The inspiration for this ticket originated from the bug fixed by SERVER-62682. The problem was that the implicit contract of the condition variable used was violated. The underlying _state variable in PrimaryOnlyService was mutated without an accompanying call to the condition variable's notify_all. Which lead to threads waiting for a change in _state to not be woken up.

Instead of relying on the developer remembering to adhere to the contract, we should create a helper class that will explicitly enforce this contract. This would be for a simple condition variable use case that is relatively common: one single state variable that changes and when it changes all threads waiting on a change should be notified.

From my investigation I think that the class should expose the following functions:

• setState(State newState, )
• waitForStateChange(Function callback)
  The current refactor PrimaryOnlyService serves as a good basis for that class. Specifically the _setState and _waitForStateNotRebuilding.

Though other solutions that will explicitly enforce the contract for that common pattern will also work.