feat(copyparty): add read-only root volume

configs/nixos/AGENTS.md

@@ -46,6 +46,10 @@
 - `sops.secrets.<name>.owner` must name a user known on the decrypting host.
   Do not set the owner to a guest-only service user unless the host also declares that user.
   Use a host-visible owner plus a shared group mode when the guest service can read the decrypted secret file through its group.
+- A read-only guest bind mount cannot contain nested writable bind mounts unless their mount-point
+  directories already exist inside the read-only source.
+  For aggregate Copyparty volumes such as `/srv` with nested `/srv/inbox`, keep the guest bind mount
+  writable and enforce read-only behavior through Copyparty access plus service `ReadWritePaths`.
 - Pikachu deploys from the monorepo checkout copied to `~/dev`.
   Do not use the old `~/wrk/dotfiles` path for remote rebuild commands.
 - Do not hand-author `hardware-configuration.nix` for a host that has not been generated on that machine.

configs/nixos/guests/copyparty.nix

@@ -24,6 +24,12 @@ let
         description = "Copyparty access map from permission string to account names.";
       };
 
+      readOnly = lib.mkOption {
+        type = lib.types.bool;
+        default = false;
+        description = "Serve this Copyparty volume read-only.";
+      };
+
       flags = lib.mkOption {
         type = lib.types.listOf lib.types.str;
         default = [ ];
@@ -94,7 +100,7 @@ in
           inherit (cfg.reverseProxy) trustedSources;
         };
         volumes = lib.mapAttrs (_urlPath: volume: {
-          inherit (volume) path access flags;
+          inherit (volume) path access flags readOnly;
         }) cfg.volumes;
       };
 

configs/nixos/hosts/pikachu/default.nix

@@ -158,6 +158,12 @@
       trustedSources = [ "${registry.address.${config.networking.hostName}."reverse-proxy"}/32" ];
     };
     volumes = {
+      "/" = {
+        hostPath = "/pikachu/data/copyparty/root";
+        path = "/srv";
+        access.r = [ "alexion" ];
+        readOnly = true;
+      };
       "/inbox" = {
         hostPath = "/pikachu/__inbox";
         path = "/srv/inbox";

configs/nixos/modules/services/copyparty.nix

@@ -23,6 +23,12 @@ let
         description = "Copyparty access map from permission string to account names.";
       };
 
+      readOnly = lib.mkOption {
+        type = lib.types.bool;
+        default = false;
+        description = "Serve this volume read-only.";
+      };
+
       flags = lib.mkOption {
         type = lib.types.listOf lib.types.str;
         default = [ ];
@@ -196,7 +202,7 @@ in
           PrivateTmp = true;
           ProtectSystem = "strict";
           ProtectHome = true;
-          ReadWritePaths = [ cfg.stateDir ] ++ map (volume: volume.path) (lib.attrValues cfg.volumes);
+          ReadWritePaths = [ cfg.stateDir ] ++ map (volume: volume.path) (lib.filter (volume: !volume.readOnly) (lib.attrValues cfg.volumes));
         };
       };