systemctl: Implement --wait for kill command

TEST-26-SYSTEMCTL is racy as we call systemctl is-active immediately
after systemctl kill. Let's implement --wait for systemctl kill and
use it in TEST-26-SYSTEMCTL to avoid the race.

src/systemctl/systemctl-kill.c

@@ -2,11 +2,13 @@
 
 #include "bus-error.h"
 #include "bus-locator.h"
+#include "bus-wait-for-units.h"
 #include "systemctl-kill.h"
 #include "systemctl-util.h"
 #include "systemctl.h"
 
 int verb_kill(int argc, char *argv[], void *userdata) {
+        _cleanup_(bus_wait_for_units_freep) BusWaitForUnits *w = NULL;
         _cleanup_strv_free_ char **names = NULL;
         const char *kill_whom;
         sd_bus *bus;
@@ -16,6 +18,12 @@ int verb_kill(int argc, char *argv[], void *userdata) {
         if (r < 0)
                 return r;
 
+        if (arg_wait) {
+                r = bus_wait_for_units_new(bus, &w);
+                if (r < 0)
+                        return log_error_errno(r, "Failed to allocate unit watch context: %m");
+        }
+
         polkit_agent_open_maybe();
 
         kill_whom = arg_kill_whom ?: "all";
@@ -48,10 +56,23 @@ int verb_kill(int argc, char *argv[], void *userdata) {
                                         NULL,
                                         "ssi", *name, kill_whom, arg_signal);
                 if (q < 0) {
-                        log_error_errno(q, "Failed to kill unit %s: %s", *name, bus_error_message(&error, q));
-                        if (r == 0)
-                                r = q;
+                        RET_GATHER(r, log_error_errno(q, "Failed to kill unit %s: %s", *name, bus_error_message(&error, q)));
+                        continue;
                 }
+
+                if (w) {
+                        q = bus_wait_for_units_add_unit(w, *name, BUS_WAIT_FOR_INACTIVE|BUS_WAIT_NO_JOB, NULL, NULL);
+                        if (q < 0)
+                                RET_GATHER(r, log_error_errno(q, "Failed to watch unit %s: %m", *name));
+                }
+        }
+
+        if (w) {
+                q = bus_wait_for_units_run(w);
+                if (q < 0)
+                        return log_error_errno(q, "Failed to wait for units: %m");
+                if (q == BUS_WAIT_FAILURE)
+                        RET_GATHER(r, -EIO);
         }
 
         return r;