From a6e6c85f06395a92062eb31753a8dd8f3b7e733c Mon Sep 17 00:00:00 2001
From: "William A. Kennington III" <william@wkennington.com>
Date: Tue, 29 Apr 2014 22:53:11 -0500
Subject: [PATCH] grub: Add support for detecting btrfs subvolumes

---
 nixos/modules/system/boot/loader/grub/grub.nix        | 2 +-
 nixos/modules/system/boot/loader/grub/install-grub.pl | 7 +++++++
 2 files changed, 8 insertions(+), 1 deletion(-)

diff --git a/nixos/modules/system/boot/loader/grub/grub.nix b/nixos/modules/system/boot/loader/grub/grub.nix
index bc19dd8c62fa..581b76f8fb21 100644
--- a/nixos/modules/system/boot/loader/grub/grub.nix
+++ b/nixos/modules/system/boot/loader/grub/grub.nix
@@ -28,7 +28,7 @@ let
         extraEntriesBeforeNixOS extraPrepareConfig configurationLimit copyKernels timeout
         default devices fsIdentifier;
       path = (makeSearchPath "bin" [
-        pkgs.coreutils pkgs.gnused pkgs.gnugrep pkgs.findutils pkgs.diffutils
+        pkgs.coreutils pkgs.gnused pkgs.gnugrep pkgs.findutils pkgs.diffutils pkgs.btrfsProgs
       ]) + ":" + (makeSearchPath "sbin" [
         pkgs.mdadm pkgs.utillinux
       ]);
diff --git a/nixos/modules/system/boot/loader/grub/install-grub.pl b/nixos/modules/system/boot/loader/grub/install-grub.pl
index 2615bc3ae11e..814ae7e05f7c 100644
--- a/nixos/modules/system/boot/loader/grub/install-grub.pl
+++ b/nixos/modules/system/boot/loader/grub/install-grub.pl
@@ -111,6 +111,13 @@ sub GrubFs {
                     die "invalid fs identifier type\n";
                 }
             }
+            if ($fs->type eq "btrfs") {
+                $subvol = `mount | sed -n 's,^$fs->device on .*subvol=\([^,)]*\).*$,\1,p'`
+                if ($subvol eq "") {
+                    $subvol = `btrfs subvol get-default $fs->mount | sed -n 's,^.*path \([^ ]*\) .*$,\1,p'`
+                }
+                $path = "/$subvol";
+            }
         }
         if (not $search eq "") {
             $search = "search --set=drive$driveid " . $search;