capture: Wait for StartTransientUnit() job finish (bsc#1217897)

The parent may not be migrated when StartTransientUnit() DBus call
returns. Wait until proper notification via JobRemoved() DBus signal
arrives to be sure parent was migrated as expected.

Use sd_bus_add_match() instead of sd_bus_match_signal_async() since the
latter is only available from libsystemd v236.

Makefile.am

@@ -5,4 +5,4 @@ sapwmp_capture_CFLAGS = \
 	$(LIBSYSTEMD_CGLAGS)
 sapwmp_capture_LDADD = \
 	$(LIBSYSTEMD_LIBS)
-sapwmp_capture_SOURCES = src/main.c src/config.c src/log.c
+sapwmp_capture_SOURCES = src/main.c src/config.c src/log.c src/dbus-job.c

src/dbus-job.c

@@ -0,0 +1,117 @@
+#include <assert.h>
+#include <errno.h>
+#include <string.h>
+#include <stdlib.h>
+
+#include "dbus-job.h"
+#include "log.h"
+
+/*
+ * The code waiting for org.freedesktop.systemd1.Manager.JobRemoved signal is
+ * based on systemd's src/shared/bus-wait-for-jobs.c
+ */
+
+struct waited_job {
+	const char *job;
+	char *result;
+};
+
+static int match_disconnected(sd_bus_message *m, void *userdata, sd_bus_error *error) {
+        assert(m);
+
+        log_info("D-Bus connection terminated while waiting for jobs.");
+        sd_bus_close(sd_bus_message_get_bus(m));
+
+        return 0;
+}
+
+static int match_job_removed(sd_bus_message *m, void *userdata, sd_bus_error *error) {
+	struct waited_job *wj = userdata;
+        const char *path, *result;
+        int r;
+
+        assert(m);
+
+        r = sd_bus_message_read(m, "uoss", /* id = */ NULL, &path, /* unit = */ NULL, &result);
+        if (r < 0) {
+		log_error("DBus signal parsing error: %s", strerror(r));
+                return 0;
+        }
+
+	if (strcmp(path, wj->job))
+		return 0;
+
+	wj->result = strdup(result);
+	/* best effort upon ENOMEM */
+	if (!wj->result)
+		wj->result = "";
+
+        return 0;
+}
+
+int bus_setup_wait(sd_bus *bus, struct waited_job *wj) {
+        int r;
+
+        /* When we are a bus client we match by sender. Direct connections OTOH have no initialized sender
+         * field, and hence we ignore the sender then */
+        r = sd_bus_add_match(
+                        bus,
+                        NULL, /* slot removed eventually with sd_bus */
+                        "type='signal',"
+                        "sender='org.freedesktop.systemd1',"
+                        "interface='org.freedesktop.systemd1.Manager',"
+                        "member='JobRemoved',"
+                        "path='/org/freedesktop/systemd1'",
+                        match_job_removed, wj);
+        if (r < 0)
+                return r;
+
+        r = sd_bus_add_match(
+                        bus,
+                        NULL, /* slot removed eventually with sd_bus */
+                        "type='signal',"
+                        "sender='org.freedesktop.DBus.Local',"
+                        "interface='org.freedesktop.DBus.Local',"
+                        "member='Disconnected'",
+                        match_disconnected, wj);
+        if (r < 0)
+                return r;
+
+        return 0;
+}
+
+static int bus_process_wait(sd_bus *bus, struct waited_job *wj) {
+        int r;
+
+        for (;;) {
+                r = sd_bus_process(bus, NULL);
+                if (r < 0)
+                        return r;
+                if (r > 0 && wj->result) {
+			if (wj->result[0] == '\0')
+				return -ENOMEM;
+			return 0;
+		}
+
+                r = sd_bus_wait(bus, UINT64_MAX);
+                if (r < 0)
+                        return r;
+        }
+}
+
+int wait_for_job(sd_bus *bus, const char *job) {
+	struct waited_job wj = { .job = job };
+	int r;
+
+	r = bus_setup_wait(bus, &wj);
+	if (r < 0)
+		return r;
+
+	r = bus_process_wait(bus, &wj);
+	if (r < 0)
+		return r;
+	log_debug("Job %s finished, result=%s", job, wj.result);
+	free(wj.result);
+	return r;
+}
+

src/dbus-job.h

@@ -0,0 +1,4 @@
+#include <systemd/sd-bus.h>
+
+int wait_for_job(sd_bus *bus, const char *job);
+

src/main.c

@@ -10,6 +10,7 @@
 
 #include "config.h"
 #include "log.h"
+#include "dbus-job.h"
 
 #define MAX_PIDS		16
 #define CONF_FILE		"/etc/sapwmp.conf"
@@ -76,8 +77,9 @@ int already_in_slice(pid_t pid, const char *target_slice) {
 int migrate(sd_bus *bus, const char *target_unit, const char *target_slice,
             struct unit_config *properties,
             size_t n_pids, pid_t *pids) {
-	_cleanup_(sd_bus_message_unrefp) sd_bus_message *m = NULL;
+	_cleanup_(sd_bus_message_unrefp) sd_bus_message *m = NULL, *reply = NULL;
 	_cleanup_(sd_bus_error_free) sd_bus_error bus_error = SD_BUS_ERROR_NULL;
+	const char *job;
 	int r;
 
 	r = sd_bus_message_new_method_call(
@@ -166,11 +168,20 @@ int migrate(sd_bus *bus, const char *target_unit, const char *target_slice,
         if (r < 0)
 		return r;
 
-        r = sd_bus_call(bus, m, 0, &bus_error, NULL);
+        r = sd_bus_call(bus, m, 0, &bus_error, &reply);
 	if (r < 0) {
 		log_info("DBus call error: %s", strerror(sd_bus_error_get_errno(&bus_error)));
+		return r;
+	}
+	r = sd_bus_message_read(reply, "o", &job);
+	if (r < 0) {
+		log_info("DBus read error: %s", strerror(sd_bus_error_get_errno(&bus_error)));
+		return r;
+	}
+	r = wait_for_job(bus, job);
+	if (r < 0) {
+		log_info("Start job failed: %s", strerror(sd_bus_error_get_errno(&bus_error)));
 	}
-	/* ignore reply, i.e. don't wait for the job to finish */
 	return r;
 }