importd: allow activation in early boot, and make it socket activatable

Previously, importd was only accessible via D-Bus, which required it to
be a late boot service. Now that we have Varlink we can rearrange things
to become early-boot activated, just after the image directories are
mounted.

This will later allow us to have generator that auto-downloads images on
boot.

src/import/importd.c

@@ -1743,7 +1743,7 @@ static int manager_connect_bus(Manager *m) {
         assert(m->event);
         assert(!m->bus);
 
-        r = sd_bus_default_system(&m->bus);
+        r = bus_open_system_watch_bind(&m->bus);
         if (r < 0)
                 return log_error_errno(r, "Failed to get system bus connection: %m");
 

units/meson.build

@@ -361,6 +361,11 @@ units = [
           'conditions' : ['ENABLE_IMPORTD'],
           'symlinks' : ['dbus-org.freedesktop.import1.service'],
         },
+        {
+          'file' : 'systemd-importd.socket',
+          'conditions' : ['ENABLE_IMPORTD'],
+          'symlinks' : ['sockets.target.wants/'],
+        },
         {
           'file' : 'systemd-initctl.service.in',
           'conditions' : ['HAVE_SYSV_COMPAT'],

units/systemd-importd.service.in

@@ -8,9 +8,15 @@
 #  (at your option) any later version.
 
 [Unit]
-Description=Virtual Machine and Container Download Service
+Description=Disk Image Download Service
 Documentation=man:systemd-importd.service(8)
 Documentation=man:org.freedesktop.import1(5)
+DefaultDependencies=no
+After=systemd-importd.socket
+WantsMountsFor=/var/lib/machines /var/lib/portables /var/lib/extensions /var/lib/confexts
+After=systemd-remount-fs.service
+Before=shutdown.target
+Conflicts=shutdown.target
 
 [Service]
 Type=notify

units/systemd-importd.socket

@@ -0,0 +1,24 @@
+#  SPDX-License-Identifier: LGPL-2.1-or-later
+#
+#  This file is part of systemd.
+#
+#  systemd is free software; you can redistribute it and/or modify it
+#  under the terms of the GNU Lesser General Public License as published by
+#  the Free Software Foundation; either version 2.1 of the License, or
+#  (at your option) any later version.
+
+[Unit]
+Description=Disk Image Download Service Socket
+Documentation=man:systemd-importd.service(8)
+Documentation=man:org.freedesktop.import1(5)
+DefaultDependencies=no
+WantsMountsFor=/var/lib/machines /var/lib/portables /var/lib/extensions /var/lib/confexts
+After=systemd-remount-fs.service
+Before=sockets.target
+Conflicts=shutdown.target
+Before=shutdown.target
+
+[Socket]
+ListenStream=/run/systemd/io.systemd.Import
+FileDescriptorName=varlink
+SocketMode=0666