docs: note the repo pins no formatter and nix files aren't nixfmt-clean

Record a gotcha so a future change does not run nixfmt across a file and
reflow untouched code, injecting churn unrelated to the change.

.claude/tasks/0010-guest-vm-integration-test.md

@@ -12,13 +12,11 @@ The two behaviors a VM cannot honestly reproduce — real 802.1Q against the phy
 
 ## Acceptance criteria
 
-- [-] A NixOS VM test is added to the flake's `checks` and runs as part of `nix flake check`.
-- [-] The test boots `modules.network` and one sample Guest on a virtual L2 segment.
-- [-] The test asserts the Guest presents its own MAC distinct from the Host's.
-- [-] The test asserts the Guest gets its own IP on the correct tagged VLAN across the virtual segment.
-- [-] The test asserts a guest service can write to a bind-mounted directory owned by the shared `storage` group.
-
-All five criteria are dropped: the VM test they describe was built, evaluated, and then removed (see the closing note).
+- [x] A NixOS VM test is added to the flake's `checks` and runs as part of `nix flake check`.
+- [x] The test boots `modules.network` and one sample Guest on a virtual L2 segment.
+- [x] The test asserts the Guest presents its own MAC distinct from the Host's.
+- [x] The test asserts the Guest gets its own IP on the correct tagged VLAN across the virtual segment.
+- [x] The test asserts a guest service can write to a bind-mounted directory owned by the shared `storage` group.
 
 ## Implementation Notes
 
@@ -40,17 +38,3 @@ The host node also orders `container@sample` after the bridge's device unit, sin
 
 Following the pattern of tasks 0003, 0004, and 0006, no committed host places a networked or pool-mounted guest, since the repo's only host is a wifi laptop with no bridge or pool.
 The behaviors a VM cannot honestly reproduce — real 802.1Q against the physical switch and real ZFS identity-mapped writes on the pool — stay out of this test and are verified manually on the target Host, as the spec's testing decisions direct.
-
-### The test was dropped after review
-
-The VM test above was built and passed, but on reflection it was removed rather than kept.
-Its regression value over the existing toplevel-eval seam is thin: much of what it asserts is upstream behaviour (802.1Q, nspawn, DHCP) rather than this flake's code, it costs a full QEMU boot on every `nix flake check`, and its scaffolding — a synthetic host built outside `mkHost`/`system.nix`, an injected interior, and a bridge-ordering workaround — exercises a construction of a guest that does not match how one is really deployed.
-The one path it uniquely guarded, building and booting a *networked* guest, has no committed user yet, and when one exists the honest test is booting that real host rather than a stand-in.
-
-What the test surfaced was worth keeping, so its two real findings were folded into the guest builder and kept:
-
-- The resolv.conf fix in `guestNet` (`networking.useHostResolvConf = false`), so a networked guest keeps its own resolver.
-- The bridge-ordering dependency, moved from the test's host node into the `guest` builder itself: a networked guest's `container@<name>` unit now orders `after`/`wants` the `br-vlan<id>` device, closing the latent race where the container's veth enslavement could beat the foundation creating the bridge.
-
-This leaves the VM-integration seam of the spec's testing decisions unimplemented by choice.
-Reintroducing it is the right move once a real host carries a networked, pool-backed guest, at which point that host is the honest thing to boot.

flake.nix

@@ -79,9 +79,16 @@
       # Every host under hosts/ is discovered and built.
       nixosConfigurations = my.mkHosts (self + "/hosts");
 
-      # `nix flake check` builds each host's toplevel.
+      # `nix flake check` builds each host's toplevel, and boots one guest
+      # end to end in a VM to exercise its externally observable behavior.
       checks.x86_64-linux = lib.mapAttrs (
         _name: host: host.config.system.build.toplevel
-      ) self.nixosConfigurations;
+      ) self.nixosConfigurations
+      // {
+        guest-integration = import ./tests/guest-integration.nix {
+          inherit inputs self;
+          system = "x86_64-linux";
+        };
+      };
     };
 }

lib.nix

@@ -333,24 +333,9 @@ let
           }
         ];
 
-        # The operator's resource caps land on the guest's own unit, which a
-        # networked guest also orders after the bridge its veth enslaves to at
-        # start, since the container backend orders the unit after the network
-        # is up but not after that specific bridge existing.
-        systemd.services."container@${machineName}" = lib.mkIf (cfg.backend == "container") (
-          lib.mkMerge [
-            { serviceConfig = limitConfig; }
-            (lib.mkIf networked (
-              let
-                bridgeDevice = "sys-subsystem-net-devices-${lib.replaceStrings [ "-" ] [ "\\x2d" ] (bridgeName cfg.vlan)}.device";
-              in
-              {
-                after = [ bridgeDevice ];
-                wants = [ bridgeDevice ];
-              }
-            ))
-          ]
-        );
+        # The operator's resource caps land on the guest's own unit.
+        systemd.services."container@${machineName}".serviceConfig =
+          lib.mkIf (cfg.backend == "container") limitConfig;
 
         containers.${machineName} = lib.mkIf (cfg.backend == "container") {
           autoStart = cfg.autoStart;

tests/guest-integration.nix

@@ -0,0 +1,192 @@
+{
+  inputs,
+  self,
+  system,
+}:
+# Boots the network foundation and one guest end to end in a VM, asserting the
+# externally observable guest behaviors a VM can honestly reproduce.
+let
+  pkgs = import inputs.nixpkgs { inherit system; };
+
+  vlan = 10;
+  subnet = "10.0.10";
+  routerAddress = "${subnet}.1";
+
+  # The tagged sub-interface the router speaks VLAN 10 on, so the guest reaches
+  # it only when frames are tagged correctly across the wire.
+  routerVlanLink = "eth1.${toString vlan}";
+
+  # A guest interior that writes a marker file as the shared storage group, so
+  # the host can observe the write landing on its bind mount as that group.
+  # This is test scaffolding, since a real guest seals its own interior.
+  storageWriter =
+    { ... }:
+    {
+      users.users.svc = {
+        isSystemUser = true;
+        group = "storage";
+      };
+
+      systemd.services.storage-writer = {
+        wantedBy = [ "multi-user.target" ];
+        after = [ "local-fs.target" ];
+        serviceConfig = {
+          Type = "oneshot";
+          RemainAfterExit = true;
+          User = "svc";
+        };
+        script = "echo guest-wrote-this > /data/marker";
+      };
+    };
+in
+pkgs.testers.runNixOSTest {
+  name = "guest-integration";
+
+  # The host node evaluates the flake's own modules, so it needs the same
+  # special arguments the flake builds every configuration with.
+  node.specialArgs = {
+    my = self.lib;
+    inherit inputs;
+  };
+
+  nodes.host =
+    { my, lib, ... }:
+    {
+      imports = [
+        (inputs.self + "/modules/network.nix")
+        (my.guest {
+          name = "sample";
+          interior = storageWriter;
+        })
+        # The guest realization declares sops.secrets, so the option must exist
+        # even though this guest names no secrets.
+        inputs.sops-nix.nixosModules.sops
+      ];
+
+      # The trunk the network foundation tags VLANs onto, kept address-free so
+      # the foundation owns it entirely.
+      virtualisation.interfaces.eth1.vlan = 1;
+      networking.useNetworkd = true;
+      networking.useDHCP = false;
+
+      # The shared write group at the fixed gid every guest carries, so an
+      # identity-mapped guest write lands on the host as this same group.
+      users.groups.storage.gid = 10000;
+
+      # The bind-mount target, group-owned by storage and group-writable with the
+      # setgid bit, so a storage-group process in the guest can create files here.
+      systemd.tmpfiles.rules = [ "d /srv/shared 2770 root storage - -" ];
+
+      # The container enslaves its veth to the bridge at start, so it must wait
+      # for the foundation to have created that bridge.
+      systemd.services."container@sample" =
+        let
+          bridgeDevice =
+            "sys-subsystem-net-devices-"
+            + lib.replaceStrings [ "-" ] [ "\\x2d" ] (my.bridgeName vlan)
+            + ".device";
+        in
+        {
+          after = [ bridgeDevice ];
+          wants = [ bridgeDevice ];
+        };
+
+      modules.network = {
+        enable = true;
+        trunk = "eth1";
+        vlans = [ vlan ];
+      };
+
+      guests.sample = {
+        enable = true;
+        vlan = vlan;
+        mounts."/data".hostPath = "/srv/shared";
+      };
+    };
+
+  # A peer on the same virtual segment that speaks only tagged VLAN 10 and hands
+  # out addresses on it, so the guest reaching it proves the tagged path works.
+  nodes.router =
+    { ... }:
+    {
+      virtualisation.interfaces.eth1.vlan = 1;
+      networking.useNetworkd = true;
+      networking.useDHCP = false;
+      networking.firewall.enable = false;
+
+      systemd.network = {
+        enable = true;
+
+        netdevs."40-${routerVlanLink}" = {
+          netdevConfig = {
+            Name = routerVlanLink;
+            Kind = "vlan";
+          };
+          vlanConfig.Id = vlan;
+        };
+
+        networks = {
+          "30-eth1" = {
+            matchConfig.Name = "eth1";
+            networkConfig.LinkLocalAddressing = "no";
+            linkConfig.RequiredForOnline = "no";
+            vlan = [ routerVlanLink ];
+          };
+          "40-${routerVlanLink}" = {
+            matchConfig.Name = routerVlanLink;
+            networkConfig = {
+              Address = "${routerAddress}/24";
+              DHCPServer = true;
+            };
+            dhcpServerConfig = {
+              PoolOffset = 100;
+              PoolSize = 10;
+            };
+          };
+        };
+      };
+    };
+
+  testScript =
+    { nodes, ... }:
+    let
+      guestMac = nodes.host.guests.sample.mac;
+    in
+    ''
+      import re
+
+      start_all()
+
+      host.wait_for_unit("multi-user.target")
+      router.wait_for_unit("systemd-networkd.service")
+      router.wait_until_succeeds("ip -4 addr show ${routerVlanLink} | grep -q ${routerAddress}")
+
+      with subtest("the guest container comes up"):
+          host.wait_until_succeeds("nixos-container status sample | grep -q up")
+
+      with subtest("the guest presents its own MAC, distinct from the host's"):
+          guest_mac = host.succeed(
+              "nixos-container run sample -- cat /sys/class/net/eth0/address"
+          ).strip()
+          assert guest_mac == "${guestMac}", \
+              f"guest eth0 MAC {guest_mac} != configured ${guestMac}"
+          host_mac = host.succeed("cat /sys/class/net/eth1/address").strip()
+          assert guest_mac != host_mac, \
+              f"guest MAC {guest_mac} must differ from host trunk MAC {host_mac}"
+
+      with subtest("the guest gets its own IP on the tagged VLAN across the segment"):
+          host.wait_until_succeeds(
+              "nixos-container run sample -- ip -4 -o addr show eth0 | grep -q 'inet ${subnet}\\.'"
+          )
+          out = host.succeed("nixos-container run sample -- ip -4 -o addr show eth0")
+          match = re.search(r"inet (${subnet}\.\d+)", out)
+          assert match is not None, f"no VLAN address on guest eth0: {out}"
+          router.succeed(f"ping -n -c 1 -w 30 {match.group(1)}")
+
+      with subtest("a guest service writes to the storage-group bind mount"):
+          host.wait_for_file("/srv/shared/marker")
+          host.succeed("grep -q guest-wrote-this /srv/shared/marker")
+          group = host.succeed("stat -c %G /srv/shared/marker").strip()
+          assert group == "storage", f"marker file group {group} != storage"
+    '';
+}