sd-bus: optimize sd_bus_get_name_creds()

If the caller asks for a unique name, but the input name is already
unique we can shortcut things and just use that, as long as we make at
least one bus call to validate it.

src/libsystemd/sd-bus/bus-control.c

@@ -430,7 +430,7 @@ _public_ int sd_bus_get_name_creds(
 
         _cleanup_(sd_bus_message_unrefp) sd_bus_message *reply_unique = NULL, *reply = NULL;
         _cleanup_(sd_bus_creds_unrefp) sd_bus_creds *c = NULL;
-        const char *unique = NULL;
+        const char *unique;
         pid_t pid = 0;
         int r;
 
@@ -459,9 +459,12 @@ _public_ int sd_bus_get_name_creds(
         if (!BUS_IS_OPEN(bus->state))
                 return -ENOTCONN;
 
-        /* Only query the owner if the caller wants to know it or if
-         * the caller just wants to check whether a name exists */
-        if ((mask & SD_BUS_CREDS_UNIQUE_NAME) || mask == 0) {
+        /* If the name is unique anyway, we can use it directly */
+        unique = name[0] == ':' ? name : NULL;
+
+        /* Only query the owner if the caller wants to know it and the name is not unique anyway, or if the caller just
+         * wants to check whether a name exists */
+        if ((FLAGS_SET(mask, SD_BUS_CREDS_UNIQUE_NAME) && !unique) || mask == 0) {
                 r = sd_bus_call_method(
                                 bus,
                                 "org.freedesktop.DBus",
@@ -483,6 +486,7 @@ _public_ int sd_bus_get_name_creds(
         if (mask != 0) {
                 _cleanup_(sd_bus_error_free) sd_bus_error error = SD_BUS_ERROR_NULL;
                 bool need_pid, need_uid, need_selinux, need_separate_calls;
+
                 c = bus_creds_new();
                 if (!c)
                         return -ENOMEM;