feat: rewrite syschdemd

Mostly just a big pile of cleanups.

Important changes:
- refactor and clean up the code, fix all the shellcheck complaints
- stop trying to guess paths in scripts and wrap them correctly instead
- use nsfs instead of trying to figure out the right PID to copy namespaces from
- clean up $WSLPATH to remove extra impure entries
- don't try to restart systemd if it died
- make sure the store is read-only before we do anything, so systemd can't mess with it
- reformat shell scripts with shfmt

.github/workflows/main.yml

@@ -59,7 +59,7 @@ jobs:
       - uses: actions/download-artifact@v2
         with:
           name: installer
-          
+
       - name: Generate checksums
         run: |
           for x in *.tar.gz; do

flake.nix

@@ -40,16 +40,16 @@
           pkgs = import nixpkgs { inherit system; };
         in
         {
-          checks.check-format = pkgs.runCommand "check-format"
-            {
-              buildInputs = with pkgs; [ nixpkgs-fmt ];
-            } ''
-            nixpkgs-fmt --check ${./.}
-            mkdir $out # success
-          '';
+          checks = {
+            check-format = pkgs.runCommand "check-format" { nativeBuildInputs = with pkgs; [ nixpkgs-fmt shfmt ]; } ''
+              nixpkgs-fmt --check ${./.}
+              shfmt -i 2 -d ${./scripts}/*.sh
+              mkdir $out # success
+            '';
+          };
 
           devShell = pkgs.mkShell {
-            nativeBuildInputs = with pkgs; [ nixpkgs-fmt ];
+            nativeBuildInputs = with pkgs; [ nixpkgs-fmt shfmt ];
           };
         }
       );

modules/wsl-distro.nix

@@ -33,7 +33,7 @@ with builtins; with lib;
   config =
     let
       cfg = config.wsl;
-      syschdemd = import ../syschdemd.nix { inherit lib pkgs config; inherit (cfg) automountPath defaultUser; defaultUserHome = config.users.users.${cfg.defaultUser}.home; };
+      syschdemd = pkgs.callPackage ../scripts/syschdemd.nix { inherit (cfg) automountPath defaultUser; };
     in
     mkIf cfg.enable {
 

scripts/syschdemd.nix

@@ -0,0 +1,51 @@
+{ runCommand
+, makeWrapper
+, lib
+, coreutils
+, daemonize
+, glibc
+, gnugrep
+, systemd
+, util-linux
+, defaultUser
+, automountPath
+,
+}:
+let
+  mkWrappedScript =
+    { name
+    , src
+    , path
+    , ...
+    } @ args:
+    runCommand name ({ nativeBuildInputs = [ makeWrapper ]; } // args) ''
+      install -Dm755 ${src} $out/bin/${name}
+      patchShebangs $out/bin/${name}
+      substituteAllInPlace $out/bin/${name}
+      wrapProgram $out/bin/${name} --prefix PATH ':' ${lib.escapeShellArg path}
+    '';
+
+  wrapper = mkWrappedScript {
+    name = "nixos-wsl-systemd-wrapper";
+    src = ./wrapper.sh;
+    path = lib.makeSearchPath "" [
+      "/run/wrappers/bin" # mount
+      "${systemd}/lib/systemd" # systemd
+    ];
+  };
+in
+mkWrappedScript {
+  name = "syschdemd";
+  src = ./syschdemd.sh;
+  path = lib.makeBinPath [
+    "/run/wrappers" # mount
+    coreutils
+    daemonize
+    glibc # getent
+    gnugrep
+    systemd # machinectl
+    util-linux # nsenter
+    wrapper
+  ];
+  inherit defaultUser automountPath;
+}

scripts/syschdemd.sh

@@ -0,0 +1,138 @@
+#!/usr/bin/env bash
+set -euo pipefail
+
+[ "${NIXOS_WSL_DEBUG:-}" == "1" ] && set -x
+
+rundir="/run/nixos-wsl"
+pidfile="$rundir/unshare.pid"
+
+ensure_root() {
+  if [ $EUID -ne 0 ]; then
+    echo "[ERROR] Requires root! :( Make sure the WSL default user is set to root" >&2
+    exit 1
+  fi
+}
+
+activate() {
+  mount --bind -o ro /nix/store /nix/store
+
+  mount --bind "@automountPath@" "@automountPath@"
+  mount --make-rshared "@automountPath@"
+
+  LANG="C.UTF-8" /nix/var/nix/profiles/system/activate
+}
+
+create_rundir() {
+  if [ ! -d $rundir ]; then
+    mkdir -p $rundir/ns
+    touch $rundir/ns/{pid,mount}
+  fi
+}
+
+is_unshare_alive() {
+  [ -e $pidfile ] && [ -d "/proc/$(<$pidfile)" ]
+}
+
+run_in_namespace() {
+  nsenter \
+    --pid=$rundir/ns/pid \
+    --mount=$rundir/ns/mount \
+    -- "$@"
+}
+
+start_systemd() {
+  daemonize \
+    -o $rundir/stdout \
+    -e $rundir/stderr \
+    -l $rundir/systemd.lock \
+    -p $pidfile \
+    -E LOCALE_ARCHIVE=/run/current-system/sw/lib/locale/locale-archive \
+    "$(command -v unshare)" \
+    --fork \
+    --pid=$rundir/ns/pid \
+    --mount=$rundir/ns/mount \
+    --mount-proc=/proc \
+    nixos-wsl-systemd-wrapper
+
+  # Wait for systemd to start
+  while ! (run_in_namespace systemctl is-system-running -q --wait) &>/dev/null; do
+    sleep 1
+
+    if ! is_unshare_alive; then
+      echo "[ERROR] systemd startup failed!"
+
+      echo "[ERROR] stderr:"
+      cat $rundir/stderr
+
+      echo "[ERROR] stdout:"
+      cat $rundir/stdout
+
+      exit 1
+    fi
+  done
+}
+
+get_shell() {
+  getent passwd "$1" | cut -d: -f7
+}
+
+get_home() {
+  getent passwd "$1" | cut -d: -f6
+}
+
+is_in_container() {
+  [ "${INSIDE_NAMESPACE:-}" == "true" ]
+}
+
+clean_wslpath() {
+  echo "$PATH" | tr ':' '\n' | grep -E "^@automountPath@" | tr '\n' ':'
+}
+
+main() {
+  ensure_root
+
+  if [ ! -e "/run/current-system" ]; then
+    activate
+  fi
+
+  if [ ! -e "$rundir" ]; then
+    create_rundir
+  fi
+
+  if ! is_in_container && ! is_unshare_alive; then
+    start_systemd
+  fi
+
+  if [ $# -gt 0 ]; then
+    # wsl seems to prefix with "-c"
+    shift
+    command="$*"
+  else
+    command=$(get_shell @defaultUser@)
+  fi
+
+  # If we're executed from inside the container, e.g. sudo
+  if is_in_container; then
+    exec $command
+  fi
+
+  # If we are currently in /root, this is probably because the directory that WSL was started is inaccessible
+  # cd to the user's home to prevent a warning about permission being denied on /root
+  if [ "$PWD" == "/root" ]; then
+    cd "$(get_home @defaultUser@)"
+  fi
+
+  # Pass external environment but filter variables specific to root user.
+  exportCmd="$(export -p | grep -vE ' (HOME|LOGNAME|SHELL|USER)=')"
+
+  run_in_namespace \
+    machinectl \
+    --quiet \
+    --uid=@defaultUser@ \
+    --setenv=INSIDE_NAMESPACE=true \
+    --setenv=WSLPATH="$(clean_wslpath)" \
+    shell .host \
+    /bin/sh -c "cd \"$PWD\"; $exportCmd; source /etc/set-environment; exec $command"
+}
+
+main "$@"

scripts/wrapper.sh

@@ -0,0 +1,6 @@
+#!/usr/bin/env bash
+set -euxo pipefail
+
+mount -t binfmt_misc binfmt_misc /proc/sys/fs/binfmt_misc
+
+exec systemd