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

1 files changed, 22 insertions, 0 deletions

diff --git a/src/core/shutdown.c b/src/core/shutdown.c
index 8cc6efc5b8..5296efce1d 100644
--- a/src/core/shutdown.c
+++ b/src/core/shutdown.c
@@ -48,6 +48,7 @@
 #define FINALIZE_ATTEMPTS 50
 
 static char* arg_verb;
+static uint8_t arg_exit_code;
 
 static int parse_argv(int argc, char *argv[]) {
         enum {
@@ -55,6 +56,7 @@ static int parse_argv(int argc, char *argv[]) {
                 ARG_LOG_TARGET,
                 ARG_LOG_COLOR,
                 ARG_LOG_LOCATION,
+                ARG_EXIT_CODE,
         };
 
         static const struct option options[] = {
@@ -62,6 +64,7 @@ static int parse_argv(int argc, char *argv[]) {
                 { "log-target",    required_argument, NULL, ARG_LOG_TARGET   },
                 { "log-color",     optional_argument, NULL, ARG_LOG_COLOR    },
                 { "log-location",  optional_argument, NULL, ARG_LOG_LOCATION },
+                { "exit-code",     required_argument, NULL, ARG_EXIT_CODE    },
                 {}
         };
 
@@ -110,6 +113,13 @@ static int parse_argv(int argc, char *argv[]) {
 
                         break;
 
+                case ARG_EXIT_CODE:
+                        r = safe_atou8(optarg, &arg_exit_code);
+                        if (r < 0)
+                                log_error("Failed to parse exit code %s, ignoring", optarg);
+
+                        break;
+
                 case '\001':
                         if (!arg_verb)
                                 arg_verb = optarg;
@@ -183,6 +193,8 @@ int main(int argc, char *argv[]) {
                 cmd = RB_HALT_SYSTEM;
         else if (streq(arg_verb, "kexec"))
                 cmd = LINUX_REBOOT_CMD_KEXEC;
+        else if (streq(arg_verb, "exit"))
+                cmd = 0; /* ignored, just checking that arg_verb is valid */
         else {
                 r = -EINVAL;
                 log_error("Unknown action '%s'.", arg_verb);
@@ -339,6 +351,16 @@ int main(int argc, char *argv[]) {
         if (!in_container)
                 sync();
 
+        if (streq(arg_verb, "exit")) {
+                if (in_container)
+                        exit(arg_exit_code);
+                else {
+                        /* We cannot exit() on the host, fallback on another
+                         * method. */
+                        cmd = RB_POWER_OFF;
+                }
+        }
+
         switch (cmd) {
 
         case LINUX_REBOOT_CMD_KEXEC: