Add filesystem option to automatically grow to the maximum size

This is primarily for EC2 and other cloud environments, where the disk
may be bigger than the original image.

nixos/modules/tasks/filesystems.nix

@@ -7,7 +7,7 @@ let
 
   fileSystems = attrValues config.fileSystems;
 
-  prioOption = prio: optionalString (prio !=null) " pri=${toString prio}";
+  prioOption = prio: optionalString (prio != null) " pri=${toString prio}";
 
   fileSystemOpts = { name, config, ... }: {
 
@@ -43,7 +43,7 @@ let
       options = mkOption {
         default = "defaults";
         example = "data=journal";
-        type = types.commas;
+        type = types.commas; # FIXME: should be a list
         description = "Options used to mount the file system.";
       };
 
@@ -58,6 +58,17 @@ let
         '';
       };
 
+      autoResize = mkOption {
+        default = false;
+        type = types.bool;
+        description = ''
+          If set, the filesystem is grown to its maximum size before
+          being mounted. (This is typically the size of the containing
+          partition.) This is currently only supported for ext2/3/4
+          filesystems that are mounted during early boot.
+        '';
+      };
+
       noCheck = mkOption {
         default = false;
         type = types.bool;
@@ -69,6 +80,7 @@ let
     config = {
       mountPoint = mkDefault name;
       device = mkIf (config.fsType == "tmpfs") (mkDefault config.fsType);
+      options = mkIf config.autoResize "x-nixos.autoresize";
     };
 
   };