Server Selection logic bug. Assume a primary P and secondary S, and a read preference with mode SecondaryPreferred and a tag set. S doesn't match the tag set, so the driver should fall back to using the primary.

There's a bug if P happens to match the read preference's tag set. The driver selects no servers instead of using the primary.

mongoc_topology_description_suitable_servers's logic is:

1. Set has_secondary true, because there is a secondary S.
2. Select "eligible" servers according to the tag set: this leaves only P.
3. Since there is at least one eligible server, leave has_secondary true.
4. Since has_secondary is true and mode is SecondaryPreferred, remove P from suitable servers.
5. Now there are no suitable servers, selection fails.

The bug is near #3: has_secondary should only be true if there is an eligible secondary, rather than if there is any eligible server?

Another option is to partly rewrite the C Driver's server selection based on how other clients do SecondaryPreferred: try selection with mode Secondary, if that fails then select with mode Primary. For PrimaryPreferred, vice versa.