feat: add a recovery shell

2 files changed, 39 insertions, 0 deletions

diff --git a/README.md b/README.md
index 45a948b..e2dad8f 100644
--- a/README.md
+++ b/README.md
@@ -40,6 +40,26 @@ If you want to make NixOS your default distribution, you can do so with
 wsl -s NixOS
 ```
 
+## Troubleshooting
+
+A recovery shell can be started with
+
+```sh
+wsl -d NixOS --system --user root -- /mnt/wslg/distro/bin/nixos-wsl-recovery
+```
+
+This will load the WSL "system" distribution, activate your configuration,
+then chroot into your NixOS system, similar to what `nixos-enter` would do
+on a normal NixOS install.
+
+You can choose an older generation to load with
+
+```sh
+wsl -d NixOS --system --user root -- /mnt/wslg/distro/bin/nixos-wsl-recovery --system /nix/var/nix/profiles/system-42-link
+```
+
+(note that the path is relative to the new root)
+
 ## Building your own system tarball
 
 This requires access to a system that already has Nix installed. Please refer to the [Nix installation guide](https://nixos.org/guides/install-nix.html) if that\'s not the case.
diff --git a/modules/wsl-distro.nix b/modules/wsl-distro.nix
index ac763d7..0c5a15f 100644
--- a/modules/wsl-distro.nix
+++ b/modules/wsl-distro.nix
@@ -8,6 +8,22 @@ let
     exec ${pkgs.bashInteractive}/bin/sh "$@"
   '';
 
+  nixos-enter' = config.system.build.nixos-enter.overrideAttrs (_: {
+    runtimeShell = "/bin/bash";
+  });
+
+  recovery = pkgs.writeScriptBin "nixos-wsl-recovery" ''
+    #! /bin/sh
+    if [ -f /etc/NIXOS ]; then
+      echo "nixos-wsl-recovery should only be run from the WSL system distribution."
+      echo "Example:"
+      echo "    wsl --system --distribution NixOS --user root -- /nix/var/nix/profiles/system/bin/nixos-wsl-recovery"
+      exit 1
+    fi
+    mount -o remount,rw /mnt/wslg/distro
+    exec /mnt/wslg/distro/${nixos-enter'}/bin/nixos-enter --root /mnt/wslg/distro "$@"
+  '';
+
   cfg = config.wsl;
 in
 {
@@ -125,6 +141,9 @@ in
               ln -sf /init /bin/wslpath
               ln -sf ${cfg.binShPkg}/bin/sh /bin/sh
               ln -sf ${pkgs.util-linux}/bin/mount /bin/mount
+
+              # needs to be a copy, not a symlink, to be executable from outside
+              cp -f ${recovery}/bin/nixos-wsl-recovery /bin/nixos-wsl-recovery
             '');
             update-entrypoint.text = ''
               mkdir -p /nix/nixos-wsl