Comment 5 for bug 1899657

2020-10-13 14:48:12 ERROR juju.worker.dependency engine.go:671 "migration-minion" manifold worker returned unexpected error: txn watcher sync error
2020-10-13 14:48:12 DEBUG juju.worker.dependency engine.go:673 stack trace:
txn watcher sync error

^- this is a rather serious error, where while Juju is trying to read the active transactions going on in Mongo, it gets disconnected. This sort of error generally causes a full restart of everything going on in Juju (all the workers get restarted, because Juju has lost track of what is going on in the model at this point).

While "txn watcher sync" gets all the noise in the logs, somewhere else there should be a separate log line indicating the underlying error. This might be something like EOF (which just indicates that Mongo hung up on us).

  added, err := w.sync()
  if err != nil {
   w.hub.Publish(txnWatcherSyncErr, nil)
   return errors.Trace(err)

That should be in the controller logs (vs the unit agent logs).