servo: relocate ext device to /mnt/impermanence/ext and fixup deps

hosts/servo/fs.nix

@@ -27,7 +27,7 @@
   };
 
   # slow, external storage (for archiving, etc)
-  fileSystems."/nix/persist/ext" = {
+  fileSystems."/mnt/impermanence/ext" = {
     device = "/dev/disk/by-uuid/aa272cff-0fcc-498e-a4cb-0d95fb60631b";
     fsType = "btrfs";
     options = [
@@ -37,9 +37,10 @@
   };
 
   sane.impermanence.stores."ext" = {
-    mountpt = "/nix/persist/ext/persist";
+    mountpt = "/mnt/impermanence/ext/persist";
     storeDescription = "external HDD storage";
   };
+  sane.fs."/mnt/impermanence/ext".mount = {};
 
   sane.impermanence.dirs.sys.plaintext = [
     # TODO: this is overly broad; only need media and share directories to be persisted

modules/fs/default.nix

@@ -195,6 +195,13 @@ let
     underlying = cfg."${device}";
     isBind = opt.mount.bind != null;
     ifBind = lib.mkIf isBind;
+    # before mounting:
+    # - create the target directory
+    # - prepare the source directory -- assuming it's not an external device
+    # - satisfy any user-specified prerequisites ("depends")
+    requires = [ opt.generated.unit ]
+      ++ (if lib.hasPrefix "/dev/disk/" device then [] else [ underlying.unit ])
+      ++ opt.mount.depends;
   in {
     fileSystems."${path}" = {
       device = ifBind opt.mount.bind;
@@ -206,13 +213,8 @@ let
           "nofail"
           # x-systemd options documented here:
           # - <https://www.freedesktop.org/software/systemd/man/systemd.mount.html>
-          # we can't mount this until after the underlying path is prepared.
-          # if the underlying path disappears, this mount will be stopped.
-          "x-systemd.requires=${underlying.unit}"
-          # the mount depends on its target directory being prepared
-          "x-systemd.requires=${opt.generated.unit}"
         ]
-        ++ (builtins.map (unit: "x-systemd.requires=${unit}") opt.mount.depends)
+        ++ (builtins.map (unit: "x-systemd.requires=${unit}") requires)
         ++ (builtins.map (unit: "x-systemd.before=${unit}") opt.wantedBeforeBy)
         ++ (builtins.map (unit: "x-systemd.wanted-by=${unit}") (opt.wantedBy ++ opt.wantedBeforeBy));
       noCheck = ifBind true;

modules/impermanence/stores/plaintext.nix

@@ -6,4 +6,6 @@ in lib.mkIf cfg.enable {
   sane.impermanence.stores."plaintext" = {
     mountpt = "/nix/persist";
   };
+  # TODO: needed?
+  # sane.fs."/nix".mount = {};
 }

modules/impermanence/stores/private.nix

@@ -37,8 +37,9 @@ lib.mkIf config.sane.impermanence.enable
     noCheck = true;
   };
 
-  # let sane.fs know about the endpoints
+  # let sane.fs know about the mount
   sane.fs."/home/colin/private".mount = {};
+  # it also needs to know that the underlying device is an ordinary folder
   sane.fs."/nix/persist/home/colin/private".dir = {};
 
   # TODO: could add this *specifically* to the .mount file for the encrypted fs?