Got my NixOS flake to run under WSL!

profiles/wsl/nixos-wsl/syschdemd.sh

@@ -0,0 +1,78 @@
+#! @shell@
+
+set -e
+
+sw="/nix/var/nix/profiles/system/sw/bin"
+systemPath=$(${sw}/readlink -f /nix/var/nix/profiles/system)
+
+function start_systemd {
+    echo "Starting systemd..." >&2
+
+    PATH=/run/current-system/systemd/lib/systemd:@fsPackagesPath@ \
+        LOCALE_ARCHIVE=/run/current-system/sw/lib/locale/locale-archive \
+        @daemonize@/bin/daemonize /run/current-system/sw/bin/unshare -fp --mount-proc @systemdWrapper@
+
+    # Wait until systemd has been started to prevent a race condition from occuring
+    while ! $sw/pgrep -xf systemd | $sw/tail -n1 >/run/systemd.pid; do
+        $sw/sleep 1s
+    done
+
+    # Wait for systemd to start services
+    status=1
+    while [[ $status -gt 0 ]]; do
+        $sw/sleep 1
+        status=0
+        $sw/nsenter -t $(</run/systemd.pid) -p -m -- \
+            $sw/systemctl is-system-running -q --wait 2>/dev/null ||
+            status=$?
+    done
+}
+
+# Needs root to work
+if [[ $EUID -ne 0 ]]; then
+    echo "[ERROR] Requires root! :( Make sure the WSL default user is set to root" >&2
+    exit 1
+fi
+
+if [ ! -e "/run/current-system" ]; then
+    LANG="C.UTF-8" /nix/var/nix/profiles/system/activate
+fi
+
+if [ ! -e "/run/systemd.pid" ]; then
+    start_systemd
+fi
+
+userShell=$($sw/getent passwd @defaultUser@ | $sw/cut -d: -f7)
+if [[ $# -gt 0 ]]; then
+    # wsl seems to prefix with "-c"
+    shift
+    cmd="$@"
+else
+    cmd="$userShell"
+fi
+
+# Pass external environment but filter variables specific to root user.
+exportCmd="$(export -p | $sw/grep -vE ' (HOME|LOGNAME|SHELL|USER)='); export WSLPATH=\"$PATH\"; export INSIDE_NAMESPACE=true"
+
+if [[ -z "${INSIDE_NAMESPACE:-}" ]]; then
+
+    # Test whether systemd is still alive if it was started previously
+    if ! [ -d "/proc/$(</run/systemd.pid)" ]; then
+        # Clear systemd pid if the process is not alive anymore
+        $sw/rm /run/systemd.pid
+        start_systemd
+    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 @defaultUserHome@
+    fi
+
+    exec $sw/nsenter -t $(</run/systemd.pid) -p -m -- $sw/machinectl -q \
+        --uid=@defaultUser@ shell .host /bin/sh -c \
+        "cd \"$PWD\"; $exportCmd; source /etc/set-environment; exec $cmd"
+
+else
+    exec $cmd
+fi