timesync: try to reload DBus configuration when RequestName() fails

If dbus.service starts earlier than the dynamic user systemd-timesync
is realized, then the dbus policy file for timesyncd does not loaded
and timesyncd fails to request name.
To support such case, try to reload dbus configuration when requesting
name fails.

src/timesync/timesyncd-bus.c

@@ -3,6 +3,8 @@
 #include "sd-bus.h"
 
 #include "alloc-util.h"
+#include "bus-internal.h"
+#include "bus-protocol.h"
 #include "bus-util.h"
 #include "in-addr-util.h"
 #include "log.h"
@@ -109,6 +111,100 @@ static const sd_bus_vtable manager_vtable[] = {
         SD_BUS_VTABLE_END
 };
 
+static int reload_dbus_handler(sd_bus_message *m, void *userdata, sd_bus_error *ret_error) {
+        const sd_bus_error *e;
+        int r;
+
+        assert(m);
+
+        e = sd_bus_message_get_error(m);
+        if (e) {
+                log_error_errno(sd_bus_error_get_errno(e), "Failed to reload DBus configuration: %s", e->message);
+                return 1;
+        }
+
+        /* Here, use the default request name handler to avoid an infinite loop of reloading and requesting. */
+        r = sd_bus_request_name_async(sd_bus_message_get_bus(m), NULL, "org.freedesktop.timesync1", 0, NULL, NULL);
+        if (r < 0)
+                log_error_errno(r, "Failed to request name: %m");
+
+        return 1;
+}
+
+static int request_name_handler(sd_bus_message *m, void *userdata, sd_bus_error *ret_error) {
+        uint32_t ret;
+        int r;
+
+        assert(m);
+
+        if (sd_bus_message_is_method_error(m, NULL)) {
+                const sd_bus_error *e = sd_bus_message_get_error(m);
+
+                if (!sd_bus_error_has_name(e, SD_BUS_ERROR_ACCESS_DENIED)) {
+                        log_debug_errno(sd_bus_error_get_errno(e),
+                                        "Unable to request name, failing connection: %s",
+                                        e->message);
+
+                        bus_enter_closing(sd_bus_message_get_bus(m));
+                        return 1;
+                }
+
+                log_debug_errno(sd_bus_error_get_errno(e),
+                                "Unable to request name, retry after reloading DBus configuration: %s",
+                                e->message);
+
+                /* If systemd-timesyncd.service enables DynamicUser= and dbus.service
+                 * started before the dynamic user is realized, then the DBus policy
+                 * about timesyncd has not been enabled yet. So, let's try to reload
+                 * DBus configuration, and after that request name again. Note that it
+                 * seems that no privileges are necessary to call the following method. */
+
+                r = sd_bus_call_method_async(
+                                sd_bus_message_get_bus(m),
+                                NULL,
+                                "org.freedesktop.DBus",
+                                "/org/freedesktop/DBus",
+                                "org.freedesktop.DBus",
+                                "ReloadConfig",
+                                reload_dbus_handler,
+                                NULL, NULL);
+                if (r < 0) {
+                        log_error_errno(r, "Failed to reload DBus configuration: %m");
+                        bus_enter_closing(sd_bus_message_get_bus(m));
+                }
+
+                return 1;
+        }
+
+        r = sd_bus_message_read(m, "u", &ret);
+        if (r < 0)
+                return r;
+
+        switch (ret) {
+
+        case BUS_NAME_ALREADY_OWNER:
+                log_debug("Already owner of requested service name, ignoring.");
+                return 1;
+
+        case BUS_NAME_IN_QUEUE:
+                log_debug("In queue for requested service name.");
+                return 1;
+
+        case BUS_NAME_PRIMARY_OWNER:
+                log_debug("Successfully acquired requested service name.");
+                return 1;
+
+        case BUS_NAME_EXISTS:
+                log_debug("Requested service name already owned, failing connection.");
+                bus_enter_closing(sd_bus_message_get_bus(m));
+                return 1;
+        }
+
+        log_debug("Unexpected response from RequestName(), failing connection.");
+        bus_enter_closing(sd_bus_message_get_bus(m));
+        return 1;
+}
+
 int manager_connect_bus(Manager *m) {
         int r;
 
@@ -125,7 +221,7 @@ int manager_connect_bus(Manager *m) {
         if (r < 0)
                 return log_error_errno(r, "Failed to add manager object vtable: %m");
 
-        r = sd_bus_request_name_async(m->bus, NULL, "org.freedesktop.timesync1", 0, NULL, NULL);
+        r = sd_bus_request_name_async(m->bus, NULL, "org.freedesktop.timesync1", 0, request_name_handler, NULL);
         if (r < 0)
                 return log_error_errno(r, "Failed to request name: %m");