rls: change lossy GetState() and WaitForStateChange() to use grpcsync…

….PubSub (#8055)

balancer/rls/control_channel.go

@@ -29,7 +29,9 @@ import (
 	"google.golang.org/grpc/connectivity"
 	"google.golang.org/grpc/credentials/insecure"
 	"google.golang.org/grpc/internal"
+	"google.golang.org/grpc/internal/buffer"
 	internalgrpclog "google.golang.org/grpc/internal/grpclog"
+	"google.golang.org/grpc/internal/grpcsync"
 	"google.golang.org/grpc/internal/pretty"
 	rlsgrpc "google.golang.org/grpc/internal/proto/grpc_lookup_v1"
 	rlspb "google.golang.org/grpc/internal/proto/grpc_lookup_v1"
@@ -55,37 +57,58 @@ type controlChannel struct {
 	// hammering the RLS service while it is overloaded or down.
 	throttler adaptiveThrottler
 
-	cc     *grpc.ClientConn
-	client rlsgrpc.RouteLookupServiceClient
-	logger *internalgrpclog.PrefixLogger
+	cc                  *grpc.ClientConn
+	client              rlsgrpc.RouteLookupServiceClient
+	logger              *internalgrpclog.PrefixLogger
+	connectivityStateCh *buffer.Unbounded
+	unsubscribe         func()
+	monitorDoneCh       chan struct{}
 }
 
 // newControlChannel creates a controlChannel to rlsServerName and uses
 // serviceConfig, if non-empty, as the default service config for the underlying
 // gRPC channel.
 func newControlChannel(rlsServerName, serviceConfig string, rpcTimeout time.Duration, bOpts balancer.BuildOptions, backToReadyFunc func()) (*controlChannel, error) {
 	ctrlCh := &controlChannel{
-		rpcTimeout:      rpcTimeout,
-		backToReadyFunc: backToReadyFunc,
-		throttler:       newAdaptiveThrottler(),
+		rpcTimeout:          rpcTimeout,
+		backToReadyFunc:     backToReadyFunc,
+		throttler:           newAdaptiveThrottler(),
+		connectivityStateCh: buffer.NewUnbounded(),
+		monitorDoneCh:       make(chan struct{}),
 	}
 	ctrlCh.logger = internalgrpclog.NewPrefixLogger(logger, fmt.Sprintf("[rls-control-channel %p] ", ctrlCh))
 
 	dopts, err := ctrlCh.dialOpts(bOpts, serviceConfig)
 	if err != nil {
 		return nil, err
 	}
-	ctrlCh.cc, err = grpc.Dial(rlsServerName, dopts...)
+	ctrlCh.cc, err = grpc.NewClient(rlsServerName, dopts...)
 	if err != nil {
 		return nil, err
 	}
+	// Subscribe to connectivity state before connecting to avoid missing initial
+	// updates, which are only delivered to active subscribers.
+	ctrlCh.unsubscribe = internal.SubscribeToConnectivityStateChanges.(func(cc *grpc.ClientConn, s grpcsync.Subscriber) func())(ctrlCh.cc, ctrlCh)
+	ctrlCh.cc.Connect()
 	ctrlCh.client = rlsgrpc.NewRouteLookupServiceClient(ctrlCh.cc)
 	ctrlCh.logger.Infof("Control channel created to RLS server at: %v", rlsServerName)
-
-	go ctrlCh.monitorConnectivityState()
+	start := make(chan struct{})
+	go func() {
+		close(start)
+		ctrlCh.monitorConnectivityState()
+	}()
+	<-start
 	return ctrlCh, nil
 }
 
+func (cc *controlChannel) OnMessage(msg any) {
+	st, ok := msg.(connectivity.State)
+	if !ok {
+		panic(fmt.Sprintf("Unexpected message type %T , wanted connectectivity.State type", msg))
+	}
+	cc.connectivityStateCh.Put(st)
+}
+
 // dialOpts constructs the dial options for the control plane channel.
 func (cc *controlChannel) dialOpts(bOpts balancer.BuildOptions, serviceConfig string) ([]grpc.DialOption, error) {
 	// The control plane channel will use the same authority as the parent
@@ -97,7 +120,6 @@ func (cc *controlChannel) dialOpts(bOpts balancer.BuildOptions, serviceConfig st
 	if bOpts.Dialer != nil {
 		dopts = append(dopts, grpc.WithContextDialer(bOpts.Dialer))
 	}
-
 	// The control channel will use the channel credentials from the parent
 	// channel, including any call creds associated with the channel creds.
 	var credsOpt grpc.DialOption
@@ -133,6 +155,8 @@ func (cc *controlChannel) dialOpts(bOpts balancer.BuildOptions, serviceConfig st
 
 func (cc *controlChannel) monitorConnectivityState() {
 	cc.logger.Infof("Starting connectivity state monitoring goroutine")
+	defer close(cc.monitorDoneCh)
+
 	// Since we use two mechanisms to deal with RLS server being down:
 	//   - adaptive throttling for the channel as a whole
 	//   - exponential backoff on a per-request basis
@@ -154,39 +178,45 @@ func (cc *controlChannel) monitorConnectivityState() {
 	// returning only one new picker, regardless of how many backoff timers are
 	// cancelled.
 
-	// Using the background context is fine here since we check for the ClientConn
-	// entering SHUTDOWN and return early in that case.
-	ctx := context.Background()
+	// Wait for the control channel to become READY for the first time.
+	for s, ok := <-cc.connectivityStateCh.Get(); s != connectivity.Ready; s, ok = <-cc.connectivityStateCh.Get() {
+		if !ok {
+			return
+		}
+
+		cc.connectivityStateCh.Load()
+		if s == connectivity.Shutdown {
+			return
+		}
+	}
+	cc.connectivityStateCh.Load()
+	cc.logger.Infof("Connectivity state is READY")
 
-	first := true
 	for {
-		// Wait for the control channel to become READY.
-		for s := cc.cc.GetState(); s != connectivity.Ready; s = cc.cc.GetState() {
-			if s == connectivity.Shutdown {
-				return
-			}
-			cc.cc.WaitForStateChange(ctx, s)
+		s, ok := <-cc.connectivityStateCh.Get()
+		if !ok {
+			return
 		}
-		cc.logger.Infof("Connectivity state is READY")
+		cc.connectivityStateCh.Load()
 
-		if !first {
+		if s == connectivity.Shutdown {
+			return
+		}
+		if s == connectivity.Ready {
 			cc.logger.Infof("Control channel back to READY")
 			cc.backToReadyFunc()
 		}
-		first = false
 
-		// Wait for the control channel to move out of READY.
-		cc.cc.WaitForStateChange(ctx, connectivity.Ready)
-		if cc.cc.GetState() == connectivity.Shutdown {
-			return
-		}
-		cc.logger.Infof("Connectivity state is %s", cc.cc.GetState())
+		cc.logger.Infof("Connectivity state is %s", s)
 	}
 }
 
 func (cc *controlChannel) close() {
-	cc.logger.Infof("Closing control channel")
+	cc.unsubscribe()
+	cc.connectivityStateCh.Close()
+	<-cc.monitorDoneCh
 	cc.cc.Close()
+	cc.logger.Infof("Shutdown")
 }
 
 type lookupCallback func(targets []string, headerData string, err error)