In mongodb/mongo-php-library#562, a user reported an issue executing a Collection::find() operation in PHPLIB. Because the library explicitly assigns the collection's read preference on the query, it triggered the following code path in _mongoc_cursor_new_with_opts():

   if (_mongoc_client_session_in_txn (cursor->client_session)) {
      if (!IS_PREF_PRIMARY (user_prefs)) {
         bson_set_error (&cursor->error,
                         MONGOC_ERROR_CURSOR,
                         MONGOC_ERROR_CURSOR_INVALID_CURSOR,
                         "Read preference in a transaction must be primary");
         GOTO (finish);
      }

      if (user_prefs) {
         bson_set_error (
            &cursor->error,
            MONGOC_ERROR_CURSOR,
            MONGOC_ERROR_CURSOR_INVALID_CURSOR,
            "Cannot set read preferences after starting transaction");
         GOTO (finish);
      }

The first check for a non-primary read preference is consistent with _mongoc_client_command_with_opts() and mongoc_cmd_parts_assemble(), but the second conditional only exists in this function. I don't believe it serves a purpose, as a primary read preference should be permitted at this point.

Note: I realize that it's a bit redundant for PHPLIB to add a read preference to a query if we're not communicating with mongos, since we would have already done server selection by this point. That is likely be something we can refactor independently of this issue.