From e4cbcab81c44367eb4680c58a4802de8830ef6a6 Mon Sep 17 00:00:00 2001
From: Rodrigo Arias Mallo <rodrigo.arias@bsc.es>
Date: Mon, 18 Sep 2023 18:53:40 +0200
Subject: [PATCH] Use a systemd mount directly for the nix store

Allows the LazyUnmount option and avoids the stage1 hack with
/nix//store.
---
 m/module/shared-nix-store.nix | 50 ++++++++++++++---------------------
 1 file changed, 20 insertions(+), 30 deletions(-)

diff --git a/m/module/shared-nix-store.nix b/m/module/shared-nix-store.nix
index 72eab92..95df035 100644
--- a/m/module/shared-nix-store.nix
+++ b/m/module/shared-nix-store.nix
@@ -38,37 +38,27 @@
   # Mount an overlay in /nix/store using as lower layer the NFS store and upper
   # layer the disk nix store. The destination is still the nix store in
   # /nix/store (confusing). We need rw access, as the daemon need to write the
-  # lock files to build derivations locally.
-  # HACK: Use /nix//store to prevent the overlay to be mounted on boot, see:
-  # https://github.com/NixOS/nixpkgs/blob/17a46d09ac123d0da3a26855bf3af7db01f9c751/nixos/lib/utils.nix#L14
-  fileSystems."/nix//store" = {
-    device = "overlay";
-    fsType = "overlay";
-    options = [
+  # lock files to build derivations locally. Use a systemd mount unit directly
+  # so we can specify the LazyUmount option and we avoid having it mounted
+  # in the stage1 before systemd.
+  systemd.mounts = [
+    {
+      what = "overlay";
+      type = "overlay";
+      where = "/nix/store";
       # We need the local-fs.target to be ready, so the network interfaces can
       # be configured to the network.target is reached. So make this a netdev
       # mount.
-      "_netdev"
-      "lowerdir=/mnt/hut-nix-store,upperdir=/nix/store,workdir=/mnt/nix-work"
-      "x-systemd.requires-mounts-for=/nix/store"
-      # We need to wait for the NFS mount
-      "x-systemd.requires-mounts-for=/mnt/hut-nix-store"
-    ];
-    depends = [ "/nix/store" "/mnt/hut-nix-store" "/mnt/nix-work" ];
-  };
-
-  # Maybe we should move it to a systemd mount, so we avoid the /nix//store
-  # hack. Example (not tested):
-  # systemd.mounts = [
-  #   {
-  #     what = "overlay";
-  #     type = "overlay";
-  #     where = "/nix/store";
-  #     options = "lowerdir=/mnt/hut-nix-store,upperdir=/nix/store,workdir=/mnt/nix-work";
-  #     description = "Overlay nix store mount";
-  #     requires = [ "hut-nix-store.mount" ];
-  #     after = [ "mnt-hut\\x2dnix\\x2dstore.mount" ];
-  #     before = [ "nix-daemon.service" ];
-  #   }
-  # ];
+      options = "_netdev,lowerdir=/mnt/hut-nix-store,upperdir=/nix/store,workdir=/mnt/nix-work";
+      description = "Overlay /nix/store mount";
+      mountConfig = {
+        LazyUnmount = true;
+      };
+      unitConfig = {
+        # We need to wait for the NFS mount
+        RequiresMountsFor = "/nix/store /mnt/hut-nix-store"; 
+        Before = "remote-fs.target";
+      };
+    }
+  ];
 }