In the exchange operator, it's possible that consumer A may fill its buffer, and consumer B is waiting for A to consume its documents before execution can continue. It seems that the maxTimeMS option is not handled properly when that happens; the cursor will hang indefinitely rather than returning when maxTimeMS is reached. 

charlie.swanson@mongodb.com suggested using something like this to wait on condition variables so the maxTimeMS can be respected, rather than hanging on the mutex as happens now.