nixos/stage-1-systemd: Handover between the systemds directly

nixos/modules/system/activation/top-level.nix

@@ -55,11 +55,15 @@ let
       substituteInPlace $out/dry-activate --subst-var out
       chmod u+x $out/activate $out/dry-activate
       unset activationScript dryActivationScript
-      ${pkgs.stdenv.shellDryRun} $out/activate
-      ${pkgs.stdenv.shellDryRun} $out/dry-activate
 
+      ${if config.boot.initrd.systemd.enable then ''
+        cp ${config.system.build.bootStage2} $out/prepare-root
+        substituteInPlace $out/prepare-root --subst-var-by systemConfig $out
+        ln -s "$systemd/lib/systemd/systemd" $out/init
+      '' else ''
         cp ${config.system.build.bootStage2} $out/init
         substituteInPlace $out/init --subst-var-by systemConfig $out
+      ''}
 
       ln -s ${config.system.build.etc}/etc $out/etc
       ln -s ${config.system.path} $out/sw

nixos/modules/system/boot/stage-2-init.sh

@@ -5,6 +5,7 @@ systemConfig=@systemConfig@
 export HOME=/root PATH="@path@"
 
 
+if [ "${IN_NIXOS_SYSTEMD_STAGE1:-}" != true ]; then
     # Process the kernel command line.
     for o in $(</proc/cmdline); do
         case $o in
@@ -28,6 +29,7 @@ echo
     if [ -z "$container" ]; then
         mount -n -o remount,rw none /
     fi
+fi
 
 
 # Likewise, stage 1 mounts /proc, /dev and /sys, so if we don't have a
@@ -39,6 +41,12 @@ if [ ! -e /proc/1 ]; then
         local options="$3"
         local fsType="$4"
 
+        # We must not overwrite this mount because it's bind-mounted
+        # from stage 1's /run
+        if [ "${IN_NIXOS_SYSTEMD_STAGE1:-}" = true ] && [ "${mountPoint}" = /run ]; then
+            return
+        fi
+
         install -m 0755 -d "$mountPoint"
         mount -n -t "$fsType" -o "$options" "$device" "$mountPoint"
     }
@@ -46,7 +54,11 @@ if [ ! -e /proc/1 ]; then
 fi
 
 
+if [ "${IN_NIXOS_SYSTEMD_STAGE1:-}" = true ]; then
+    echo "booting system configuration ${systemConfig}"
+else
     echo "booting system configuration $systemConfig" > /dev/kmsg
+fi
 
 
 # Make /nix/store a read-only bind mount to enforce immutability of
@@ -68,6 +80,7 @@ if [ -n "@readOnlyStore@" ]; then
 fi
 
 
+if [ "${IN_NIXOS_SYSTEMD_STAGE1:-}" != true ]; then
     # Use /etc/resolv.conf supplied by systemd-nspawn, if applicable.
     if [ -n "@useHostResolvConf@" ] && [ -e /etc/resolv.conf ]; then
         resolvconf -m 1000 -a host </etc/resolv.conf
@@ -87,6 +100,7 @@ else
         mkdir -p /run/log
         exec > >(tee -i /run/log/stage-2-init.log) 2>&1
     fi
+fi
 
 
 # Required by the activation script
@@ -116,6 +130,9 @@ ln -sfn "$systemConfig" /run/booted-system
 : >> /etc/machine-id
 
 
+# No need to restore the stdout/stderr streams we never redirected and
+# especially no need to start systemd
+if [ "${IN_NIXOS_SYSTEMD_STAGE1:-}" != true ]; then
     # Reset the logging file descriptors.
     exec 1>&$logOutFd 2>&$logErrFd
     exec {logOutFd}>&- {logErrFd}>&-
@@ -124,3 +141,4 @@ exec {logOutFd}>&- {logErrFd}>&-
     # Start systemd in a clean environment.
     echo "starting systemd..."
     exec @systemdExecutable@ "$@"
+fi

nixos/modules/system/boot/systemd/initrd.nix

@@ -445,6 +445,67 @@ in {
       '')];
       services."systemd-makefs@".unitConfig.IgnoreOnIsolate = true;
       services."systemd-growfs@".unitConfig.IgnoreOnIsolate = true;
+
+      services.initrd-nixos-activation = {
+        after = [ "initrd-fs.target" ];
+        requiredBy = [ "initrd.target" ];
+        unitConfig.AssertPathExists = "/etc/initrd-release";
+        serviceConfig.Type = "oneshot";
+        description = "NixOS Activation";
+
+        script = /* bash */ ''
+          set -uo pipefail
+          export PATH="/bin:${cfg.package.util-linux}/bin"
+
+          # Figure out what closure to boot
+          closure=
+          for o in $(< /proc/cmdline); do
+              case $o in
+                  init=*)
+                      IFS== read -r -a initParam <<< "$o"
+                      closure="$(dirname "''${initParam[1]}")"
+                      ;;
+              esac
+          done
+
+          # Sanity check
+          if [ -z "''${closure:-}" ]; then
+            echo 'No init= parameter on the kernel command line' >&2
+            exit 1
+          fi
+
+          # If we are not booting a NixOS closure (e.g. init=/bin/sh),
+          # we don't know what root to prepare so we don't do anything
+          if ! [ -x "/sysroot$closure/prepare-root" ]; then
+            echo "NEW_INIT=''${initParam[1]}" > /etc/switch-root.conf
+            echo "$closure does not look like a NixOS installation - not activating"
+            exit 0
+          fi
+          echo 'NEW_INIT=' > /etc/switch-root.conf
+
+
+          # We need to propagate /run for things like /run/booted-system
+          # and /run/current-system.
+          mkdir -p /sysroot/run
+          mount --bind /run /sysroot/run
+
+          # Initialize the system
+          export IN_NIXOS_SYSTEMD_STAGE1=true
+          exec chroot /sysroot $closure/prepare-root
+        '';
+      };
+
+      # This will either call systemctl with the new init as the last parameter (which
+      # is the case when not booting a NixOS system) or with an empty string, causing
+      # systemd to bypass its verification code that checks whether the next file is a systemd
+      # and using its compiled-in value
+      services.initrd-switch-root.serviceConfig = {
+        EnvironmentFile = "-/etc/switch-root.conf";
+        ExecStart = [
+          ""
+          ''systemctl --no-block switch-root /sysroot "''${NEW_INIT}"''
+        ];
+      };
     };
   };
 }

nixos/tests/systemd-initrd-simple.nix

@@ -14,6 +14,23 @@ import ./make-test-python.nix ({ lib, pkgs, ... }: {
   testScript = ''
     import subprocess
 
+    with subtest("handover to stage-2 systemd works"):
+        machine.wait_for_unit("multi-user.target")
+        machine.succeed("systemd-analyze | grep -q '(initrd)'")  # direct handover
+        machine.succeed("touch /testfile")  # / is writable
+        machine.fail("touch /nix/store/testfile")  # /nix/store is not writable
+        # Special filesystems are mounted by systemd
+        machine.succeed("[ -e /run/booted-system ]") # /run
+        machine.succeed("[ -e /sys/class ]") # /sys
+        machine.succeed("[ -e /dev/null ]") # /dev
+        machine.succeed("[ -e /proc/1 ]") # /proc
+        # stage-2-init mounted more special filesystems
+        machine.succeed("[ -e /dev/shm ]") # /dev/shm
+        machine.succeed("[ -e /dev/pts/ptmx ]") # /dev/pts
+        machine.succeed("[ -e /run/keys ]") # /run/keys
+
+
+    with subtest("growfs works"):
         oldAvail = machine.succeed("df --output=avail / | sed 1d")
         machine.shutdown()