logind: Add fallback for when the PIDFDs= property is not available

logind is not zero-downtime restartable yet, specifically it's not yet
restarted in the Fedora spec, so we can end up in situations where we're
running newer logind with older pid1 which doesn't know about the PIDFDs=
property, so let's make sure we have a fallback in place for when that
happens.

(cherry picked from commit 8ba3efe)

src/login/logind-dbus.c

@@ -4126,6 +4126,7 @@ int manager_start_scope(
                 Manager *manager,
                 const char *scope,
                 const PidRef *pidref,
+                bool allow_pidfd,
                 const char *slice,
                 const char *description,
                 char **wants,
@@ -4191,7 +4192,10 @@ int manager_start_scope(
         if (r < 0)
                 return r;
 
-        r = bus_append_scope_pidref(m, pidref);
+        if (allow_pidfd)
+                r = bus_append_scope_pidref(m, pidref);
+        else
+                r = sd_bus_message_append(m, "(sv)", "PIDs", "au", 1, (uint32_t) pidref->pid);
         if (r < 0)
                 return r;
 
@@ -4222,8 +4226,27 @@ int manager_start_scope(
                 return r;
 
         r = sd_bus_call(manager->bus, m, 0, error, &reply);
-        if (r < 0)
+        if (r < 0) {
+                /* If this failed with a property we couldn't write, this is quite likely because the server
+                 * doesn't support PIDFDs yet, let's try without. */
+                if (allow_pidfd &&
+                    sd_bus_error_has_names(error, SD_BUS_ERROR_UNKNOWN_PROPERTY, SD_BUS_ERROR_PROPERTY_READ_ONLY))
+                        return manager_start_scope(
+                                        manager,
+                                        scope,
+                                        pidref,
+                                        /* allow_pidfd = */ false,
+                                        slice,
+                                        description,
+                                        wants,
+                                        after,
+                                        requires_mounts_for,
+                                        more_properties,
+                                        error,
+                                        job);
+
                 return r;
+        }
 
         return strdup_job(reply, job);
 }

src/login/logind-dbus.h

@@ -24,7 +24,19 @@ int match_reloading(sd_bus_message *message, void *userdata, sd_bus_error *error
 
 int manager_send_changed(Manager *manager, const char *property, ...) _sentinel_;
 
-int manager_start_scope(Manager *manager, const char *scope, const PidRef *pidref, const char *slice, const char *description, char **wants, char **after, const char *requires_mounts_for, sd_bus_message *more_properties, sd_bus_error *error, char **job);
+int manager_start_scope(
+                Manager *manager,
+                const char *scope,
+                const PidRef *pidref,
+                bool allow_pidfd,
+                const char *slice,
+                const char *description,
+                char **wants,
+                char **after,
+                const char *requires_mounts_for,
+                sd_bus_message *more_properties,
+                sd_bus_error *error,
+                char **job);
 int manager_start_unit(Manager *manager, const char *unit, sd_bus_error *error, char **job);
 int manager_stop_unit(Manager *manager, const char *unit, const char *job_mode, sd_bus_error *error, char **job);
 int manager_abandon_scope(Manager *manager, const char *scope, sd_bus_error *error);

src/login/logind-session.c

@@ -686,6 +686,7 @@ static int session_start_scope(Session *s, sd_bus_message *properties, sd_bus_er
                                 s->manager,
                                 scope,
                                 &s->leader,
+                                /* allow_pidfd = */ true,
                                 s->user->slice,
                                 description,
                                 /* These two have StopWhenUnneeded= set, hence add a dep towards them */