nixos/nix-containers: bind container units to machined and dbus

[pennae]

Motivation for this change

fix #109695 (containers don't stop properly during system shutdown). not sure whether this is the best way to do it, but can't think of something better that doesn't require moving away from machinectl commands again

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS linux)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Ensured that relevant documentation is up to date
• Fits CONTRIBUTING.md.

[pennae]

looked at this a bit more due to "shutdown during startup: issues: when a container is stopped before it has reached the in-container systemd it won't shut down properly. setting --kill-signal=SIGRTMIN+3 on nspawn should be sufficient to solve both that and what we tried to solve here, maybe without needing a stop command on the container unit at all. will have to test this some more.

[pennae]

investigated some more. it seems that we'll want this instead to fix both problems:

diff --git a/nixos/modules/system/boot/stage-2-init.sh b/nixos/modules/system/boot/stage-2-init.sh
index 50ee0b8841e..3f87c5c920c 100644
--- a/nixos/modules/system/boot/stage-2-init.sh
+++ b/nixos/modules/system/boot/stage-2-init.sh
@@ -1,5 +1,8 @@
 #! @shell@
 
+# Exit early if we're a container and asked to shut down.
+trap "exit 0" SIGRTMIN+3
+
 systemConfig=@systemConfig@
 
 export HOME=/root PATH="@path@"
diff --git a/nixos/modules/virtualisation/nixos-containers.nix b/nixos/modules/virtualisation/nixos-containers.nix
index f15d5875841..b7074e4460f 100644
--- a/nixos/modules/virtualisation/nixos-containers.nix
+++ b/nixos/modules/virtualisation/nixos-containers.nix
@@ -35,6 +35,9 @@ let
       ''
         #! ${pkgs.runtimeShell} -e
 
+        # Exit early if we're asked to shut down.
+        trap "exit 0" SIGRTMIN+3
+
         # Initialise the container side of the veth pair.
         if [ -n "$HOST_ADDRESS" ]   || [ -n "$HOST_ADDRESS6" ]  ||
            [ -n "$LOCAL_ADDRESS" ]  || [ -n "$LOCAL_ADDRESS6" ] ||
@@ -127,12 +130,16 @@ let
       ''}
 
       # Run systemd-nspawn without startup notification (we'll
-      # wait for the container systemd to signal readiness).
+      # wait for the container systemd to signal readiness)
+      # Kill signal handling means systemd-nspawn will pass a system-halt signal
+      # to the container systemd when it receives SIGTERM for container shutdown;
+      # containerInit and stage2 have to handle this as well.
       exec ${config.systemd.package}/bin/systemd-nspawn \
         --keep-unit \
         -M "$INSTANCE" -D "$root" $extraFlags \
         $EXTRA_NSPAWN_FLAGS \
         --notify-ready=yes \
+        --kill-signal=SIGRTMIN+3 \
         --bind-ro=/nix/store \
         --bind-ro=/nix/var/nix/db \
         --bind-ro=/nix/var/nix/daemon-socket \
@@ -259,13 +266,10 @@ let
     Slice = "machine.slice";
     Delegate = true;
 
-    # Hack: we don't want to kill systemd-nspawn, since we call
-    # "machinectl poweroff" in preStop to shut down the
-    # container cleanly. But systemd requires sending a signal
-    # (at least if we want remaining processes to be killed
-    # after the timeout). So send an ignored signal.
+    # We rely on systemd-nspawn turning a SIGTERM to itself into a shutdown
+    # signal (SIGRTMIN+3) for the inner container.
     KillMode = "mixed";
-    KillSignal = "WINCH";
+    KillSignal = "TERM";
 
     DevicePolicy = "closed";
     DeviceAllow = map (d: "${d.node} ${d.modifier}") cfg.allowedDevices;
@@ -752,8 +756,6 @@ in
 
       postStart = postStartScript dummyConfig;
 
-      preStop = "machinectl poweroff $INSTANCE";
-
       restartIfChanged = false;
 
       serviceConfig = serviceDirectives dummyConfig;

that will not eliminate the race between starting up a container and a stop job that replaces the running start job, but it makes it about as small as i can think of making it. also completely removes the need for machined or dbus during container shutdown. modifying stage2 doesn't feel quite right, but i really can't find a better way to do it.