Patch nix instead of using an override unit

m/module/shared-nix-store.nix

@@ -11,9 +11,17 @@
   # One solution is to make the nix-daemon.socket depend only on the socket
   # patch (which is already covered by ConditionPathIsReadWrite =
   # /nix/var/nix/daemon-socket), instead on the /nix/store.
-  systemd.sockets.nix-daemon.unitConfig = {
-    RequiresMountsFor = "/nix/var/nix/daemon-socket";
-  };
+  #
+  # Using systemd.sockets.nix-daemon.unitConfig.RequiresMountsFor =
+  # "/nix/var/nix/daemon-socket" doesn't work, as the the mount options get
+  # added by systemd when the override config is merged with the one that Nix
+  # provides:
+  #
+  # owl2% sudo systemctl show nix-daemon.socket | grep RequiresMountsFor
+  # RequiresMountsFor=/nix/store /nix/var/nix/daemon-socket/socket /nix/var/nix/daemon-socket
+  # 
+  # To fix this, the Nix package is patched to only depend on /nix/var instead.
+  # See ../../pkgs/overlay.nix for details.
 
   # Mount the hut nix store via NFS in read-only mode.
   fileSystems."/mnt/hut-nix-store" = {

pkgs/nix-socket.patch

@@ -0,0 +1,11 @@
+--- a/misc/systemd/nix-daemon.socket.in	1970-01-01 01:00:01.000000000 +0100
++++ b/misc/systemd/nix-daemon.socket.in	2023-09-18 17:53:32.351760208 +0200
+@@ -1,7 +1,7 @@
+ [Unit]
+ Description=Nix Daemon Socket
+ Before=multi-user.target
+-RequiresMountsFor=@storedir@
++RequiresMountsFor=@localstatedir@
+ ConditionPathIsReadWrite=@localstatedir@/nix/daemon-socket
+ 
+ [Socket]

pkgs/overlay.nix

@@ -32,4 +32,8 @@ final: prev:
     lua = prev.lua5_4;
     fmt = prev.fmt_8;
   }) ceph ceph-client;
+
+  nix = prev.nix.overrideAttrs (old: {
+    patches = old.patches ++ [ ./nix-socket.patch ];
+  });
 }