Attempt to use systemd transient units through dbus

README

Several problems.  First, after I create the scope, it never goes away so I can't recreate it:

```
serge@jerom ~/test/sdbus$ ./a
currently I am in 0::/user.slice/user-1000.slice/session-136.scope

Queued service job as /org/freedesktop/systemd1/job/7212.
I am 1429460
currently I am in 0::/user.slice/user-1000.slice/session-136.scope

serge@jerom ~/test/sdbus$ ./a
currently I am in 0::/user.slice/user-1000.slice/session-136.scope

Failed sending message: Unit lxctest7.scope already exists.
currently I am in 0::/user.slice/user-1000.slice/session-136.scope
```

But I can't set RemainAfterExit.  If I leave that in the properties in client.c, I get:

```
serge@jerom ~/test/sdbus$ ./a
currently I am in 0::/user.slice/user-1000.slice/session-136.scope

Failed sending message: Cannot set property RemainAfterExit, or unknown property.
currently I am in 0::/user.slice/user-1000.slice/session-136.scope
```

If I try to set it after that fact through busctl, I get:

```
serge@jerom ~$ busctl --user call org.freedesktop.systemd1 /org/freedesktop/systemd1 org.freedesktop.systemd1.Manager GetUnit s lxctest.scope
o "/org/freedesktop/systemd1/unit/lxctest_2escope"

serge@jerom ~$ busctl --user call org.freedesktop.systemd1 /org/freedesktop/systemd1 org.freedesktop.systemd1.Manager SetUnitProperties "sba(sv)" lxctest.scope 1 1 RemainAfterExit b 0
Call failed: Cannot set property RemainAfterExit, or unknown property.
```

client.c

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <stdbool.h>
#include <systemd/sd-bus.h>

void print_current_cgroup(void)
{
	FILE *f;
	char *line = NULL;
	size_t len = 0;

	f = fopen("/proc/self/cgroup", "r");
	if (!f)
		exit(1);
	while (getline(&line, &len, f) != -1) {
		printf("currently I am in %s\n", line);
	}
	fclose(f);
	free(line);
}

/*
 * from https://www.man7.org/linux/man-pages/man5/org.freedesktop.systemd1.5.html :
 * StartTransientUnit(in  s name,
 *    in  s mode,
 *    in  a(sv) properties,
 *    in  a(sa(sv)) aux,
 *    out o job);
 */
int enter_new_cgroup()
{
        sd_bus_error error = SD_BUS_ERROR_NULL;
        sd_bus_message *reply = NULL, *m = NULL;
        sd_bus *bus = NULL;
        const char *path;
        int r;
	pid_t pid = getpid();

        /* Connect to the system bus */
        r = sd_bus_open_user(&bus);
        if (r < 0) {
                fprintf(stderr, "Failed to connect to system bus: %s\n", strerror(-r));
                goto finish;
        }

#define DESTINATION "org.freedesktop.systemd1"
#define PATH "/org/freedesktop/systemd1"
#define INTERFACE "org.freedesktop.systemd1.Manager"
#define MEMBER "StartTransientUnit"

	r = sd_bus_message_new_method_call(bus, &m,
		DESTINATION, PATH, INTERFACE, MEMBER);
	if (r < 0) {
		fprintf(stderr, "Failed creating bus message\n");
		goto finish;
	}

	char *scope = "lxctest7.scope";
	r = sd_bus_message_append(m, "ss", scope, "replace");
	if (r < 0) {
		fprintf(stderr, "Failed setting scope name\n");
		goto finish;
	}
	r = sd_bus_message_open_container(m, 'a', "(sv)");
	if (r < 0) {
		fprintf(stderr, "Failed allocating properties\n");
		goto finish;
	}

#if 0
	r = sd_bus_message_append(m, "(sv)(sv)(sv)",
		"PIDs", "au", 1, pid,
		"Delegate", "b", 1,
		"RemainAfterExit", "b", false);
#else
	r = sd_bus_message_append(m, "(sv)(sv)",
		"PIDs", "au", 1, pid,
		"Delegate", "b", 1);
#endif
	if (r < 0) {
		fprintf(stderr, "Failed setting delegation\n");
		goto finish;
	}
	r = sd_bus_message_close_container(m);
	if (r < 0) {
		fprintf(stderr, "Failed closing properties\n");
		goto finish;
	}

	r = sd_bus_message_append(m, "a(sa(sv))", 0);
	if (r < 0) {
		fprintf(stderr, "Failed appending aux boilerplate\n");
		goto finish;
	}

	r = sd_bus_call(NULL, m, 0, &error, &reply);
	if (r < 0) {
		fprintf(stderr, "Failed sending message: %s\n", error.message);
		goto finish;
	}

        /* Parse the response message */
        r = sd_bus_message_read(reply, "o", &path);
        if (r < 0) {
                fprintf(stderr, "Failed to parse response message: %s\n", strerror(-r));
                goto finish;
        }

        printf("Queued service job as %s.\n", path);
	printf("I am %d\n", pid);

finish:
        sd_bus_error_free(&error);
        sd_bus_message_unref(reply);
        sd_bus_message_unref(m);
        sd_bus_unref(bus);

        return r < 0 ? EXIT_FAILURE : EXIT_SUCCESS;
}

int main(int argc, char *argv[]) {
	print_current_cgroup();

	enter_new_cgroup("lxc/test");

	print_current_cgroup();
}

Makefile

a: a.c
	gcc a.c -o a `pkg-config --cflags --libs libsystemd`