implement handling of serial consoles for autoterminal

serial consoles have separate systemd services (serial-getty@.service). We need to adapt
the handling and also need some different options for them than for regular gettys.

To allow sysrescue-initialize.py to differentiate between regular consoles and serial ones
they must be configured with the prefix "serial:" in the yaml, for example like this:

autoterminal:
    "serial:ttyS0": "/usr/bin/bash"

airootfs/etc/systemd/scripts/sysrescue-initialize.py

@@ -208,27 +208,47 @@ if ('autoterminal' in config) and (config['autoterminal'] is not None) and \
     print("====> Configuring autoterminal ...")
     with open('/usr/share/sysrescue/template/autoterminal.service', 'r') as template_file:
         conf_template = template_file.read()
+    with open('/usr/share/sysrescue/template/serial-autoterminal.service', 'r') as template_file:
+        serial_conf_template = template_file.read()
     start_services = []
     for terminal, command in sorted(config['autoterminal'].items()):
+        if m := re.match(r"^serial:([a-zA-Z0-9_-]+)$", terminal):
+            serial=True
+            terminal = m.group(1)
+        else:
+            serial=False
+
         if not re.match(r"^[a-zA-Z0-9_-]+$", terminal):
             print (f"Ignoring invalid terminal name '{terminal}'")
             errcnt+=1
             continue
         # do not check if terminal or command exists: an autorun could create them later on
+        if serial:
+            print (f"setting serial terminal '{terminal}' to '{command}'")
+        else:
             print (f"setting terminal '{terminal}' to '{command}'")
         with open(f"/etc/systemd/system/autoterminal-{terminal}.service", "w") as terminal_conf:
             # write service config, based on the template config we loaded above
             # don't use getty@{terminal}.service name to not use autovt@{terminal}.service on-demand logic
+            if serial:
+                conf_data=serial_conf_template.replace("%TTY%",terminal)
+            else:
                 conf_data=conf_template.replace("%TTY%",terminal)
+
             conf_data=conf_data.replace("%EXEC%",command)
             terminal_conf.write(conf_data)
         # enable service: always start it, do not wait for the user to switch to the terminal
         # means other programs (like X.org) can't allocate it away, also allows for longer running init sequences
         symlink_overwrite(f"/etc/systemd/system/autoterminal-{terminal}.service",
                         f"/etc/systemd/system/getty.target.wants/autoterminal-{terminal}.service")
+
         # mask the regular getty for this terminal
+        if serial:
+            symlink_overwrite("/dev/null",f"/etc/systemd/system/serial-getty@{terminal}.service")
+        else:
             symlink_overwrite("/dev/null",f"/etc/systemd/system/getty@{terminal}.service")
             symlink_overwrite("/dev/null",f"/etc/systemd/system/autovt@{terminal}.service")
+
         start_services.append(f"autoterminal-{terminal}.service")
     # reload systemd to allow the new config to take effect
     subprocess.run(["/usr/bin/systemctl", "daemon-reload"])

airootfs/usr/share/sysrescue/template/serial-autoterminal.service

@@ -0,0 +1,54 @@
+#  SPDX-License-Identifier: LGPL-2.1-or-later
+#
+#  This file is part of SystemRescue, based on serial-getty@.service from 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=SystemRescue serial autoterminal %TTY%
+Documentation=https://www.system-rescue.org/manual/autoterminal/
+BindsTo=dev-%TTY%.device
+After=dev-%TTY%.device systemd-user-sessions.service plymouth-quit-wait.service \
+      getty-pre.target serial-getty@%TTY%.service
+
+# If additional gettys are spawned during boot then we should make
+# sure that this is synchronized before getty.target, even though
+# getty.target didn't actually pull it in.
+Before=getty.target
+IgnoreOnIsolate=yes
+
+# IgnoreOnIsolate causes issues with sulogin, if someone isolates
+# rescue.target or starts rescue.service from multi-user.target or
+# graphical.target.
+Conflicts=rescue.service serial-getty@%TTY%.service
+Before=rescue.service
+
+[Service]
+ExecStart=-%EXEC%
+
+# do not wait 5 seconds as for Type=idle before starting the service
+Type=simple
+
+Restart=always
+RestartSec=1
+UtmpIdentifier=%TTY%
+StandardInput=tty
+StandardOutput=tty
+TTYPath=/dev/%TTY%
+TTYReset=yes
+TTYVHangup=yes
+IgnoreSIGPIPE=no
+SendSIGHUP=yes
+
+# make this a systemd-logind session without needing a getty
+User=root
+PAMName=login
+
+# generate all utmp/wtmp entries and don't expect the program to do it
+UtmpMode=user
+
+[Install]
+WantedBy=getty.target