core: use an AF_UNIX/SOCK_DGRAM socket for cgroup agent notification

dbus-daemon currently uses a backlog of 30 on its D-bus system bus socket. On
overloaded systems this means that only 30 connections may be queued without
dbus-daemon processing them before further connection attempts fail. Our
cgroups-agent binary so far used D-Bus for its messaging, and hitting this
limit hence may result in us losing cgroup empty messages.

This patch adds a seperate cgroup agent socket of type AF_UNIX/SOCK_DGRAM.
Since sockets of these types need no connection set up, no listen() backlog
applies. Our cgroup-agent binary will hence simply block as long as it can't
enqueue its datagram message, so that we won't lose cgroup empty messages as
likely anymore.

This also rearranges the ordering of the processing of SIGCHLD signals, service
notification messages (sd_notify()...) and the two types of cgroup
notifications (inotify for the unified hierarchy support, and agent for the
classic hierarchy support). We now always process events for these in the
following order:

  1. service notification messages  (SD_EVENT_PRIORITY_NORMAL-7)
  2. SIGCHLD signals (SD_EVENT_PRIORITY_NORMAL-6)
  3. cgroup inotify and cgroup agent (SD_EVENT_PRIORITY_NORMAL-5)

This is because when receiving SIGCHLD we invalidate PID information, which we
need to process the service notification messages which are bound to PIDs.
Hence the order between the first two items. And we want to process SIGCHLD
metadata to detect whether a service is gone, before using cgroup
notifications, to decide when a service is gone, since the former carries more
useful metadata.

Related to this:
https://bugs.freedesktop.org/show_bug.cgi?id=95264
https://github.com/systemd/systemd/issues/1961

1 files changed, 29 insertions, 44 deletions

diff --git a/src/core/dbus.c b/src/core/dbus.c
index 263955d874..c8375a0475 100644
--- a/src/core/dbus.c
+++ b/src/core/dbus.c
@@ -71,28 +71,42 @@ int bus_send_queued_message(Manager *m) {
         return 0;
 }
 
+int bus_forward_agent_released(Manager *m, const char *path) {
+        int r;
+
+        assert(m);
+        assert(path);
+
+        if (!MANAGER_IS_SYSTEM(m))
+                return 0;
+
+        if (!m->system_bus)
+                return 0;
+
+        /* If we are running a system instance we forward the agent message on the system bus, so that the user
+         * instances get notified about this, too */
+
+        r = sd_bus_emit_signal(m->system_bus,
+                               "/org/freedesktop/systemd1/agent",
+                               "org.freedesktop.systemd1.Agent",
+                               "Released",
+                               "s", path);
+        if (r < 0)
+                return log_warning_errno(r, "Failed to propagate agent release message: %m");
+
+        return 1;
+}
+
 static int signal_agent_released(sd_bus_message *message, void *userdata, sd_bus_error *error) {
         _cleanup_(sd_bus_creds_unrefp) sd_bus_creds *creds = NULL;
-        const char *cgroup, *me;
         Manager *m = userdata;
+        const char *cgroup;
         uid_t sender_uid;
-        sd_bus *bus;
         int r;
 
         assert(message);
         assert(m);
 
-        /* ignore recursive events sent by us on the system/user bus */
-        bus = sd_bus_message_get_bus(message);
-        if (!sd_bus_is_server(bus)) {
-                r = sd_bus_get_unique_name(bus, &me);
-                if (r < 0)
-                        return r;
-
-                if (streq_ptr(sd_bus_message_get_sender(message), me))
-                        return 0;
-        }
-
         /* only accept org.freedesktop.systemd1.Agent from UID=0 */
         r = sd_bus_query_sender_creds(message, SD_BUS_CREDS_EUID, &creds);
         if (r < 0)
@@ -110,16 +124,6 @@ static int signal_agent_released(sd_bus_message *message, void *userdata, sd_bus
         }
 
         manager_notify_cgroup_empty(m, cgroup);
-
-        /* if running as system-instance, forward under our name */
-        if (MANAGER_IS_SYSTEM(m) && m->system_bus) {
-                r = sd_bus_message_rewind(message, 1);
-                if (r >= 0)
-                        r = sd_bus_send(m->system_bus, message, NULL);
-                if (r < 0)
-                        log_warning_errno(r, "Failed to forward Released message: %m");
-        }
-
         return 0;
 }
 
@@ -690,25 +694,6 @@ static int bus_on_connection(sd_event_source *s, int fd, uint32_t revents, void
                 return 0;
         }
 
-        if (MANAGER_IS_SYSTEM(m)) {
-                /* When we run as system instance we get the Released
-                 * signal via a direct connection */
-
-                r = sd_bus_add_match(
-                                bus,
-                                NULL,
-                                "type='signal',"
-                                "interface='org.freedesktop.systemd1.Agent',"
-                                "member='Released',"
-                                "path='/org/freedesktop/systemd1/agent'",
-                                signal_agent_released, m);
-
-                if (r < 0) {
-                        log_warning_errno(r, "Failed to register Released match on new connection bus: %m");
-                        return 0;
-                }
-        }
-
         r = bus_setup_disconnected_match(m, bus);
         if (r < 0)
                 return 0;
@@ -906,8 +891,8 @@ static int bus_setup_system(Manager *m, sd_bus *bus) {
         assert(m);
         assert(bus);
 
-        /* On kdbus or if we are a user instance we get the Released message via the system bus */
-        if (MANAGER_IS_USER(m) || m->kdbus_fd >= 0) {
+        /* if we are a user instance we get the Released message via the system bus */
+        if (MANAGER_IS_USER(m)) {
                 r = sd_bus_add_match(
                                 bus,
                                 NULL,