Generic health check via producer

balancer/balancer.go

@@ -152,6 +152,8 @@ type SubConn interface {
 	// indicate the shutdown operation.  This may be delivered before
 	// in-progress RPCs are complete and the actual connection is closed.
 	Shutdown()
+	RegisterConnectivityListner(StateListener)
+	UnregisterConnectivityListner(StateListener)
 }
 
 // NewSubConnOptions contains options to create new SubConn.
@@ -261,6 +263,8 @@ type BuildOptions struct {
 	// metrics. Balancer implementations which do not register metrics on
 	// metrics registry and record on them can ignore this field.
 	MetricsRecorder estats.MetricsRecorder
+
+	HealthCheckOptions HealthCheckOptions
 }
 
 // Builder creates a balancer.
@@ -462,3 +466,15 @@ type ProducerBuilder interface {
 // other methods to provide additional functionality, e.g. configuration or
 // subscription registration.
 type Producer any
+
+type StateListener interface {
+	OnStateChange(SubConnState)
+}
+
+// HealthCheckOptions are the options to configure the health check producer.
+type HealthCheckOptions struct {
+	DisableHealthCheckDialOpt bool
+	ServiceName               func() string
+	HealthCheckFunc           internal.HealthChecker
+	EnableHealthCheck         bool
+}

balancer/base/balancer_test.go

@@ -41,6 +41,7 @@ func (c *testClientConn) NewSubConn(addrs []resolver.Address, opts balancer.NewS
 func (c *testClientConn) UpdateState(balancer.State) {}
 
 type testSubConn struct {
+	balancer.SubConn
 	updateState func(balancer.SubConnState)
 }
 

balancer/endpointsharding/endpointsharding.go

@@ -116,7 +116,13 @@ func (es *endpointSharding) UpdateClientConnState(state balancer.ClientConnState
 			bal = child.(*balancerWrapper)
 		} else {
 			bal = &balancerWrapper{
-				childState: ChildState{Endpoint: endpoint},
+				childState: ChildState{
+					Endpoint: endpoint,
+					State: balancer.State{
+						ConnectivityState: connectivity.Connecting,
+						Picker:            base.NewErrPicker(balancer.ErrNoSubConnAvailable),
+					},
+				},
 				ClientConn: es.cc,
 				es:         es,
 			}