containers: systemd exits with non-zero code

When a systemd service running in a container exits with a non-zero
code, it can be useful to terminate the container immediately and get
the exit code back to the host, when systemd-nspawn returns. This was
not possible to do. This patch adds the following to make it possible:

- Add a read-only "ExitCode" property on PID 1's "Manager" bus object.
  By default, it is 0 so the behaviour stays the same as previously.
- Add a method "SetExitCode" on the same object. The method fails when
  called on baremetal: it is only allowed in containers or in user
  session.
- Add support in systemctl to call "systemctl exit 42". It reuses the
  existing code for user session.
- Add exit.target and systemd-exit.service to the system instance.
- Change main() to actually call systemd-shutdown to exit() with the
  correct value.
- Add verb 'exit' in systemd-shutdown with parameter --exit-code
- Update systemctl manpage.

I used the following to test it:

| $ sudo rkt --debug --insecure-skip-verify run \
|            --mds-register=false --local docker://busybox \
|            --exec=/bin/chroot -- /proc/1/root \
|            systemctl --force exit 42
| ...
| Container rkt-895a0cba-5c66-4fa5-831c-e3f8ddc5810d failed with error code 42.
| $ echo $?
| 42

Fixes https://github.com/systemd/systemd/issues/1290

2 files changed, 28 insertions, 27 deletions

diff --git a/man/systemctl.xml b/man/systemctl.xml
index d8d433e4d3..c1359d1678 100644
--- a/man/systemctl.xml
+++ b/man/systemctl.xml
@@ -1633,13 +1633,17 @@ kobject-uevent 1 systemd-udevd-kernel.socket systemd-udevd.service
         </varlistentry>
 
         <varlistentry>
-          <term><command>exit</command></term>
+          <term><command>exit <optional><replaceable>EXIT_CODE</replaceable></optional></command></term>
 
           <listitem>
             <para>Ask the systemd manager to quit. This is only
             supported for user service managers (i.e. in conjunction
-            with the <option>--user</option> option) and will fail
-            otherwise.</para>
+            with the <option>--user</option> option) or in containers
+            and is equivalent to <command>poweroff</command> otherwise.</para>
+
+            <para>The systemd manager can exit with a non-zero exit
+            code if the optional argument
+            <replaceable>EXIT_CODE</replaceable> is given.</para>
           </listitem>
         </varlistentry>
 
diff --git a/man/systemd.special.xml b/man/systemd.special.xml
index e4700d950b..6e0dff9b47 100644
--- a/man/systemd.special.xml
+++ b/man/systemd.special.xml
@@ -212,6 +212,26 @@
         </listitem>
       </varlistentry>
       <varlistentry>
+        <term><filename>exit.target</filename></term>
+        <listitem>
+          <para>A special service unit for shutting down the system or
+          user service manager. It also works in containers and is
+          equivalent to <filename>poweroff.target</filename> on
+          non-container systems.</para>
+
+          <para>Applications wanting to terminate the user service
+          manager should start this unit. If systemd receives
+          <constant>SIGTERM</constant> or <constant>SIGINT</constant>
+          when running as user service daemon, it will start this
+          unit.</para>
+
+          <para>Normally, this pulls in
+          <filename>shutdown.target</filename> which in turn should be
+          conflicted by all units that want to be shut down on user
+          service manager exit.</para>
+        </listitem>
+      </varlistentry>
+      <varlistentry>
         <term><filename>final.target</filename></term>
         <listitem>
           <para>A special target unit that is used during the shutdown
@@ -797,6 +817,7 @@
     <para>When systemd runs as a user instance, the following special
     units are available, which have similar definitions as their
     system counterparts:
+    <filename>exit.target</filename>,
     <filename>default.target</filename>,
     <filename>shutdown.target</filename>,
     <filename>sockets.target</filename>,
@@ -806,30 +827,6 @@
     <filename>printer.target</filename>,
     <filename>smartcard.target</filename>,
     <filename>sound.target</filename>.</para>
-
-    <para>In addition, the following special unit is understood only
-    when systemd runs as service instance:</para>
-
-    <variablelist>
-      <varlistentry>
-        <term><filename>exit.target</filename></term>
-        <listitem>
-          <para>A special service unit for shutting down the user
-          service manager.</para>
-
-          <para>Applications wanting to terminate the user service
-          manager should start this unit. If systemd receives
-          <constant>SIGTERM</constant> or <constant>SIGINT</constant>
-          when running as user service daemon, it will start this
-          unit.</para>
-
-          <para>Normally, this pulls in
-          <filename>shutdown.target</filename> which in turn should be
-          conflicted by all units that want to be shut down on user
-          service manager exit.</para>
-        </listitem>
-      </varlistentry>
-    </variablelist>
   </refsect1>
 
   <refsect1>