imageBuilder: add btrfs support

lib/image-builder/default.nix

@@ -16,6 +16,7 @@ makeScope newScope (self:
     # Use stand-alone (outside of a disk image) is supported.
     fileSystem = {
       makeExt4 = callPackage ./makeExt4.nix {};
+      makeBtrfs = callPackage ./makeBtrfs.nix {};
       makeFAT32 = callPackage ./makeFAT32.nix {};
       # Specialization of `makeFAT32` with (1) filesystemType showing as ESP,
       # and (2) the name defaults to ESP.

lib/image-builder/makeBtrfs.nix

@@ -0,0 +1,15 @@
+{ lib, imageBuilder, libfaketime, btrfs-progs }:
+{ partitionID, ... }@args:
+imageBuilder.makeFilesystem (args // {
+  filesystemType = "btrfs";
+  blockSize = 4096; # dummy
+  nativeBuildInputs = [btrfs-progs];
+  copyPhase = ''
+    mkfs.btrfs \
+      -r . \
+      -L "$partName" \
+      -U "$partitionID" \
+      --shrink \
+      "$img"
+  '';
+})

lib/image-builder/makeGPT.nix

@@ -18,6 +18,7 @@ let
     "ext2"  = "0FC63DAF-8483-4772-8E79-3D69D8477DE4";
     "ext3"  = "0FC63DAF-8483-4772-8E79-3D69D8477DE4";
     "ext4"  = "0FC63DAF-8483-4772-8E79-3D69D8477DE4";
+    "btrfs" = "0FC63DAF-8483-4772-8E79-3D69D8477DE4";
   };
 in
 {

modules/generated-filesystems.nix

@@ -8,13 +8,14 @@ let
 
   filesystemFunctions = {
     "ext4" = pkgs.imageBuilder.fileSystem.makeExt4;
+    "btrfs" = pkgs.imageBuilder.fileSystem.makeBtrfs;
   };
 
   filesystemSubmodule =
     { name, config, ... }: {
       options = {
         type = lib.mkOption {
-          type = types.enum [ "ext4" ];
+          type = types.enum [ "ext4" "btrfs" ];
           description = ''
             Type of the generated filesystem.
           '';