nixos/nspawn-container: hash name, remove assertion

the code comment is copied from the commit that added the assertion

https://github.com/NixOS/nixpkgs/commit/af0238c70efc80925c31d293da3bf8affca6437f

nixos/modules/virtualisation/nspawn-container/default.nix

@@ -81,34 +81,6 @@ in
           which is prohibited within the Nix build sandbox where the test is run.
         '';
       }
-      {
-        # Check every interface defined in allInterfaces.
-        # Containers try to create a bridge "${config.system.name}-${interfaceName}"
-        assertion = lib.all (
-          iface:
-          let
-            hostName = "${config.system.name}-${iface.name}";
-          in
-          lib.stringLength hostName <= 15
-        ) (lib.attrValues cfg.allInterfaces);
-
-        message =
-          let
-            offendingInterfaces = lib.filter (
-              iface: lib.stringLength "${config.system.name}-${iface.name}" > 15
-            ) (lib.attrValues cfg.allInterfaces);
-            offenderList = map (
-              i:
-              "${config.system.name}-${i.name} (${toString (lib.stringLength "${config.system.name}-${i.name}")} chars)"
-            ) offendingInterfaces;
-          in
-          ''
-            The following generated host interface names exceed the Linux 15-character limit:
-              ${lib.concatStringsSep "\n            " offenderList}
-
-            Please shorten 'config.system.name' or the interface names in 'virtualisation.interfaces'.
-          '';
-      }
     ];
 
     virtualisation.systemd-nspawn.options = [

nixos/modules/virtualisation/nspawn-container/run-nspawn/src/run_nspawn/__init__.py

@@ -1,6 +1,7 @@
 import contextlib
 import dataclasses
 import fcntl
+import hashlib
 import logging
 import os
 import signal
@@ -124,6 +125,13 @@ def mk_veth(
     vlan: int,
 ) -> typing.Generator[None, None, None]:
     host_intf_name = f"{container_name}-{container_intf_name}"
+    # If the names for systemd-nspawn containers are too long,
+    # the generated bridge interface names will surpass the
+    # kernel limit IFNAMSIZ (15 characters + '\0').
+    if len(host_intf_name) > 15:
+        hashed = hashlib.sha256(host_intf_name.encode()).hexdigest()[:6]
+        host_intf_name = f"{host_intf_name[:8]}-{hashed}"
+
     with ensure_vlan_bridge(vlan) as bridge_name:
         logger.info("creating interface %s", host_intf_name)
         run_ip(