Major - P3 Documentation for Read preference tags (tag_sets) seems to be missing.
Examples on how to provide them (and how they are parsed) through the uri would be nice:
http://mongoc.org/libmongoc/current/mongoc_uri_t.html
The Connection String Spec has this to say:
- Lists:
Repeated keys represent a list in the Connection String consisting of the corresponding values in the same order as they appear in the Connection String. For example::
?readPreferenceTags=dc:ny,rack:1&readPreferenceTags=dc:ny&readPreferenceTags=
- Key value pairs:
A value that represents one or more key and value pairs. Multiple key value pairs are delimited by a comma (","). The key is everything up to the first colon sign (":") and the value is everything afterwards. If any keys or values containing a comma (",") or a colon (":") they must be URL enco ded. For example::
?readPreferenceTags=dc:ny,rack:1
The mongoc_read_prefs_set_tags could also use an example as its not entirely immediately obvious how multiple tags works.
http://mongoc.org/libmongoc/current/mongoc_read_prefs_set_tags.html