user-sessions: rely on PID 1 to kill sessions

As we want to centralized cgroup access we should stop killing the user
sessions directly from the systemd-user-sessions service. Instead, rely
on PID 1 doing this by adding the right ordering dependencies to the
session scope units.

src/core/dbus-unit.c

@@ -844,6 +844,63 @@ static int bus_unit_set_transient_property(
                         unit_write_drop_in(u, mode, "Slice", contents);
                 }
                 return 1;
+
+        } else if (streq(name, "Requires") ||
+                   streq(name, "RequiresOverridable") ||
+                   streq(name, "Requisite") ||
+                   streq(name, "RequisiteOverridable") ||
+                   streq(name, "Wants") ||
+                   streq(name, "BindsTo") ||
+                   streq(name, "Conflicts") ||
+                   streq(name, "Before") ||
+                   streq(name, "After") ||
+                   streq(name, "OnFailure") ||
+                   streq(name, "PropagatesReloadTo") ||
+                   streq(name, "ReloadPropagatedFrom") ||
+                   streq(name, "PartOf")) {
+
+                UnitDependency d;
+                DBusMessageIter sub;
+
+                d = unit_dependency_from_string(name);
+                if (d < 0)
+                        return -EINVAL;
+
+                if (dbus_message_iter_get_arg_type(i) != DBUS_TYPE_ARRAY ||
+                    dbus_message_iter_get_element_type(i) != DBUS_TYPE_STRING)
+                        return -EINVAL;
+
+                dbus_message_iter_recurse(i, &sub);
+                while (dbus_message_iter_get_arg_type(&sub) == DBUS_TYPE_STRING) {
+                        const char *other;
+
+                        dbus_message_iter_get_basic(&sub, &other);
+
+                        if (!unit_name_is_valid(other, false))
+                                return -EINVAL;
+
+                        if (mode != UNIT_CHECK) {
+                                _cleanup_free_ char *label = NULL, *contents = NULL;
+
+                                r = unit_add_dependency_by_name(u, d, other, NULL, true);
+                                if (r < 0)
+                                        return r;
+
+                                label = strjoin(name, "-", other, NULL);
+                                if (!label)
+                                        return -ENOMEM;
+
+                                contents = strjoin("[Unit]\n", name, "=", other, "\n", NULL);
+                                if (!contents)
+                                        return -ENOMEM;
+
+                                unit_write_drop_in(u, mode, label, contents);
+                        }
+
+                        dbus_message_iter_next(&sub);
+                }
+
+                return 1;
         }
 
         return 0;

src/login/logind-dbus.c

@@ -2518,6 +2518,7 @@ int manager_start_scope(
                 pid_t pid,
                 const char *slice,
                 const char *description,
+                const char *after,
                 DBusError *error,
                 char **job) {
 
@@ -2575,6 +2576,20 @@ int manager_start_scope(
                         return log_oom();
         }
 
+        if (!isempty(after)) {
+                const char *after_property = "After";
+
+                if (!dbus_message_iter_open_container(&sub, DBUS_TYPE_STRUCT, NULL, &sub2) ||
+                    !dbus_message_iter_append_basic(&sub2, DBUS_TYPE_STRING, &after_property) ||
+                    !dbus_message_iter_open_container(&sub2, DBUS_TYPE_VARIANT, "as", &sub3) ||
+                    !dbus_message_iter_open_container(&sub3, DBUS_TYPE_ARRAY, "s", &sub4) ||
+                    !dbus_message_iter_append_basic(&sub4, DBUS_TYPE_STRING, &after) ||
+                    !dbus_message_iter_close_container(&sub3, &sub4) ||
+                    !dbus_message_iter_close_container(&sub2, &sub3) ||
+                    !dbus_message_iter_close_container(&sub, &sub2))
+                        return log_oom();
+        }
+
         /* cgroup empty notification is not available in containers
          * currently. To make this less problematic, let's shorten the
          * stop timeout for sessions, so that we don't wait

src/login/logind-session.c

@@ -474,7 +474,7 @@ static int session_start_scope(Session *s) {
 
                 description = strjoin("Session ", s->id, " of user ", s->user->name, NULL);
 
-                r = manager_start_scope(s->manager, scope, s->leader, s->user->slice, description, &error, &job);
+                r = manager_start_scope(s->manager, scope, s->leader, s->user->slice, description, "systemd-user-sessions.service", &error, &job);
                 if (r < 0) {
                         log_error("Failed to start session scope: %s %s", bus_error(&error, r), error.name);
                         dbus_error_free(&error);

src/login/logind.h

@@ -178,7 +178,7 @@ int manager_send_changed(Manager *manager, const char *properties);
 
 int manager_dispatch_delayed(Manager *manager);
 
-int manager_start_scope(Manager *manager, const char *scope, pid_t pid, const char *slice, const char *description, DBusError *error, char **job);
+int manager_start_scope(Manager *manager, const char *scope, pid_t pid, const char *slice, const char *description, const char *after, DBusError *error, char **job);
 int manager_start_unit(Manager *manager, const char *unit, DBusError *error, char **job);
 int manager_stop_unit(Manager *manager, const char *unit, DBusError *error, char **job);
 int manager_kill_unit(Manager *manager, const char *unit, KillWho who, int signo, DBusError *error);

src/login/user-sessions.c

@@ -25,109 +25,8 @@
 
 #include "log.h"
 #include "util.h"
-#include "cgroup-util.h"
 #include "fileio.h"
 
-static int kill_all_users(void) {
-        _cleanup_closedir_ DIR *d = NULL;
-        struct dirent *de;
-        int r = 0;
-
-        d = opendir("/run/systemd/users");
-        if (!d) {
-                if (errno == ENOENT)
-                        return 0;
-
-                log_error("Failed to open /run/systemd/users: %m");
-                return -errno;
-        }
-
-        FOREACH_DIRENT(de, d, return -errno) {
-                _cleanup_free_ char *cgroup = NULL;
-                char *a;
-                int k;
-
-                if (!dirent_is_file(de))
-                        continue;
-
-                a = strappenda("/run/systemd/users/", de->d_name);
-
-                k = parse_env_file(a, NEWLINE, "CGROUP", &cgroup, NULL);
-                if (k < 0) {
-                        if (k != -ENOENT) {
-                                log_error("Failed to read user data: %s", strerror(-k));
-                                r = k;
-                        }
-
-                        continue;
-                }
-
-                if (!cgroup) {
-                        log_error("User data did not contain cgroup field.");
-                        r = -ENOENT;
-                        continue;
-                }
-
-                k = cg_kill_recursive_and_wait(SYSTEMD_CGROUP_CONTROLLER, cgroup, true);
-                if (k < 0) {
-                        log_error("Failed to kill cgroup %s: %s", cgroup, strerror(-k));
-                        r = k;
-                }
-        }
-
-        return r;
-}
-
-static int kill_all_sessions(void) {
-        _cleanup_closedir_ DIR *d = NULL;
-        struct dirent *de;
-        int r = 0;
-
-        d = opendir("/run/systemd/sessions");
-        if (!d) {
-                if (errno == ENOENT)
-                        return 0;
-
-                log_error("Failed to open /run/systemd/sessions: %m");
-                return -errno;
-        }
-
-        FOREACH_DIRENT(de, d, return -errno) {
-                _cleanup_free_ char *cgroup = NULL;
-                char *a;
-                int k;
-
-                if (!dirent_is_file(de))
-                        continue;
-
-                a = strappenda("/run/systemd/sessions/", de->d_name);
-
-                k = parse_env_file(a, NEWLINE, "CGROUP", &cgroup, NULL);
-                if (k < 0) {
-                        if (k != -ENOENT) {
-                                log_error("Failed to read session data: %s", strerror(-k));
-                                r = k;
-                        }
-
-                        continue;
-                }
-
-                if (!cgroup) {
-                        log_error("Session data did not contain cgroup field.");
-                        r = -ENOENT;
-                        continue;
-                }
-
-                k = cg_kill_recursive_and_wait(SYSTEMD_CGROUP_CONTROLLER, cgroup, true);
-                if (k < 0) {
-                        log_error("Failed to kill cgroup %s: %s", cgroup, strerror(-k));
-                        r = k;
-                }
-        }
-
-        return r;
-}
-
 int main(int argc, char*argv[]) {
         int ret = EXIT_FAILURE;
 
@@ -167,20 +66,12 @@ int main(int argc, char*argv[]) {
                         goto finish;
 
         } else if (streq(argv[1], "stop")) {
-                int r, q;
+                int r;
 
                 r = write_string_file_atomic("/run/nologin", "System is going down.");
                 if (r < 0)
                         log_error("Failed to create /run/nologin: %s", strerror(-r));
 
-                q = kill_all_users();
-                if (q < 0 && r >= 0)
-                        r = q;
-
-                q = kill_all_sessions();
-                if (q < 0 && r >= 0)
-                        r = q;
-
         } else {
                 log_error("Unknown verb %s.", argv[1]);
                 goto finish;