rls: only reset backoff on recovery from TRANSIENT_FAILURE

Fix control channel connectivity monitoring to track TRANSIENT_FAILURE
state explicitly. Only reset backoff timers when transitioning from
TRANSIENT_FAILURE to READY, not for benign state changes like
READY → IDLE → READY.

Fixes #8693

Author: ulascansenturk

balancer/rls/control_channel.go

@@ -187,6 +187,11 @@ func (cc *controlChannel) monitorConnectivityState() {
 	cc.connectivityStateCh.Load()
 	cc.logger.Infof("Connectivity state is READY")
 
+	// Track whether we've seen TRANSIENT_FAILURE since the last READY state.
+	// We only want to reset backoff when recovering from an actual failure,
+	// not when transitioning through benign states like IDLE.
+	seenTransientFailure := false
+
 	for {
 		s, ok := <-cc.connectivityStateCh.Get()
 		if !ok {
@@ -197,9 +202,21 @@ func (cc *controlChannel) monitorConnectivityState() {
 		if s == connectivity.Shutdown {
 			return
 		}
+
+		// Track if we've entered TRANSIENT_FAILURE state
+		if s == connectivity.TransientFailure {
+			seenTransientFailure = true
+		}
+
+		// Only reset backoff if we're returning to READY after a failure
 		if s == connectivity.Ready {
-			cc.logger.Infof("Control channel back to READY")
-			cc.backToReadyFunc()
+			if seenTransientFailure {
+				cc.logger.Infof("Control channel back to READY after TRANSIENT_FAILURE")
+				cc.backToReadyFunc()
+				seenTransientFailure = false
+			} else {
+				cc.logger.Infof("Control channel back to READY (no prior failure)")
+			}
 		}
 
 		cc.logger.Infof("Connectivity state is %s", s)

balancer/rls/control_channel_test.go

@@ -26,13 +26,15 @@ import (
 	"fmt"
 	"os"
 	"regexp"
+	"sync"
 	"testing"
 	"time"
 
 	"github.com/google/go-cmp/cmp"
 	"google.golang.org/grpc"
 	"google.golang.org/grpc/balancer"
 	"google.golang.org/grpc/codes"
+	"google.golang.org/grpc/connectivity"
 	"google.golang.org/grpc/credentials"
 	"google.golang.org/grpc/internal"
 	rlspb "google.golang.org/grpc/internal/proto/grpc_lookup_v1"
@@ -463,3 +465,94 @@ func (s) TestNewControlChannelUnsupportedCredsBundle(t *testing.T) {
 		t.Fatal("newControlChannel succeeded when expected to fail")
 	}
 }
+
+// TestControlChannelConnectivityStateTransitions verifies that the control
+// channel only resets backoff when recovering from TRANSIENT_FAILURE, not
+// when going through benign state changes like READY → IDLE → READY.
+func (s) TestControlChannelConnectivityStateTransitions(t *testing.T) {
+	tests := []struct {
+		name              string
+		states            []connectivity.State
+		wantCallbackCount int
+	}{
+		{
+			name: "READY → TRANSIENT_FAILURE → READY triggers callback",
+			states: []connectivity.State{
+				connectivity.TransientFailure,
+				connectivity.Ready,
+			},
+			wantCallbackCount: 1,
+		},
+		{
+			name: "READY → IDLE → READY does not trigger callback",
+			states: []connectivity.State{
+				connectivity.Idle,
+				connectivity.Ready,
+			},
+			wantCallbackCount: 0,
+		},
+		{
+			name: "Multiple failures trigger callback each time",
+			states: []connectivity.State{
+				connectivity.TransientFailure,
+				connectivity.Ready,
+				connectivity.TransientFailure,
+				connectivity.Ready,
+			},
+			wantCallbackCount: 2,
+		},
+		{
+			name: "IDLE between failures doesn't affect callback",
+			states: []connectivity.State{
+				connectivity.TransientFailure,
+				connectivity.Idle,
+				connectivity.Ready,
+			},
+			wantCallbackCount: 1,
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			// Start an RLS server
+			rlsServer, _ := rlstest.SetupFakeRLSServer(t, nil)
+
+			// Setup callback to count invocations
+			callbackCount := 0
+			var mu sync.Mutex
+			callback := func() {
+				mu.Lock()
+				callbackCount++
+				mu.Unlock()
+			}
+
+			// Create control channel
+			ctrlCh, err := newControlChannel(rlsServer.Address, "", defaultTestTimeout, balancer.BuildOptions{}, callback)
+			if err != nil {
+				t.Fatalf("Failed to create control channel: %v", err)
+			}
+			defer ctrlCh.close()
+
+			// Give the channel time to reach initial READY state
+			time.Sleep(100 * time.Millisecond)
+
+			// Inject the test state sequence
+			for _, state := range tt.states {
+				ctrlCh.OnMessage(state)
+				// Give time for the monitoring goroutine to process the state
+				time.Sleep(50 * time.Millisecond)
+			}
+
+			// Give extra time for any pending callbacks
+			time.Sleep(100 * time.Millisecond)
+
+			mu.Lock()
+			gotCallbackCount := callbackCount
+			mu.Unlock()
+
+			if gotCallbackCount != tt.wantCallbackCount {
+				t.Errorf("Got %d callback invocations, want %d", gotCallbackCount, tt.wantCallbackCount)
+			}
+		})
+	}
+}