pid1: lookup owning PID of BusName= name of services asynchronously

A first step of removing blocking calls to the D-Bus broker from PID 1.
There's a lot more to got (i.e. grep src/core/ for sd_bus_creds
basically), but it's a start.

Removing blocking calls to D-Bus broker deals systematicallly with
deadlocks caused by dbus-daemon blocking on synchronous IPC calls back
to PID1 (e.g. Varlink calls through nss-systemd). Bugs such as #15316.

Also-see: https://github.com/systemd/systemd/pull/22038#issuecomment-1042958390

src/core/service.c

@@ -398,6 +398,8 @@ static void service_done(Unit *u) {
         s->timer_event_source = sd_event_source_disable_unref(s->timer_event_source);
         s->exec_fd_event_source = sd_event_source_disable_unref(s->exec_fd_event_source);
 
+        s->bus_name_pid_lookup_slot = sd_bus_slot_unref(s->bus_name_pid_lookup_slot);
+
         service_release_resources(u);
 }
 
@@ -4322,6 +4324,60 @@ static int service_get_timeout(Unit *u, usec_t *timeout) {
         return 1;
 }
 
+static bool pick_up_pid_from_bus_name(Service *s) {
+        assert(s);
+
+        /* If the service is running but we have no main PID yet, get it from the owner of the D-Bus name */
+
+        return !pid_is_valid(s->main_pid) &&
+                IN_SET(s->state,
+                       SERVICE_START,
+                       SERVICE_START_POST,
+                       SERVICE_RUNNING,
+                       SERVICE_RELOAD);
+}
+
+static int bus_name_pid_lookup_callback(sd_bus_message *reply, void *userdata, sd_bus_error *ret_error) {
+        const sd_bus_error *e;
+        Unit *u = userdata;
+        uint32_t pid;
+        Service *s;
+        int r;
+
+        assert(reply);
+        assert(u);
+
+        s = SERVICE(u);
+        s->bus_name_pid_lookup_slot = sd_bus_slot_unref(s->bus_name_pid_lookup_slot);
+
+        if (!s->bus_name || !pick_up_pid_from_bus_name(s))
+                return 1;
+
+        e = sd_bus_message_get_error(reply);
+        if (e) {
+                r = sd_bus_error_get_errno(e);
+                log_warning_errno(r, "GetConnectionUnixProcessID() failed: %s", bus_error_message(e, r));
+                return 1;
+        }
+
+        r = sd_bus_message_read(reply, "u", &pid);
+        if (r < 0) {
+                bus_log_parse_error(r);
+                return 1;
+        }
+
+        if (!pid_is_valid(pid)) {
+                log_debug_errno(SYNTHETIC_ERRNO(EINVAL), "GetConnectionUnixProcessID() returned invalid PID");
+                return 1;
+        }
+
+        log_unit_debug(u, "D-Bus name %s is now owned by process " PID_FMT, s->bus_name, (pid_t) pid);
+
+        service_set_main_pid(s, pid);
+        unit_watch_pid(UNIT(s), pid, false);
+        return 1;
+}
+
 static void service_bus_name_owner_change(Unit *u, const char *new_owner) {
 
         Service *s = SERVICE(u);
@@ -4352,28 +4408,25 @@ static void service_bus_name_owner_change(Unit *u, const char *new_owner) {
                 else if (s->state == SERVICE_START && new_owner)
                         service_enter_start_post(s);
 
-        } else if (new_owner &&
-                   s->main_pid <= 0 &&
-                   IN_SET(s->state,
-                          SERVICE_START,
-                          SERVICE_START_POST,
-                          SERVICE_RUNNING,
-                          SERVICE_RELOAD)) {
-
-                _cleanup_(sd_bus_creds_unrefp) sd_bus_creds *creds = NULL;
-                pid_t pid;
+        } else if (new_owner && pick_up_pid_from_bus_name(s)) {
 
                 /* Try to acquire PID from bus service */
 
-                r = sd_bus_get_name_creds(u->manager->api_bus, s->bus_name, SD_BUS_CREDS_PID, &creds);
-                if (r >= 0)
-                        r = sd_bus_creds_get_pid(creds, &pid);
-                if (r >= 0) {
-                        log_unit_debug(u, "D-Bus name %s is now owned by process " PID_FMT, s->bus_name, pid);
+                s->bus_name_pid_lookup_slot = sd_bus_slot_unref(s->bus_name_pid_lookup_slot);
 
-                        service_set_main_pid(s, pid);
-                        unit_watch_pid(UNIT(s), pid, false);
-                }
+                r = sd_bus_call_method_async(
+                                u->manager->api_bus,
+                                &s->bus_name_pid_lookup_slot,
+                                "org.freedesktop.DBus",
+                                "/org/freedesktop/DBus",
+                                "org.freedesktop.DBus",
+                                "GetConnectionUnixProcessID",
+                                bus_name_pid_lookup_callback,
+                                s,
+                                "s",
+                                s->bus_name);
+                if (r < 0)
+                        log_debug_errno(r, "Failed to request owner PID of service name, ignoring: %m");
         }
 }
 

src/core/service.h

@@ -195,6 +195,8 @@ struct Service {
         NotifyAccess notify_access;
         NotifyState notify_state;
 
+        sd_bus_slot *bus_name_pid_lookup_slot;
+
         sd_event_source *exec_fd_event_source;
 
         ServiceFDStore *fd_store;