WIP: temmie/userweb: use IPC to proxy sendmail requests out of sandbox

hosts/temmie/services/userweb/mail.nix

@@ -1,4 +1,4 @@
-{ config, lib, ... }:
+{ config, lib, pkgs, ... }:
 {
   services.postfix.enable = lib.mkForce false;
 
@@ -9,4 +9,26 @@
       remotes = "mail.pvv.ntnu.no smtp --port=25";
     };
   };
+
+  systemd.sockets.userweb-sendmail-sandbox-proxy = {
+    wantedBy = [ "sockets.target" ];
+    listenStreams = [ "/run/userweb-sendmail-sandbox-proxy.sock" ];
+    socketConfig = {
+      # Accept = true;
+      SocketUser = "httpd";
+      SocketGroup = "httpd"; # TODO: is wwwrun(54) in this group?
+      SocketMode = "0660";
+    };
+  };
+
+  systemd.services.userweb-sendmail-sandbox-proxy = {
+    serviceConfig = {
+      User = "root";
+      Group = "root";
+      Sockets = [
+        "userweb-sendmail-sandbox-proxy.socket"
+      ];
+      ExecStart = "${lib.getExe pkgs.hello}";
+    };
+  };
 }