refactor(nixos): derive ZFS host IDs

configs/nixos/hosts/pikachu/default.nix

@@ -60,7 +60,6 @@
 
   modules.zfs = {
     enable = true;
-    hostId = "2346edbd";
     pools.pikachu = { };
   };
 

configs/nixos/modules/zfs.nix

@@ -11,18 +11,6 @@ in
   options.modules.zfs = {
     enable = lib.mkEnableOption "importing durable ZFS pools that hold service state";
 
-    hostId = lib.mkOption {
-      type = lib.types.strMatching "[0-9a-f]{8}";
-      example = "deadbeef";
-      description = ''
-        This host's 8-hex-digit ZFS host id, written to `networking.hostId`. ZFS
-        stamps an imported pool with the importing host's id, so a pool still
-        held by another machine is refused rather than silently dual-mounted. It
-        must be fixed for the machine and distinct across machines that can reach
-        the same pool.
-      '';
-    };
-
     pools = lib.mkOption {
       type = lib.types.attrsOf (lib.types.attrsOf lib.types.path);
       default = { };
@@ -49,8 +37,10 @@ in
     # The ZFS stack in the kernel and boot, needed even where the root filesystem is another kind.
     boot.supportedFilesystems = [ "zfs" ];
 
-    # ZFS refuses to import a pool without a host id to stamp its ownership onto.
-    networking.hostId = cfg.hostId;
+    # Derive a stable 32-bit host ID from the configured hostname.
+    networking.hostId = builtins.substring 0 8 (
+      builtins.hashString "sha256" config.networking.hostName
+    );
 
     # The declared pools are imported at boot, distinct from any pool backing the root filesystem.
     boot.zfs.extraPools = lib.attrNames cfg.pools;