Initially we thought our clients lost their connection and couldn't recover, but
we seem to run out of client connections to our primary after a while.

We run a replicaset with secondary preferred and use requestStart/requestDone. In the testing we did, our server requests return correctly, however it seems the mongodb connections are never released.

More details of the problem:

https://groups.google.com/forum/?fromgroups=#!topic/mongodb-user/4lWrfaOB_EI

Full threaddump attached from after the facts, where the client is "stuck" waiting for a connection while it's connectionsPerHost is maxed out.