Restore oldestRegisteredTimestamp optimization

ring/ring.go

@@ -726,6 +726,16 @@ func (r *Ring) shuffleShard(identifier string, size int, lookbackPeriod time.Dur
 	r.mtx.RLock()
 	defer r.mtx.RUnlock()
 
+	// If all instances have RegisteredTimestamp within the lookback period,
+	// then all instances would be included in the resulting ring, so we can
+	// simply return this ring.
+	//
+	// If any instance had RegisteredTimestamp equal to 0 (it would not cause additional lookup of next instance),
+	// then r.oldestRegisteredTimestamp is zero too, and we skip this optimization.
+	if lookbackPeriod > 0 && r.oldestRegisteredTimestamp > 0 && r.oldestRegisteredTimestamp >= lookbackUntil {
+		return r
+	}
+
 	var numInstancesPerZone int
 	var actualZones []string
 
@@ -801,7 +811,7 @@ func (r *Ring) shuffleShard(identifier string, size int, lookbackPeriod time.Dur
 				// If the lookback is enabled, and this instance has switched its read-only state within the lookback period,
 				// then we should include it in the subring, but continue selecting more instances.
 				//
-				// * If instance switched to read-only state within the lookback period, then next instance is currently receiving data that belonged to this instance before.
+				// * If instance switched to read-only state within the lookback period, then next instance is currently receiving data that previously belonged to this instance.
 				// * If instance switched to read-write state (read-only=false) within the lookback period, then there was another instance that received data that now belongs back to this instance.
 				if lookbackPeriod > 0 && instance.ReadOnlyUpdatedTimestamp >= lookbackUntil {
 					continue