Use custom for of grpc-go and unify client and server

vendor/google.golang.org/grpc/balancer/balancer.go

@@ -226,3 +226,45 @@ type Balancer interface {
 	// ClientConn.RemoveSubConn for its existing SubConns.
 	Close()
 }
+
+// ConnectivityStateEvaluator takes the connectivity states of multiple SubConns
+// and returns one aggregated connectivity state.
+//
+// It's not thread safe.
+type ConnectivityStateEvaluator struct {
+	numReady            uint64 // Number of addrConns in ready state.
+	numConnecting       uint64 // Number of addrConns in connecting state.
+	numTransientFailure uint64 // Number of addrConns in transientFailure.
+}
+
+// RecordTransition records state change happening in subConn and based on that
+// it evaluates what aggregated state should be.
+//
+//  - If at least one SubConn in Ready, the aggregated state is Ready;
+//  - Else if at least one SubConn in Connecting, the aggregated state is Connecting;
+//  - Else the aggregated state is TransientFailure.
+//
+// Idle and Shutdown are not considered.
+func (cse *ConnectivityStateEvaluator) RecordTransition(oldState, newState connectivity.State) connectivity.State {
+	// Update counters.
+	for idx, state := range []connectivity.State{oldState, newState} {
+		updateVal := 2*uint64(idx) - 1 // -1 for oldState and +1 for new.
+		switch state {
+		case connectivity.Ready:
+			cse.numReady += updateVal
+		case connectivity.Connecting:
+			cse.numConnecting += updateVal
+		case connectivity.TransientFailure:
+			cse.numTransientFailure += updateVal
+		}
+	}
+
+	// Evaluate.
+	if cse.numReady > 0 {
+		return connectivity.Ready
+	}
+	if cse.numConnecting > 0 {
+		return connectivity.Connecting
+	}
+	return connectivity.TransientFailure
+}