.claude/tasks/0006-guest-storage-placement.md

@@ -0,0 +1,35 @@
+---
+spec: guests
+blocked-by: [0002-guest-walking-skeleton, 0005-storage-zfs-foundation]
+---
+
+## What to build
+
+The Host-side placement that lets a Guest mount shared pool paths at any granularity and write to them without permission errors.
+A Host sets `mounts`, a map of guest path to host path, each with a per-mount `readOnly`, defaulting to read-write to match the migration reality that services must write to pools.
+Each entry is realized as a bind mount of the nested container, so a Guest sees exactly the data it should — a single folder or a whole pool, read-only or read-write.
+Because the container backend uses identity mapping, a guest service that writes as the shared `storage` group lands on the pool as that same group, which is the entire "no permission errors" mechanism and the privileged-container equivalent the operator already trusts from Proxmox.
+
+## Acceptance criteria
+
+- [x] A Host setting `guests.<path>.mounts` bind-mounts each host path at its guest path inside the nested container.
+- [x] Each mount honours its per-mount `readOnly`, defaulting to read-write.
+- [x] Identity mapping is configured so a guest service writing as the `storage` group lands on the host pool as that same group with no permission error.
+- [x] A Host with a Guest that has pool mounts builds, and the resolved bind mounts are verifiable by `nix eval`. Verified by ad-hoc mounts on the sample guest; no host commits mounts (see notes).
+
+## Implementation Notes
+
+`mounts` is a placement option on the `guest` builder: an attribute set keyed by the guest-interior path, each value carrying `hostPath` and a `readOnly` flag that defaults to `false`.
+It is realized as the nested container's `bindMounts`, where the container option's `mountPoint` defaults to the attribute key, so the guest path is stated once as the key.
+The upstream `bindMounts` `isReadOnly` defaults to read-only, so it is driven explicitly from `readOnly` to make read-write the default here, matching the migration reality that services must write to their pools.
+
+Identity mapping is pinned with `privateUsers = "no"`, which runs the container in the host's uid and gid space one to one.
+That is the current NixOS default, but the option documents `"pick"` (a shifting map that would break pool writes) as its recommended value, so the property is pinned rather than left to a default that may drift.
+With this mapping and the shared `storage` group from the storage foundation (identical gid on host and guest), a guest process writing as that group lands on a bind-mounted pool as the same group.
+The write itself is verified manually on the target host per the spec's testing decisions, since a real identity-mapped ZFS write cannot be reproduced in the build.
+
+No host commits pool mounts.
+The repo's only host, `neogaia`, is a laptop with no ZFS pools, so a committed mount would point at a host path that does not exist and would fail the bind at container start.
+This mirrors the storage and networking foundations (tasks 0005 and 0003), which verify by ad-hoc enablement rather than committing a placement a host cannot honestly carry.
+The realization was verified through `nixosConfigurations.neogaia.extendModules`, setting two mounts on the sample guest: the resolved `containers.sample.bindMounts` carried the right `hostPath`, `mountPoint`, and per-mount `isReadOnly` (read-write and read-only), `privateUsers` resolved to `"no"`, the `storage` gid was identical on host and guest interior, and the full toplevel built.
+`nix flake check` passes on the committed tree, where the sample guest declares no mounts.

lib.nix

@@ -160,6 +160,43 @@ let
             guest takes its address by DHCP, keeping IP management at the router.
           '';
         };
+        mounts = lib.mkOption {
+          type = lib.types.attrsOf (
+            lib.types.submodule {
+              options = {
+                hostPath = lib.mkOption {
+                  type = lib.types.str;
+                  example = "/srv/media";
+                  description = "The path on the host bind-mounted into the guest.";
+                };
+                readOnly = lib.mkOption {
+                  type = lib.types.bool;
+                  default = false;
+                  description = ''
+                    Mount the path read-only. Read-write by default, since a
+                    service must write to the pool data it owns.
+                  '';
+                };
+              };
+            }
+          );
+          default = { };
+          example = lib.literalExpression ''
+            {
+              "/data/media" = { hostPath = "/srv/media"; };
+              "/data/config" = {
+                hostPath = "/srv/config/jellyfin";
+                readOnly = true;
+              };
+            }
+          '';
+          description = ''
+            Host paths bind-mounted into the guest, keyed by the path they appear
+            at inside the guest, so a guest sees exactly the data it should at any
+            granularity — a single folder or a whole pool. Each mount is
+            read-write unless `readOnly` is set.
+          '';
+        };
       };
 
       config = lib.mkIf cfg.enable {
@@ -189,6 +226,16 @@ let
           # a first-class L2 citizen on that segment.
           hostBridge = lib.mkIf networked (bridgeName cfg.vlan);
 
+          # The container shares the host's uid and gid space one to one.
+          # A guest process writing as the shared storage group then lands on a bind-mounted pool as that same group, with no permission juggling.
+          # A private-user mapping would shift the ids and reintroduce those errors, so it stays off.
+          privateUsers = lib.mkDefault "no";
+
+          bindMounts = lib.mapAttrs (_guestPath: m: {
+            inherit (m) hostPath;
+            isReadOnly = m.readOnly;
+          }) cfg.mounts;
+
           inherit specialArgs;
 
           config = {