Create the run context once in NewEngine instead of in Start. This
keeps e.cancel valid for the engine's whole lifetime, so Stop can
cancel a Start that is blocked waiting on the network while holding
syncMsgMux: Stop now cancels before taking the lock, unblocking that
Start so it can release the mutex.
Reject re-entry into Start: a non-nil wgInterface means a prior Start
already ran (ErrEngineAlreadyStarted), and a cancelled run context
means the engine was stopped (ErrEngineAlreadyStopped). Both checks run
before the cleanup defer so a duplicate call cannot tear down the
running engine's state.
Zoltán Papp
c370c72d93