core: enforce that scope units can be started only once

Scope units are populated from PIDs specified by the bus client. We do
that when a scope is started. We really shouldn't allow scopes to be
started multiple times, as the PIDs then might be heavily out of date.
Moreover, clients should have the guarantee that any scope they allocate
has a clear runtime cycle which is not repetitive.

src/shared/bus-unit-util.c

@@ -1900,8 +1900,6 @@ finish:
 }
 
 static int check_wait_response(BusWaitForJobs *d, bool quiet, const char* const* extra_args) {
-        int r = 0;
-
         assert(d->result);
 
         if (!quiet) {
@@ -1919,6 +1917,8 @@ static int check_wait_response(BusWaitForJobs *d, bool quiet, const char* const*
                         log_error("Operation on or unit type of %s not supported on this system.", strna(d->name));
                 else if (streq(d->result, "collected"))
                         log_error("Queued job for %s was garbage collected.", strna(d->name));
+                else if (streq(d->result, "once"))
+                        log_error("Unit %s was started already once and can't be started again.", strna(d->name));
                 else if (!STR_IN_SET(d->result, "done", "skipped")) {
                         if (d->name) {
                                 _cleanup_free_ char *result = NULL;
@@ -1935,21 +1935,24 @@ static int check_wait_response(BusWaitForJobs *d, bool quiet, const char* const*
         }
 
         if (STR_IN_SET(d->result, "canceled", "collected"))
-                r = -ECANCELED;
+                return -ECANCELED;
         else if (streq(d->result, "timeout"))
-                r = -ETIME;
+                return -ETIME;
         else if (streq(d->result, "dependency"))
-                r = -EIO;
+                return -EIO;
         else if (streq(d->result, "invalid"))
-                r = -ENOEXEC;
+                return -ENOEXEC;
         else if (streq(d->result, "assert"))
-                r = -EPROTO;
+                return -EPROTO;
         else if (streq(d->result, "unsupported"))
-                r = -EOPNOTSUPP;
-        else if (!STR_IN_SET(d->result, "done", "skipped"))
-                r = -EIO;
+                return -EOPNOTSUPP;
+        else if (streq(d->result, "once"))
+                return -ESTALE;
+        else if (STR_IN_SET(d->result, "done", "skipped"))
+                return 0;
 
-        return r;
+        log_debug("Unexpected job result, assuming server side newer than us: %s", d->result);
+        return -EIO;
 }
 
 int bus_wait_for_jobs(BusWaitForJobs *d, bool quiet, const char* const* extra_args) {