While inspecting mongoc-read-prefs.c during implementation of hedged reads, it was discovered that the C driver does not properly handle sending read preference to mongos for secondary preferred mode with a specified max_staleness_seconds.

Prior to the hedged reads change, the server selection spec stated:

  - For mode 'secondaryPreferred', drivers MUST set the ``slaveOK`` wire protocol flag.
    If the read preference contains a non-empty ``tag_sets`` parameter,
    or ``maxStalenessSeconds`` is a positive integer, drivers MUST
    use ``$readPreference``; otherwise, drivers MUST NOT use ``$readPreference``

However, the logic in the _apply_read_preferences_mongos function was never updated for the implementation of the max_staleness_seconds option.

The logic is currently:

   if (mode == MONGOC_READ_SECONDARY_PREFERRED && bson_empty0 (tags)) {
       result->flags |= MONGOC_QUERY_SLAVE_OK;
 
    } else if (mode != MONGOC_READ_PRIMARY) {

Therefore, a read preference of mode MONGOC_READ_SECONDARY_PREFERRED with a positive integer value for max_staleness_seconds will NOT be communicated to mongos as required by the spec. However, if the read preference happens to have specified the tags parameter, then correct behavior will be observed.