIf cfg.enable {
     environment = {
-      etc."exim.conf".text = ''
+      etc."exim.conf".source = pkgs.writeEximConfig "exim.conf" ''
         exim_user = ${cfg.user.name}
         exim_group = ${cfg.group.name}
         exim_path = /var/setuid-wrappers/exim

krebs/5pkgs/builders.nix

@@ -56,6 +56,14 @@ rec {
     '';
   };
 
+  writeEximConfig = name: text: pkgs.runCommand name {
+    inherit text;
+    passAsFile = [ "text" ];
+  } ''
+    ${pkgs.exim}/bin/exim -C "$textPath" -bV >/dev/null
+    mv "$textPath" $out
+  '';
+
   writeNixFromCabal = name: path: pkgs.runCommand name {} ''
     ${pkgs.cabal2nix}/bin/cabal2nix ${path} > $out
   '';