questions over userStatesMtx

[bboreham]

Exhibit 1:

func (i *Ingester) query(ctx context.Context, from, through model.Time, matchers []*labels.Matcher) (model.Matrix, error) {
	i.queries.Inc()

	i.userStatesMtx.RLock()
	defer i.userStatesMtx.RUnlock()
	state, err := i.userStates.getOrCreate(ctx)

In what sense is this a Read lock if we can Create a user state inside it?
And why do we hold this lock (over all user states) for the duration of the query over one?
There are five other RLock calls which are very similar.

Exhibit 2:

// TransferChunks receives all the chunks from another ingester.
func (i *Ingester) TransferChunks(stream client.Ingester_TransferChunksServer) error {
[...]
	i.userStatesMtx.Lock()
	defer i.userStatesMtx.Unlock()

	if err := i.ChangeState(ring.ACTIVE); err != nil {
		return err
	}
	i.userStates = userStates

	return nil

This is the only place a write-lock is taken on userStatesMtx, and only for as long as it takes to CAS the state change.

Since there is a lock mtx inside userStates protecting its data, I suspect that userStatesMtx is left-over from some earlier implementation that needed it.

[tomwilkie]

I think this has been answered in #859? The lock protects the pointer to userStates, which is only ever changed in TransferChunks.