nixos/lib, nixos/filesystems: Make fsBefore more stable, and add `dep…

…ends` option

nixos/lib/utils.nix

@@ -7,8 +7,20 @@ rec {
                      || elem fs.mountPoint [ "/" "/nix" "/nix/store" "/var" "/var/log" "/var/lib" "/etc" ];
 
   # Check whenever `b` depends on `a` as a fileSystem
-  fsBefore = a: b: a.mountPoint == b.device
-                || hasPrefix "${a.mountPoint}${optionalString (!(hasSuffix "/" a.mountPoint)) "/"}" b.mountPoint;
+  fsBefore = a: b:
+    let
+      normalisePath = path: "${path}${optionalString (!(hasSuffix "/" path)) "/"}";
+      normalise = mount: mount // { device = normalisePath mount.device;
+                                    mountPoint = normalisePath mount.mountPoint;
+                                    depends = map normalisePath mount.depends;
+                                  };
+
+      a' = normalise a;
+      b' = normalise b;
+
+    in hasPrefix a'.mountPoint b'.device
+    || hasPrefix a'.mountPoint b'.mountPoint
+    || any (dependency: hasPrefix a'.mountPoint dependency) b'.depends;
 
   # Escape a path according to the systemd rules, e.g. /dev/xyzzy
   # becomes dev-xyzzy.  FIXME: slow.

nixos/modules/tasks/filesystems.nix

@@ -56,6 +56,16 @@ let
         type = types.listOf nonEmptyStr;
       };
 
+      depends = mkOption {
+        default = [ ];
+        example = [ "/persist" ];
+        type = types.listOf nonEmptyStr;
+        description = ''
+          List of paths that must be mounted before this one. Note that you do
+          not need to add the value of .device or .mountPoint to this list.
+        '';
+      };
+
     };
 
     config = {