From 025055ebafb46dcdd529fc010230822fe265ed3d Mon Sep 17 00:00:00 2001
From: Arjan Bal <arjansbal@google.com>
Date: Mon, 19 May 2025 14:29:54 +0530
Subject: [PATCH 1/6] Implement ExitIdler for endpointsharding

---
 balancer/endpointsharding/endpointsharding.go | 11 ++++
 .../endpointsharding/endpointsharding_test.go | 66 +++++++++++++++++++
 balancer/leastrequest/leastrequest.go         |  2 +
 balancer/roundrobin/roundrobin.go             |  2 +
 balancer/weightedroundrobin/balancer.go       |  2 +
 .../balancer/cdsbalancer/cdsbalancer.go       |  2 +
 .../clusterresolver/clusterresolver.go        |  2 +
 7 files changed, 87 insertions(+)

diff --git a/balancer/endpointsharding/endpointsharding.go b/balancer/endpointsharding/endpointsharding.go
index cc606f4dae4e..ad03d7a726b4 100644
--- a/balancer/endpointsharding/endpointsharding.go
+++ b/balancer/endpointsharding/endpointsharding.go
@@ -205,6 +205,17 @@ func (es *endpointSharding) Close() {
 	}
 }
 
+func (es *endpointSharding) ExitIdle() {
+	es.childMu.Lock()
+	defer es.childMu.Unlock()
+	for _, bw := range es.children.Load().Values() {
+		ei, ok := bw.child.(balancer.ExitIdler)
+		if ok && !bw.isClosed {
+			ei.ExitIdle()
+		}
+	}
+}
+
 // updateState updates this component's state. It sends the aggregated state,
 // and a picker with round robin behavior with all the child states present if
 // needed.
diff --git a/balancer/endpointsharding/endpointsharding_test.go b/balancer/endpointsharding/endpointsharding_test.go
index 5567b4c9038b..93f337326df8 100644
--- a/balancer/endpointsharding/endpointsharding_test.go
+++ b/balancer/endpointsharding/endpointsharding_test.go
@@ -285,3 +285,69 @@ func (s) TestEndpointShardingReconnectDisabled(t *testing.T) {
 		}
 	}
 }
+
+// Tests that endpointsharding doesn't automatically re-connect IDLE children
+// until cc.Connect() is called. The test creates an endpoint with a single
+// address. The client is connected and the active server is closed to make the
+// child pickfirst enter IDLE state. The test verifies that the child pickfirst
+// doesn't re-connect automatically. The test calls cc.Connect() and verified
+// that the balancer connects causing the channel to enter TransientFailure.
+func (s) TestEndpointShardingExitIdle(t *testing.T) {
+	backend := stubserver.StartTestService(t, nil)
+	defer backend.Stop()
+
+	mr := manual.NewBuilderWithScheme("e2e-test")
+	defer mr.Close()
+
+	name := strings.ReplaceAll(strings.ToLower(t.Name()), "/", "")
+	bf := stub.BalancerFuncs{
+		Init: func(bd *stub.BalancerData) {
+			epOpts := endpointsharding.Options{DisableAutoReconnect: true}
+			bd.Data = endpointsharding.NewBalancer(bd.ClientConn, bd.BuildOptions, balancer.Get(pickfirstleaf.Name).Build, epOpts)
+		},
+		UpdateClientConnState: func(bd *stub.BalancerData, ccs balancer.ClientConnState) error {
+			return bd.Data.(balancer.Balancer).UpdateClientConnState(ccs)
+		},
+		Close: func(bd *stub.BalancerData) {
+			bd.Data.(balancer.Balancer).Close()
+		},
+		ExitIdle: func(bd *stub.BalancerData) {
+			bd.Data.(balancer.ExitIdler).ExitIdle()
+		},
+	}
+	stub.Register(name, bf)
+
+	json := fmt.Sprintf(`{"loadBalancingConfig": [{"%s":{}}]}`, name)
+	sc := internal.ParseServiceConfig.(func(string) *serviceconfig.ParseResult)(json)
+	mr.InitialState(resolver.State{
+		Endpoints: []resolver.Endpoint{
+			{Addresses: []resolver.Address{{Addr: backend.Address}}},
+		},
+		ServiceConfig: sc,
+	})
+
+	cc, err := grpc.NewClient(mr.Scheme()+":///", grpc.WithResolvers(mr), grpc.WithTransportCredentials(insecure.NewCredentials()))
+	if err != nil {
+		t.Fatalf("Failed to create new client: %v", err)
+	}
+	defer cc.Close()
+	ctx, cancel := context.WithTimeout(context.Background(), defaultTestTimeout)
+	defer cancel()
+	client := testgrpc.NewTestServiceClient(cc)
+	if _, err := client.EmptyCall(ctx, &testpb.Empty{}); err != nil {
+		t.Errorf("client.EmptyCall() returned unexpected error: %v", err)
+	}
+
+	// On closing the first server, the first child balancer should enter
+	// IDLE. Since endpointsharding is configured not to auto-reconnect, it will
+	// remain IDLE and will not try to re-connect
+	backend.Stop()
+	testutils.AwaitState(ctx, t, cc, connectivity.Idle)
+	shortCtx, shortCancel := context.WithTimeout(ctx, defaultTestShortTimeout)
+	defer shortCancel()
+	testutils.AwaitNoStateChange(shortCtx, t, cc, connectivity.Idle)
+
+	// The balancer should try to re-connect and fail.
+	cc.Connect()
+	testutils.AwaitState(ctx, t, cc, connectivity.TransientFailure)
+}
diff --git a/balancer/leastrequest/leastrequest.go b/balancer/leastrequest/leastrequest.go
index f758f954e9f1..0ac0f43e6b35 100644
--- a/balancer/leastrequest/leastrequest.go
+++ b/balancer/leastrequest/leastrequest.go
@@ -127,6 +127,8 @@ func (lrb *leastRequestBalancer) ResolverError(err error) {
 func (lrb *leastRequestBalancer) ExitIdle() {
 	if ei, ok := lrb.child.(balancer.ExitIdler); ok { // Should always be ok, as child is endpoint sharding.
 		ei.ExitIdle()
+	} else {
+		lrb.logger.Errorf("Child balancer doesn't implement ExitIdler.")
 	}
 }
 
diff --git a/balancer/roundrobin/roundrobin.go b/balancer/roundrobin/roundrobin.go
index 35da5d1ec9d9..d4df537fc04a 100644
--- a/balancer/roundrobin/roundrobin.go
+++ b/balancer/roundrobin/roundrobin.go
@@ -75,5 +75,7 @@ func (b *rrBalancer) ExitIdle() {
 	// Should always be ok, as child is endpoint sharding.
 	if ei, ok := b.Balancer.(balancer.ExitIdler); ok {
 		ei.ExitIdle()
+	} else {
+		b.logger.Errorf("Child balancer doesn't implement ExitIdler.")
 	}
 }
diff --git a/balancer/weightedroundrobin/balancer.go b/balancer/weightedroundrobin/balancer.go
index e6fe5a37acd7..9f38758921a3 100644
--- a/balancer/weightedroundrobin/balancer.go
+++ b/balancer/weightedroundrobin/balancer.go
@@ -406,6 +406,8 @@ func (b *wrrBalancer) Close() {
 func (b *wrrBalancer) ExitIdle() {
 	if ei, ok := b.child.(balancer.ExitIdler); ok { // Should always be ok, as child is endpoint sharding.
 		ei.ExitIdle()
+	} else {
+		b.logger.Errorf("Child balancer doesn't implement ExitIdler.")
 	}
 }
 
diff --git a/xds/internal/balancer/cdsbalancer/cdsbalancer.go b/xds/internal/balancer/cdsbalancer/cdsbalancer.go
index fa34748c8822..1e1493d9d67c 100644
--- a/xds/internal/balancer/cdsbalancer/cdsbalancer.go
+++ b/xds/internal/balancer/cdsbalancer/cdsbalancer.go
@@ -426,6 +426,8 @@ func (b *cdsBalancer) ExitIdle() {
 		// will be connected.
 		if ei, ok := b.childLB.(balancer.ExitIdler); ok {
 			ei.ExitIdle()
+		} else {
+			b.logger.Errorf("Child balancer doesn't implement ExitIdler.")
 		}
 	})
 }
diff --git a/xds/internal/balancer/clusterresolver/clusterresolver.go b/xds/internal/balancer/clusterresolver/clusterresolver.go
index fe5d93dbfbdf..60d836741c04 100644
--- a/xds/internal/balancer/clusterresolver/clusterresolver.go
+++ b/xds/internal/balancer/clusterresolver/clusterresolver.go
@@ -341,6 +341,8 @@ func (b *clusterResolverBalancer) run() {
 				// will be connected.
 				if ei, ok := b.child.(balancer.ExitIdler); ok {
 					ei.ExitIdle()
+				} else {
+					b.logger.Errorf("Child balancer doesn't implement ExitIdler.")
 				}
 			}
 		case u := <-b.resourceWatcher.updateChannel:

From aca03a5ee5f7d0af9d806855dc4bc44105c1e722 Mon Sep 17 00:00:00 2001
From: Arjan Bal <arjansbal@google.com>
Date: Mon, 19 May 2025 14:50:26 +0530
Subject: [PATCH 2/6] Log error in endpointsharding when child doesn't
 implement ExitIdler

---
 balancer/endpointsharding/endpointsharding.go | 19 +++++++++++++++++++
 1 file changed, 19 insertions(+)

diff --git a/balancer/endpointsharding/endpointsharding.go b/balancer/endpointsharding/endpointsharding.go
index ad03d7a726b4..47821e195cf2 100644
--- a/balancer/endpointsharding/endpointsharding.go
+++ b/balancer/endpointsharding/endpointsharding.go
@@ -27,6 +27,7 @@ package endpointsharding
 
 import (
 	"errors"
+	"fmt"
 	rand "math/rand/v2"
 	"sync"
 	"sync/atomic"
@@ -34,9 +35,13 @@ import (
 	"google.golang.org/grpc/balancer"
 	"google.golang.org/grpc/balancer/base"
 	"google.golang.org/grpc/connectivity"
+	"google.golang.org/grpc/grpclog"
+	internalgrpclog "google.golang.org/grpc/internal/grpclog"
 	"google.golang.org/grpc/resolver"
 )
 
+var logger = grpclog.Component("endpointsharding-lb")
+
 // ChildState is the balancer state of a child along with the endpoint which
 // identifies the child balancer.
 type ChildState struct {
@@ -73,6 +78,7 @@ func NewBalancer(cc balancer.ClientConn, opts balancer.BuildOptions, childBuilde
 		esOpts:       esOpts,
 		childBuilder: childBuilder,
 	}
+	es.logger = internalgrpclog.NewPrefixLogger(logger, fmt.Sprintf("[endpointsharding-lb %p] ", es))
 	es.children.Store(resolver.NewEndpointMap[*balancerWrapper]())
 	return es
 }
@@ -85,6 +91,7 @@ type endpointSharding struct {
 	bOpts        balancer.BuildOptions
 	esOpts       Options
 	childBuilder ChildBuilderFunc
+	logger       *internalgrpclog.PrefixLogger // Prefix logger for all logging.
 
 	// childMu synchronizes calls to any single child. It must be held for all
 	// calls into a child. To avoid deadlocks, do not acquire childMu while
@@ -209,9 +216,15 @@ func (es *endpointSharding) ExitIdle() {
 	es.childMu.Lock()
 	defer es.childMu.Unlock()
 	for _, bw := range es.children.Load().Values() {
+		// this implementation assumes the child balancer supports
+		// exitidle (but still checks for the interface's existence to
+		// avoid a panic if not). If the child does not, no subconns
+		// will be connected.
 		ei, ok := bw.child.(balancer.ExitIdler)
 		if ok && !bw.isClosed {
 			ei.ExitIdle()
+		} else if !ok {
+			es.logger.Errorf("Child balancer doesn't implement ExitIdler.")
 		}
 	}
 }
@@ -337,6 +350,10 @@ func (bw *balancerWrapper) UpdateState(state balancer.State) {
 // ExitIdle pings an IDLE child balancer to exit idle in a new goroutine to
 // avoid deadlocks due to synchronous balancer state updates.
 func (bw *balancerWrapper) ExitIdle() {
+	// this implementation assumes the child balancer supports
+	// exitidle (but still checks for the interface's existence to
+	// avoid a panic if not). If the child does not, no subconns
+	// will be connected.
 	if ei, ok := bw.child.(balancer.ExitIdler); ok {
 		go func() {
 			bw.es.childMu.Lock()
@@ -345,6 +362,8 @@ func (bw *balancerWrapper) ExitIdle() {
 			}
 			bw.es.childMu.Unlock()
 		}()
+	} else if !ok {
+		bw.es.logger.Errorf("Child balancer doesn't implement ExitIdler.")
 	}
 }
 

From 98ebf34bbef864a9263837d8ecfd0de0040c7352 Mon Sep 17 00:00:00 2001
From: Arjan Bal <arjansbal@google.com>
Date: Tue, 20 May 2025 10:54:07 +0530
Subject: [PATCH 3/6] Ensure all Balancer implementers also implement ExitIdler

---
 balancer/endpointsharding/endpointsharding_test.go       | 4 ++++
 .../pickfirst/pickfirstleaf/pickfirstleaf_ext_test.go    | 4 +---
 balancer/rls/internal/test/e2e/rls_child_policy.go       | 4 ++++
 balancer/weightedtarget/weightedtarget_test.go           | 4 ++++
 clientconn_test.go                                       | 4 ++++
 internal/balancer/gracefulswitch/gracefulswitch_test.go  | 2 ++
 internal/balancer/nop/nop.go                             | 3 +++
 internal/stats/metrics_recorder_list_test.go             | 4 ++++
 interop/orcalb.go                                        | 6 ++++++
 test/clientconn_state_transition_test.go                 | 4 ++++
 test/end2end_test.go                                     | 4 ++++
 xds/internal/balancer/wrrlocality/balancer.go            | 9 +++++++++
 12 files changed, 49 insertions(+), 3 deletions(-)

diff --git a/balancer/endpointsharding/endpointsharding_test.go b/balancer/endpointsharding/endpointsharding_test.go
index 93f337326df8..ce68a80af0dc 100644
--- a/balancer/endpointsharding/endpointsharding_test.go
+++ b/balancer/endpointsharding/endpointsharding_test.go
@@ -102,6 +102,10 @@ type fakePetiole struct {
 	bOpts balancer.BuildOptions
 }
 
+func (fp *fakePetiole) ExitIdle() {
+	fp.Balancer.(balancer.ExitIdler).ExitIdle()
+}
+
 func (fp *fakePetiole) UpdateClientConnState(state balancer.ClientConnState) error {
 	if el := state.ResolverState.Endpoints; len(el) != 2 {
 		return fmt.Errorf("UpdateClientConnState wants two endpoints, got: %v", el)
diff --git a/balancer/pickfirst/pickfirstleaf/pickfirstleaf_ext_test.go b/balancer/pickfirst/pickfirstleaf/pickfirstleaf_ext_test.go
index 160783fc27d3..be8c43fa7ab5 100644
--- a/balancer/pickfirst/pickfirstleaf/pickfirstleaf_ext_test.go
+++ b/balancer/pickfirst/pickfirstleaf/pickfirstleaf_ext_test.go
@@ -1593,9 +1593,7 @@ func (b *stateStoringBalancer) Close() {
 }
 
 func (b *stateStoringBalancer) ExitIdle() {
-	if ib, ok := b.Balancer.(balancer.ExitIdler); ok {
-		ib.ExitIdle()
-	}
+	b.Balancer.(balancer.ExitIdler).ExitIdle()
 }
 
 type stateStoringBalancerBuilder struct {
diff --git a/balancer/rls/internal/test/e2e/rls_child_policy.go b/balancer/rls/internal/test/e2e/rls_child_policy.go
index 2bae34b60887..3e06d818b047 100644
--- a/balancer/rls/internal/test/e2e/rls_child_policy.go
+++ b/balancer/rls/internal/test/e2e/rls_child_policy.go
@@ -102,6 +102,10 @@ type RLSChildPolicyConfig struct {
 	Random  string // A random field to test child policy config changes.
 }
 
+func (b *bal) ExitIdle() {
+	b.Balancer.(balancer.ExitIdler).ExitIdle()
+}
+
 func (b *bal) UpdateClientConnState(c balancer.ClientConnState) error {
 	cfg, ok := c.BalancerConfig.(*RLSChildPolicyConfig)
 	if !ok {
diff --git a/balancer/weightedtarget/weightedtarget_test.go b/balancer/weightedtarget/weightedtarget_test.go
index c9c78fc5a307..a9a1d4340b33 100644
--- a/balancer/weightedtarget/weightedtarget_test.go
+++ b/balancer/weightedtarget/weightedtarget_test.go
@@ -130,6 +130,10 @@ func getConfigKey(attr *attributes.Attributes) (string, bool) {
 	return name, ok
 }
 
+func (b *testConfigBalancer) ExitIdle() {
+	b.Balancer.(balancer.ExitIdler).ExitIdle()
+}
+
 func (b *testConfigBalancer) UpdateClientConnState(s balancer.ClientConnState) error {
 	c, ok := s.BalancerConfig.(stringBalancerConfig)
 	if !ok {
diff --git a/clientconn_test.go b/clientconn_test.go
index 9cca5a8eb7b7..96eb7ace51bd 100644
--- a/clientconn_test.go
+++ b/clientconn_test.go
@@ -1220,6 +1220,10 @@ type stateRecordingBalancer struct {
 	balancer.Balancer
 }
 
+func (b *stateRecordingBalancer) ExitIdle() {
+	b.Balancer.(balancer.ExitIdler).ExitIdle()
+}
+
 func (b *stateRecordingBalancer) UpdateSubConnState(sc balancer.SubConn, s balancer.SubConnState) {
 	panic(fmt.Sprintf("UpdateSubConnState(%v, %+v) called unexpectedly", sc, s))
 }
diff --git a/internal/balancer/gracefulswitch/gracefulswitch_test.go b/internal/balancer/gracefulswitch/gracefulswitch_test.go
index 5e6c53c74534..537f23d42761 100644
--- a/internal/balancer/gracefulswitch/gracefulswitch_test.go
+++ b/internal/balancer/gracefulswitch/gracefulswitch_test.go
@@ -1010,6 +1010,8 @@ func (vb *verifyBalancer) UpdateClientConnState(balancer.ClientConnState) error
 	return nil
 }
 
+func (vb *verifyBalancer) ExitIdle() {}
+
 func (vb *verifyBalancer) ResolverError(error) {}
 
 func (vb *verifyBalancer) UpdateSubConnState(sc balancer.SubConn, state balancer.SubConnState) {
diff --git a/internal/balancer/nop/nop.go b/internal/balancer/nop/nop.go
index 0c96f1b81186..8c5bf96f2d27 100644
--- a/internal/balancer/nop/nop.go
+++ b/internal/balancer/nop/nop.go
@@ -60,3 +60,6 @@ func (b *bal) UpdateSubConnState(_ balancer.SubConn, _ balancer.SubConnState) {}
 
 // Close is a no-op.
 func (b *bal) Close() {}
+
+// ExitIdle is a no-op.
+func (b *bal) ExitIdle() {}
diff --git a/internal/stats/metrics_recorder_list_test.go b/internal/stats/metrics_recorder_list_test.go
index 38f9472f926a..2e64cf288a00 100644
--- a/internal/stats/metrics_recorder_list_test.go
+++ b/internal/stats/metrics_recorder_list_test.go
@@ -124,6 +124,10 @@ type recordingLoadBalancer struct {
 	balancer.Balancer
 }
 
+func (b *recordingLoadBalancer) ExitIdle() {
+	b.Balancer.(balancer.ExitIdler).ExitIdle()
+}
+
 // TestMetricsRecorderList tests the metrics recorder list functionality of the
 // ClientConn. It configures a global and local stats handler Dial Option. These
 // stats handlers implement the MetricsRecorder interface. It also configures a
diff --git a/interop/orcalb.go b/interop/orcalb.go
index 572a7dfcd5cb..b19defb63242 100644
--- a/interop/orcalb.go
+++ b/interop/orcalb.go
@@ -53,6 +53,12 @@ type orcab struct {
 	report   *v3orcapb.OrcaLoadReport
 }
 
+func (o *orcab) ExitIdle() {
+	if o.sc != nil {
+		o.sc.Connect()
+	}
+}
+
 func (o *orcab) UpdateClientConnState(s balancer.ClientConnState) error {
 	if o.sc != nil {
 		o.sc.UpdateAddresses(s.ResolverState.Addresses)
diff --git a/test/clientconn_state_transition_test.go b/test/clientconn_state_transition_test.go
index 6e179c1cd59a..57249201734c 100644
--- a/test/clientconn_state_transition_test.go
+++ b/test/clientconn_state_transition_test.go
@@ -474,6 +474,10 @@ type stateRecordingBalancer struct {
 	balancer.Balancer
 }
 
+func (b *stateRecordingBalancer) ExitIdle() {
+	b.Balancer.(balancer.ExitIdler).ExitIdle()
+}
+
 func (b *stateRecordingBalancer) Close() {
 	b.Balancer.Close()
 }
diff --git a/test/end2end_test.go b/test/end2end_test.go
index 75b27f4c224d..396336fc78fa 100644
--- a/test/end2end_test.go
+++ b/test/end2end_test.go
@@ -6541,6 +6541,10 @@ type triggerRPCBlockBalancer struct {
 	balancer.Balancer
 }
 
+func (bpb *triggerRPCBlockBalancer) ExitIdle() {
+	bpb.Balancer.(balancer.ExitIdler).ExitIdle()
+}
+
 func (bpb *triggerRPCBlockBalancer) UpdateClientConnState(s balancer.ClientConnState) error {
 	err := bpb.Balancer.UpdateClientConnState(s)
 	bpb.ClientConn.UpdateState(balancer.State{
diff --git a/xds/internal/balancer/wrrlocality/balancer.go b/xds/internal/balancer/wrrlocality/balancer.go
index 065de200f40c..cfc438c5a82e 100644
--- a/xds/internal/balancer/wrrlocality/balancer.go
+++ b/xds/internal/balancer/wrrlocality/balancer.go
@@ -154,6 +154,15 @@ type wrrLocalityBalancer struct {
 	logger *grpclog.PrefixLogger
 }
 
+func (b *wrrLocalityBalancer) ExitIdle() {
+	ei, ok := b.child.(balancer.ExitIdler)
+	if ok {
+		ei.ExitIdle()
+	} else if !ok {
+		b.logger.Errorf("Child balancer doesn't implement ExitIdler.")
+	}
+}
+
 func (b *wrrLocalityBalancer) UpdateClientConnState(s balancer.ClientConnState) error {
 	lbCfg, ok := s.BalancerConfig.(*LBConfig)
 	if !ok {

From 111c588b1e2cecb34638f1fc6b69193c75f1dcd1 Mon Sep 17 00:00:00 2001
From: Arjan Bal <arjansbal@google.com>
Date: Wed, 28 May 2025 19:04:28 +0530
Subject: [PATCH 4/6] Add ExitIdle to the Balancer interface

---
 balancer/balancer.go                          |  8 +++-
 balancer/endpointsharding/endpointsharding.go | 46 ++++++++-----------
 .../endpointsharding/endpointsharding_test.go |  4 +-
 balancer/lazy/lazy.go                         |  4 +-
 balancer/lazy/lazy_ext_test.go                |  4 +-
 balancer/leastrequest/leastrequest.go         |  6 +--
 .../pickfirstleaf/pickfirstleaf_ext_test.go   |  2 +-
 balancer/ringhash/ringhash.go                 |  4 +-
 .../rls/internal/test/e2e/rls_child_policy.go |  2 +-
 balancer/roundrobin/roundrobin.go             |  7 +--
 balancer/weightedroundrobin/balancer.go       |  6 +--
 .../weightedtarget/weightedtarget_test.go     |  2 +-
 clientconn_test.go                            |  2 +-
 examples/features/orca/client/main.go         |  8 ++++
 .../balancer/gracefulswitch/gracefulswitch.go | 10 +---
 .../gracefulswitch/gracefulswitch_test.go     | 22 ++-------
 internal/stats/metrics_recorder_list_test.go  |  2 +-
 test/balancer_test.go                         |  2 +-
 test/clientconn_state_transition_test.go      |  2 +-
 test/end2end_test.go                          |  2 +-
 .../balancer/cdsbalancer/cdsbalancer.go       |  6 +--
 .../balancer/cdsbalancer/cdsbalancer_test.go  |  5 +-
 .../clusterresolver/clusterresolver.go        | 10 +---
 xds/internal/balancer/wrrlocality/balancer.go |  7 +--
 24 files changed, 59 insertions(+), 114 deletions(-)

diff --git a/balancer/balancer.go b/balancer/balancer.go
index c9b343c71564..b1264017db1f 100644
--- a/balancer/balancer.go
+++ b/balancer/balancer.go
@@ -360,6 +360,10 @@ type Balancer interface {
 	// call SubConn.Shutdown for its existing SubConns; however, this will be
 	// required in a future release, so it is recommended.
 	Close()
+	// ExitIdle instructs the LB policy to reconnect to backends / exit the
+	// IDLE state, if appropriate and possible.  Note that SubConns that enter
+	// the IDLE state will not reconnect until SubConn.Connect is called.
+	ExitIdle()
 }
 
 // ExitIdler is an optional interface for balancers to implement.  If
@@ -367,8 +371,8 @@ type Balancer interface {
 // the ClientConn is idle.  If unimplemented, ClientConn.Connect will cause
 // all SubConns to connect.
 //
-// Notice: it will be required for all balancers to implement this in a future
-// release.
+// Deprecated: All balancers must implement this interface. This interface will
+// be removed in a future release.
 type ExitIdler interface {
 	// ExitIdle instructs the LB policy to reconnect to backends / exit the
 	// IDLE state, if appropriate and possible.  Note that SubConns that enter
diff --git a/balancer/endpointsharding/endpointsharding.go b/balancer/endpointsharding/endpointsharding.go
index 47821e195cf2..eb821b13a93c 100644
--- a/balancer/endpointsharding/endpointsharding.go
+++ b/balancer/endpointsharding/endpointsharding.go
@@ -27,7 +27,6 @@ package endpointsharding
 
 import (
 	"errors"
-	"fmt"
 	rand "math/rand/v2"
 	"sync"
 	"sync/atomic"
@@ -35,13 +34,9 @@ import (
 	"google.golang.org/grpc/balancer"
 	"google.golang.org/grpc/balancer/base"
 	"google.golang.org/grpc/connectivity"
-	"google.golang.org/grpc/grpclog"
-	internalgrpclog "google.golang.org/grpc/internal/grpclog"
 	"google.golang.org/grpc/resolver"
 )
 
-var logger = grpclog.Component("endpointsharding-lb")
-
 // ChildState is the balancer state of a child along with the endpoint which
 // identifies the child balancer.
 type ChildState struct {
@@ -50,7 +45,15 @@ type ChildState struct {
 
 	// Balancer exposes only the ExitIdler interface of the child LB policy.
 	// Other methods of the child policy are called only by endpointsharding.
-	Balancer balancer.ExitIdler
+	Balancer ExitIdler
+}
+
+// ExitIdler provides access to only the ExitIdle method of the child balancer.
+type ExitIdler interface {
+	// ExitIdle instructs the LB policy to reconnect to backends / exit the
+	// IDLE state, if appropriate and possible.  Note that SubConns that enter
+	// the IDLE state will not reconnect until SubConn.Connect is called.
+	ExitIdle()
 }
 
 // Options are the options to configure the behaviour of the
@@ -78,7 +81,6 @@ func NewBalancer(cc balancer.ClientConn, opts balancer.BuildOptions, childBuilde
 		esOpts:       esOpts,
 		childBuilder: childBuilder,
 	}
-	es.logger = internalgrpclog.NewPrefixLogger(logger, fmt.Sprintf("[endpointsharding-lb %p] ", es))
 	es.children.Store(resolver.NewEndpointMap[*balancerWrapper]())
 	return es
 }
@@ -91,7 +93,6 @@ type endpointSharding struct {
 	bOpts        balancer.BuildOptions
 	esOpts       Options
 	childBuilder ChildBuilderFunc
-	logger       *internalgrpclog.PrefixLogger // Prefix logger for all logging.
 
 	// childMu synchronizes calls to any single child. It must be held for all
 	// calls into a child. To avoid deadlocks, do not acquire childMu while
@@ -220,11 +221,8 @@ func (es *endpointSharding) ExitIdle() {
 		// exitidle (but still checks for the interface's existence to
 		// avoid a panic if not). If the child does not, no subconns
 		// will be connected.
-		ei, ok := bw.child.(balancer.ExitIdler)
-		if ok && !bw.isClosed {
-			ei.ExitIdle()
-		} else if !ok {
-			es.logger.Errorf("Child balancer doesn't implement ExitIdler.")
+		if !bw.isClosed {
+			bw.child.ExitIdle()
 		}
 	}
 }
@@ -350,21 +348,13 @@ func (bw *balancerWrapper) UpdateState(state balancer.State) {
 // ExitIdle pings an IDLE child balancer to exit idle in a new goroutine to
 // avoid deadlocks due to synchronous balancer state updates.
 func (bw *balancerWrapper) ExitIdle() {
-	// this implementation assumes the child balancer supports
-	// exitidle (but still checks for the interface's existence to
-	// avoid a panic if not). If the child does not, no subconns
-	// will be connected.
-	if ei, ok := bw.child.(balancer.ExitIdler); ok {
-		go func() {
-			bw.es.childMu.Lock()
-			if !bw.isClosed {
-				ei.ExitIdle()
-			}
-			bw.es.childMu.Unlock()
-		}()
-	} else if !ok {
-		bw.es.logger.Errorf("Child balancer doesn't implement ExitIdler.")
-	}
+	go func() {
+		bw.es.childMu.Lock()
+		if !bw.isClosed {
+			bw.child.ExitIdle()
+		}
+		bw.es.childMu.Unlock()
+	}()
 }
 
 // updateClientConnStateLocked delivers the ClientConnState to the child
diff --git a/balancer/endpointsharding/endpointsharding_test.go b/balancer/endpointsharding/endpointsharding_test.go
index ce68a80af0dc..8443cfe808ab 100644
--- a/balancer/endpointsharding/endpointsharding_test.go
+++ b/balancer/endpointsharding/endpointsharding_test.go
@@ -103,7 +103,7 @@ type fakePetiole struct {
 }
 
 func (fp *fakePetiole) ExitIdle() {
-	fp.Balancer.(balancer.ExitIdler).ExitIdle()
+	fp.Balancer.ExitIdle()
 }
 
 func (fp *fakePetiole) UpdateClientConnState(state balancer.ClientConnState) error {
@@ -316,7 +316,7 @@ func (s) TestEndpointShardingExitIdle(t *testing.T) {
 			bd.Data.(balancer.Balancer).Close()
 		},
 		ExitIdle: func(bd *stub.BalancerData) {
-			bd.Data.(balancer.ExitIdler).ExitIdle()
+			bd.Data.(balancer.Balancer).ExitIdle()
 		},
 	}
 	stub.Register(name, bf)
diff --git a/balancer/lazy/lazy.go b/balancer/lazy/lazy.go
index 7368d8f4bf9c..314ccd1b0aa1 100644
--- a/balancer/lazy/lazy.go
+++ b/balancer/lazy/lazy.go
@@ -125,9 +125,7 @@ func (lb *lazyBalancer) ExitIdle() {
 	lb.mu.Lock()
 	defer lb.mu.Unlock()
 	if lb.delegate != nil {
-		if d, ok := lb.delegate.(balancer.ExitIdler); ok {
-			d.ExitIdle()
-		}
+		lb.delegate.ExitIdle()
 		return
 	}
 	lb.delegate = lb.childBuilder(lb.cc, lb.buildOptions)
diff --git a/balancer/lazy/lazy_ext_test.go b/balancer/lazy/lazy_ext_test.go
index c57ad28e1bb7..cfd4ca998463 100644
--- a/balancer/lazy/lazy_ext_test.go
+++ b/balancer/lazy/lazy_ext_test.go
@@ -82,7 +82,7 @@ func (s) TestExitIdle(t *testing.T) {
 			bd.Data = lazy.NewBalancer(bd.ClientConn, bd.BuildOptions, balancer.Get(pickfirstleaf.Name).Build)
 		},
 		ExitIdle: func(bd *stub.BalancerData) {
-			bd.Data.(balancer.ExitIdler).ExitIdle()
+			bd.Data.(balancer.Balancer).ExitIdle()
 		},
 		ResolverError: func(bd *stub.BalancerData, err error) {
 			bd.Data.(balancer.Balancer).ResolverError(err)
@@ -410,7 +410,7 @@ func (s) TestExitIdlePassthrough(t *testing.T) {
 			bd.Data = lazy.NewBalancer(bd.ClientConn, bd.BuildOptions, balancer.Get(pickfirstleaf.Name).Build)
 		},
 		ExitIdle: func(bd *stub.BalancerData) {
-			bd.Data.(balancer.ExitIdler).ExitIdle()
+			bd.Data.(balancer.Balancer).ExitIdle()
 		},
 		ResolverError: func(bd *stub.BalancerData, err error) {
 			bd.Data.(balancer.Balancer).ResolverError(err)
diff --git a/balancer/leastrequest/leastrequest.go b/balancer/leastrequest/leastrequest.go
index 0ac0f43e6b35..f9cf7ccfc1ef 100644
--- a/balancer/leastrequest/leastrequest.go
+++ b/balancer/leastrequest/leastrequest.go
@@ -125,11 +125,7 @@ func (lrb *leastRequestBalancer) ResolverError(err error) {
 }
 
 func (lrb *leastRequestBalancer) ExitIdle() {
-	if ei, ok := lrb.child.(balancer.ExitIdler); ok { // Should always be ok, as child is endpoint sharding.
-		ei.ExitIdle()
-	} else {
-		lrb.logger.Errorf("Child balancer doesn't implement ExitIdler.")
-	}
+	lrb.child.ExitIdle()
 }
 
 func (lrb *leastRequestBalancer) UpdateClientConnState(ccs balancer.ClientConnState) error {
diff --git a/balancer/pickfirst/pickfirstleaf/pickfirstleaf_ext_test.go b/balancer/pickfirst/pickfirstleaf/pickfirstleaf_ext_test.go
index be8c43fa7ab5..aca6c82e2fad 100644
--- a/balancer/pickfirst/pickfirstleaf/pickfirstleaf_ext_test.go
+++ b/balancer/pickfirst/pickfirstleaf/pickfirstleaf_ext_test.go
@@ -1593,7 +1593,7 @@ func (b *stateStoringBalancer) Close() {
 }
 
 func (b *stateStoringBalancer) ExitIdle() {
-	b.Balancer.(balancer.ExitIdler).ExitIdle()
+	b.Balancer.ExitIdle()
 }
 
 type stateStoringBalancerBuilder struct {
diff --git a/balancer/ringhash/ringhash.go b/balancer/ringhash/ringhash.go
index 8f20410ecf07..2defa5ff1301 100644
--- a/balancer/ringhash/ringhash.go
+++ b/balancer/ringhash/ringhash.go
@@ -263,7 +263,7 @@ func (b *ringhashBalancer) updatePickerLocked() {
 		sort.Slice(endpointStates, func(i, j int) bool {
 			return endpointStates[i].hashKey < endpointStates[j].hashKey
 		})
-		var idleBalancer balancer.ExitIdler
+		var idleBalancer endpointsharding.ExitIdler
 		for _, es := range endpointStates {
 			connState := es.state.ConnectivityState
 			if connState == connectivity.Connecting {
@@ -394,7 +394,7 @@ type endpointState struct {
 	// overridden, for example based on EDS endpoint metadata.
 	hashKey  string
 	weight   uint32
-	balancer balancer.ExitIdler
+	balancer endpointsharding.ExitIdler
 
 	// state is updated by the balancer while receiving resolver updates from
 	// the channel and picker updates from its children. Access to it is guarded
diff --git a/balancer/rls/internal/test/e2e/rls_child_policy.go b/balancer/rls/internal/test/e2e/rls_child_policy.go
index 3e06d818b047..bb611efe863a 100644
--- a/balancer/rls/internal/test/e2e/rls_child_policy.go
+++ b/balancer/rls/internal/test/e2e/rls_child_policy.go
@@ -103,7 +103,7 @@ type RLSChildPolicyConfig struct {
 }
 
 func (b *bal) ExitIdle() {
-	b.Balancer.(balancer.ExitIdler).ExitIdle()
+	b.Balancer.ExitIdle()
 }
 
 func (b *bal) UpdateClientConnState(c balancer.ClientConnState) error {
diff --git a/balancer/roundrobin/roundrobin.go b/balancer/roundrobin/roundrobin.go
index d4df537fc04a..03477217df60 100644
--- a/balancer/roundrobin/roundrobin.go
+++ b/balancer/roundrobin/roundrobin.go
@@ -72,10 +72,5 @@ func (b *rrBalancer) UpdateClientConnState(ccs balancer.ClientConnState) error {
 }
 
 func (b *rrBalancer) ExitIdle() {
-	// Should always be ok, as child is endpoint sharding.
-	if ei, ok := b.Balancer.(balancer.ExitIdler); ok {
-		ei.ExitIdle()
-	} else {
-		b.logger.Errorf("Child balancer doesn't implement ExitIdler.")
-	}
+	b.Balancer.ExitIdle()
 }
diff --git a/balancer/weightedroundrobin/balancer.go b/balancer/weightedroundrobin/balancer.go
index 9f38758921a3..fe8ebff1f5b5 100644
--- a/balancer/weightedroundrobin/balancer.go
+++ b/balancer/weightedroundrobin/balancer.go
@@ -404,11 +404,7 @@ func (b *wrrBalancer) Close() {
 }
 
 func (b *wrrBalancer) ExitIdle() {
-	if ei, ok := b.child.(balancer.ExitIdler); ok { // Should always be ok, as child is endpoint sharding.
-		ei.ExitIdle()
-	} else {
-		b.logger.Errorf("Child balancer doesn't implement ExitIdler.")
-	}
+	b.child.ExitIdle()
 }
 
 // picker is the WRR policy's picker.  It uses live-updating backend weights to
diff --git a/balancer/weightedtarget/weightedtarget_test.go b/balancer/weightedtarget/weightedtarget_test.go
index a9a1d4340b33..96b5bbfab368 100644
--- a/balancer/weightedtarget/weightedtarget_test.go
+++ b/balancer/weightedtarget/weightedtarget_test.go
@@ -131,7 +131,7 @@ func getConfigKey(attr *attributes.Attributes) (string, bool) {
 }
 
 func (b *testConfigBalancer) ExitIdle() {
-	b.Balancer.(balancer.ExitIdler).ExitIdle()
+	b.Balancer.ExitIdle()
 }
 
 func (b *testConfigBalancer) UpdateClientConnState(s balancer.ClientConnState) error {
diff --git a/clientconn_test.go b/clientconn_test.go
index 96eb7ace51bd..6ec4f630b92e 100644
--- a/clientconn_test.go
+++ b/clientconn_test.go
@@ -1221,7 +1221,7 @@ type stateRecordingBalancer struct {
 }
 
 func (b *stateRecordingBalancer) ExitIdle() {
-	b.Balancer.(balancer.ExitIdler).ExitIdle()
+	b.Balancer.ExitIdle()
 }
 
 func (b *stateRecordingBalancer) UpdateSubConnState(sc balancer.SubConn, s balancer.SubConnState) {
diff --git a/examples/features/orca/client/main.go b/examples/features/orca/client/main.go
index 60a91f4c0dc6..e21f30965f02 100644
--- a/examples/features/orca/client/main.go
+++ b/examples/features/orca/client/main.go
@@ -95,6 +95,7 @@ func (orcaLBBuilder) Build(cc balancer.ClientConn, _ balancer.BuildOptions) bala
 // designed to run within.
 type orcaLB struct {
 	cc balancer.ClientConn
+	sc balancer.SubConn
 }
 
 func (o *orcaLB) UpdateClientConnState(ccs balancer.ClientConnState) error {
@@ -112,6 +113,7 @@ func (o *orcaLB) UpdateClientConnState(ccs balancer.ClientConnState) error {
 		return fmt.Errorf("orcaLB: error creating SubConn: %v", err)
 	}
 	sc.Connect()
+	o.sc = sc
 
 	// Register a simple ORCA OOB listener on the SubConn.  We request a 1
 	// second report interval, but in this example the server indicated the
@@ -124,6 +126,12 @@ func (o *orcaLB) UpdateClientConnState(ccs balancer.ClientConnState) error {
 
 func (o *orcaLB) ResolverError(error) {}
 
+func (o *orcaLB) ExitIdle() {
+	if o.sc != nil {
+		o.sc.Connect()
+	}
+}
+
 // TODO: unused; remove when no longer required.
 func (o *orcaLB) UpdateSubConnState(balancer.SubConn, balancer.SubConnState) {}
 
diff --git a/internal/balancer/gracefulswitch/gracefulswitch.go b/internal/balancer/gracefulswitch/gracefulswitch.go
index fbc1ca356ab9..4c266ce2198b 100644
--- a/internal/balancer/gracefulswitch/gracefulswitch.go
+++ b/internal/balancer/gracefulswitch/gracefulswitch.go
@@ -223,15 +223,7 @@ func (gsb *Balancer) ExitIdle() {
 	// There is no need to protect this read with a mutex, as the write to the
 	// Balancer field happens in SwitchTo, which completes before this can be
 	// called.
-	if ei, ok := balToUpdate.Balancer.(balancer.ExitIdler); ok {
-		ei.ExitIdle()
-		return
-	}
-	gsb.mu.Lock()
-	defer gsb.mu.Unlock()
-	for sc := range balToUpdate.subconns {
-		sc.Connect()
-	}
+	balToUpdate.Balancer.ExitIdle()
 }
 
 // updateSubConnState forwards the update to the appropriate child.
diff --git a/internal/balancer/gracefulswitch/gracefulswitch_test.go b/internal/balancer/gracefulswitch/gracefulswitch_test.go
index 537f23d42761..9f536e8586f0 100644
--- a/internal/balancer/gracefulswitch/gracefulswitch_test.go
+++ b/internal/balancer/gracefulswitch/gracefulswitch_test.go
@@ -795,9 +795,7 @@ func (s) TestInlineCallbackInBuild(t *testing.T) {
 	}
 }
 
-// TestExitIdle tests the ExitIdle operation on the Graceful Switch Balancer for
-// both possible codepaths, one where the child implements ExitIdler interface
-// and one where the child doesn't implement ExitIdler interface.
+// TestExitIdle tests the ExitIdle operation on the Graceful Switch Balancer.
 func (s) TestExitIdle(t *testing.T) {
 	_, gsb := setup(t)
 	// switch to a balancer that implements ExitIdle{} (will populate current).
@@ -811,22 +809,6 @@ func (s) TestExitIdle(t *testing.T) {
 	if err := currBal.waitForExitIdle(ctx); err != nil {
 		t.Fatal(err)
 	}
-
-	// switch to a balancer that doesn't implement ExitIdle{} (will populate
-	// pending).
-	gsb.SwitchTo(verifyBalancerBuilder{})
-	// call exitIdle concurrently with newSubConn to make sure there is not a
-	// data race.
-	done := make(chan struct{})
-	go func() {
-		gsb.ExitIdle()
-		close(done)
-	}()
-	pendBal := gsb.balancerPending.Balancer.(*verifyBalancer)
-	for i := 0; i < 10; i++ {
-		pendBal.newSubConn([]resolver.Address{}, balancer.NewSubConnOptions{})
-	}
-	<-done
 }
 
 const balancerName1 = "mock_balancer_1"
@@ -1070,6 +1052,8 @@ func (bcb *buildCallbackBal) UpdateClientConnState(balancer.ClientConnState) err
 
 func (bcb *buildCallbackBal) ResolverError(error) {}
 
+func (bcb *buildCallbackBal) ExitIdle() {}
+
 func (bcb *buildCallbackBal) UpdateSubConnState(sc balancer.SubConn, state balancer.SubConnState) {
 	panic(fmt.Sprintf("UpdateSubConnState(%v, %+v) called unexpectedly", sc, state))
 }
diff --git a/internal/stats/metrics_recorder_list_test.go b/internal/stats/metrics_recorder_list_test.go
index 2e64cf288a00..4ad6aa4ebf92 100644
--- a/internal/stats/metrics_recorder_list_test.go
+++ b/internal/stats/metrics_recorder_list_test.go
@@ -125,7 +125,7 @@ type recordingLoadBalancer struct {
 }
 
 func (b *recordingLoadBalancer) ExitIdle() {
-	b.Balancer.(balancer.ExitIdler).ExitIdle()
+	b.Balancer.ExitIdle()
 }
 
 // TestMetricsRecorderList tests the metrics recorder list functionality of the
diff --git a/test/balancer_test.go b/test/balancer_test.go
index 8c26ea1b92cf..049aeb7f5c23 100644
--- a/test/balancer_test.go
+++ b/test/balancer_test.go
@@ -1034,7 +1034,7 @@ func (s) TestSubConn_RegisterHealthListener(t *testing.T) {
 			return bd.Data.(balancer.Balancer).UpdateClientConnState(ccs)
 		},
 		ExitIdle: func(bd *stub.BalancerData) {
-			bd.Data.(balancer.ExitIdler).ExitIdle()
+			bd.Data.(balancer.Balancer).ExitIdle()
 		},
 	}
 
diff --git a/test/clientconn_state_transition_test.go b/test/clientconn_state_transition_test.go
index 57249201734c..568618f909d6 100644
--- a/test/clientconn_state_transition_test.go
+++ b/test/clientconn_state_transition_test.go
@@ -475,7 +475,7 @@ type stateRecordingBalancer struct {
 }
 
 func (b *stateRecordingBalancer) ExitIdle() {
-	b.Balancer.(balancer.ExitIdler).ExitIdle()
+	b.Balancer.ExitIdle()
 }
 
 func (b *stateRecordingBalancer) Close() {
diff --git a/test/end2end_test.go b/test/end2end_test.go
index 396336fc78fa..8e436877db8f 100644
--- a/test/end2end_test.go
+++ b/test/end2end_test.go
@@ -6542,7 +6542,7 @@ type triggerRPCBlockBalancer struct {
 }
 
 func (bpb *triggerRPCBlockBalancer) ExitIdle() {
-	bpb.Balancer.(balancer.ExitIdler).ExitIdle()
+	bpb.Balancer.ExitIdle()
 }
 
 func (bpb *triggerRPCBlockBalancer) UpdateClientConnState(s balancer.ClientConnState) error {
diff --git a/xds/internal/balancer/cdsbalancer/cdsbalancer.go b/xds/internal/balancer/cdsbalancer/cdsbalancer.go
index 1e1493d9d67c..3a0e3f9e6be7 100644
--- a/xds/internal/balancer/cdsbalancer/cdsbalancer.go
+++ b/xds/internal/balancer/cdsbalancer/cdsbalancer.go
@@ -424,11 +424,7 @@ func (b *cdsBalancer) ExitIdle() {
 		// ExitIdle (but still checks for the interface's existence to
 		// avoid a panic if not).  If the child does not, no subconns
 		// will be connected.
-		if ei, ok := b.childLB.(balancer.ExitIdler); ok {
-			ei.ExitIdle()
-		} else {
-			b.logger.Errorf("Child balancer doesn't implement ExitIdler.")
-		}
+		b.childLB.ExitIdle()
 	})
 }
 
diff --git a/xds/internal/balancer/cdsbalancer/cdsbalancer_test.go b/xds/internal/balancer/cdsbalancer/cdsbalancer_test.go
index 87888c734d7a..2cebfe52db52 100644
--- a/xds/internal/balancer/cdsbalancer/cdsbalancer_test.go
+++ b/xds/internal/balancer/cdsbalancer/cdsbalancer_test.go
@@ -138,8 +138,7 @@ func registerWrappedClusterResolverPolicy(t *testing.T) (chan serviceconfig.Load
 			bal.ResolverError(err)
 		},
 		ExitIdle: func(bd *stub.BalancerData) {
-			bal := bd.Data.(balancer.Balancer)
-			bal.(balancer.ExitIdler).ExitIdle()
+			bd.Data.(balancer.Balancer).ExitIdle()
 			close(exitIdleCh)
 		},
 		Close: func(bd *stub.BalancerData) {
@@ -1110,7 +1109,7 @@ func (s) TestExitIdle(t *testing.T) {
 	case <-ctx.Done():
 		t.Fatal("Timeout when waiting for cds LB policy to be created")
 	}
-	cdsBal.(balancer.ExitIdler).ExitIdle()
+	cdsBal.ExitIdle()
 
 	// Wait for ExitIdle to be called on the child policy.
 	select {
diff --git a/xds/internal/balancer/clusterresolver/clusterresolver.go b/xds/internal/balancer/clusterresolver/clusterresolver.go
index 60d836741c04..f9ce57293393 100644
--- a/xds/internal/balancer/clusterresolver/clusterresolver.go
+++ b/xds/internal/balancer/clusterresolver/clusterresolver.go
@@ -335,15 +335,7 @@ func (b *clusterResolverBalancer) run() {
 					}
 					break
 				}
-				// This implementation assumes the child balancer supports
-				// ExitIdle (but still checks for the interface's existence to
-				// avoid a panic if not).  If the child does not, no subconns
-				// will be connected.
-				if ei, ok := b.child.(balancer.ExitIdler); ok {
-					ei.ExitIdle()
-				} else {
-					b.logger.Errorf("Child balancer doesn't implement ExitIdler.")
-				}
+				b.child.ExitIdle()
 			}
 		case u := <-b.resourceWatcher.updateChannel:
 			b.handleResourceUpdate(u)
diff --git a/xds/internal/balancer/wrrlocality/balancer.go b/xds/internal/balancer/wrrlocality/balancer.go
index cfc438c5a82e..2d03a9c75e7b 100644
--- a/xds/internal/balancer/wrrlocality/balancer.go
+++ b/xds/internal/balancer/wrrlocality/balancer.go
@@ -155,12 +155,7 @@ type wrrLocalityBalancer struct {
 }
 
 func (b *wrrLocalityBalancer) ExitIdle() {
-	ei, ok := b.child.(balancer.ExitIdler)
-	if ok {
-		ei.ExitIdle()
-	} else if !ok {
-		b.logger.Errorf("Child balancer doesn't implement ExitIdler.")
-	}
+	b.child.ExitIdle()
 }
 
 func (b *wrrLocalityBalancer) UpdateClientConnState(s balancer.ClientConnState) error {

From fb056d633b92c210fe1029ffbec2009c2b329319 Mon Sep 17 00:00:00 2001
From: Arjan Bal <arjansbal@google.com>
Date: Wed, 28 May 2025 23:28:57 +0530
Subject: [PATCH 5/6] Remove unecessary overrides

---
 balancer/rls/internal/test/e2e/rls_child_policy.go | 4 ----
 clientconn_test.go                                 | 4 ----
 internal/stats/metrics_recorder_list_test.go       | 4 ----
 test/clientconn_state_transition_test.go           | 4 ----
 test/end2end_test.go                               | 4 ----
 5 files changed, 20 deletions(-)

diff --git a/balancer/rls/internal/test/e2e/rls_child_policy.go b/balancer/rls/internal/test/e2e/rls_child_policy.go
index bb611efe863a..2bae34b60887 100644
--- a/balancer/rls/internal/test/e2e/rls_child_policy.go
+++ b/balancer/rls/internal/test/e2e/rls_child_policy.go
@@ -102,10 +102,6 @@ type RLSChildPolicyConfig struct {
 	Random  string // A random field to test child policy config changes.
 }
 
-func (b *bal) ExitIdle() {
-	b.Balancer.ExitIdle()
-}
-
 func (b *bal) UpdateClientConnState(c balancer.ClientConnState) error {
 	cfg, ok := c.BalancerConfig.(*RLSChildPolicyConfig)
 	if !ok {
diff --git a/clientconn_test.go b/clientconn_test.go
index 6ec4f630b92e..9cca5a8eb7b7 100644
--- a/clientconn_test.go
+++ b/clientconn_test.go
@@ -1220,10 +1220,6 @@ type stateRecordingBalancer struct {
 	balancer.Balancer
 }
 
-func (b *stateRecordingBalancer) ExitIdle() {
-	b.Balancer.ExitIdle()
-}
-
 func (b *stateRecordingBalancer) UpdateSubConnState(sc balancer.SubConn, s balancer.SubConnState) {
 	panic(fmt.Sprintf("UpdateSubConnState(%v, %+v) called unexpectedly", sc, s))
 }
diff --git a/internal/stats/metrics_recorder_list_test.go b/internal/stats/metrics_recorder_list_test.go
index 4ad6aa4ebf92..38f9472f926a 100644
--- a/internal/stats/metrics_recorder_list_test.go
+++ b/internal/stats/metrics_recorder_list_test.go
@@ -124,10 +124,6 @@ type recordingLoadBalancer struct {
 	balancer.Balancer
 }
 
-func (b *recordingLoadBalancer) ExitIdle() {
-	b.Balancer.ExitIdle()
-}
-
 // TestMetricsRecorderList tests the metrics recorder list functionality of the
 // ClientConn. It configures a global and local stats handler Dial Option. These
 // stats handlers implement the MetricsRecorder interface. It also configures a
diff --git a/test/clientconn_state_transition_test.go b/test/clientconn_state_transition_test.go
index 568618f909d6..6e179c1cd59a 100644
--- a/test/clientconn_state_transition_test.go
+++ b/test/clientconn_state_transition_test.go
@@ -474,10 +474,6 @@ type stateRecordingBalancer struct {
 	balancer.Balancer
 }
 
-func (b *stateRecordingBalancer) ExitIdle() {
-	b.Balancer.ExitIdle()
-}
-
 func (b *stateRecordingBalancer) Close() {
 	b.Balancer.Close()
 }
diff --git a/test/end2end_test.go b/test/end2end_test.go
index 8e436877db8f..75b27f4c224d 100644
--- a/test/end2end_test.go
+++ b/test/end2end_test.go
@@ -6541,10 +6541,6 @@ type triggerRPCBlockBalancer struct {
 	balancer.Balancer
 }
 
-func (bpb *triggerRPCBlockBalancer) ExitIdle() {
-	bpb.Balancer.ExitIdle()
-}
-
 func (bpb *triggerRPCBlockBalancer) UpdateClientConnState(s balancer.ClientConnState) error {
 	err := bpb.Balancer.UpdateClientConnState(s)
 	bpb.ClientConn.UpdateState(balancer.State{

From a08fdccbfb8413fbbfd9ff1111e3012669132d18 Mon Sep 17 00:00:00 2001
From: Arjan Bal <arjansbal@google.com>
Date: Fri, 30 May 2025 09:41:48 +0530
Subject: [PATCH 6/6] Remove unecessary overrides

---
 balancer/endpointsharding/endpointsharding.go              | 4 ----
 balancer/endpointsharding/endpointsharding_test.go         | 4 ----
 balancer/pickfirst/pickfirstleaf/pickfirstleaf_ext_test.go | 4 ----
 balancer/weightedtarget/weightedtarget_test.go             | 4 ----
 internal/balancer/gracefulswitch/gracefulswitch.go         | 2 +-
 5 files changed, 1 insertion(+), 17 deletions(-)

diff --git a/balancer/endpointsharding/endpointsharding.go b/balancer/endpointsharding/endpointsharding.go
index eb821b13a93c..0ad6bb1f2203 100644
--- a/balancer/endpointsharding/endpointsharding.go
+++ b/balancer/endpointsharding/endpointsharding.go
@@ -217,10 +217,6 @@ func (es *endpointSharding) ExitIdle() {
 	es.childMu.Lock()
 	defer es.childMu.Unlock()
 	for _, bw := range es.children.Load().Values() {
-		// this implementation assumes the child balancer supports
-		// exitidle (but still checks for the interface's existence to
-		// avoid a panic if not). If the child does not, no subconns
-		// will be connected.
 		if !bw.isClosed {
 			bw.child.ExitIdle()
 		}
diff --git a/balancer/endpointsharding/endpointsharding_test.go b/balancer/endpointsharding/endpointsharding_test.go
index 8443cfe808ab..0b9955211b74 100644
--- a/balancer/endpointsharding/endpointsharding_test.go
+++ b/balancer/endpointsharding/endpointsharding_test.go
@@ -102,10 +102,6 @@ type fakePetiole struct {
 	bOpts balancer.BuildOptions
 }
 
-func (fp *fakePetiole) ExitIdle() {
-	fp.Balancer.ExitIdle()
-}
-
 func (fp *fakePetiole) UpdateClientConnState(state balancer.ClientConnState) error {
 	if el := state.ResolverState.Endpoints; len(el) != 2 {
 		return fmt.Errorf("UpdateClientConnState wants two endpoints, got: %v", el)
diff --git a/balancer/pickfirst/pickfirstleaf/pickfirstleaf_ext_test.go b/balancer/pickfirst/pickfirstleaf/pickfirstleaf_ext_test.go
index aca6c82e2fad..25bb29444d8a 100644
--- a/balancer/pickfirst/pickfirstleaf/pickfirstleaf_ext_test.go
+++ b/balancer/pickfirst/pickfirstleaf/pickfirstleaf_ext_test.go
@@ -1592,10 +1592,6 @@ func (b *stateStoringBalancer) Close() {
 	b.Balancer.Close()
 }
 
-func (b *stateStoringBalancer) ExitIdle() {
-	b.Balancer.ExitIdle()
-}
-
 type stateStoringBalancerBuilder struct {
 	balancer chan *stateStoringBalancer
 }
diff --git a/balancer/weightedtarget/weightedtarget_test.go b/balancer/weightedtarget/weightedtarget_test.go
index 96b5bbfab368..c9c78fc5a307 100644
--- a/balancer/weightedtarget/weightedtarget_test.go
+++ b/balancer/weightedtarget/weightedtarget_test.go
@@ -130,10 +130,6 @@ func getConfigKey(attr *attributes.Attributes) (string, bool) {
 	return name, ok
 }
 
-func (b *testConfigBalancer) ExitIdle() {
-	b.Balancer.ExitIdle()
-}
-
 func (b *testConfigBalancer) UpdateClientConnState(s balancer.ClientConnState) error {
 	c, ok := s.BalancerConfig.(stringBalancerConfig)
 	if !ok {
diff --git a/internal/balancer/gracefulswitch/gracefulswitch.go b/internal/balancer/gracefulswitch/gracefulswitch.go
index 4c266ce2198b..ba25b8988718 100644
--- a/internal/balancer/gracefulswitch/gracefulswitch.go
+++ b/internal/balancer/gracefulswitch/gracefulswitch.go
@@ -223,7 +223,7 @@ func (gsb *Balancer) ExitIdle() {
 	// There is no need to protect this read with a mutex, as the write to the
 	// Balancer field happens in SwitchTo, which completes before this can be
 	// called.
-	balToUpdate.Balancer.ExitIdle()
+	balToUpdate.ExitIdle()
 }
 
 // updateSubConnState forwards the update to the appropriate child.