balancergroup: add method to exitIdle a sub-balancer (#4994)

This is required for the RLS LB policy. At pick time, if the RLS picker
finds one of its child policies in IDLE, it needs to be able to ask it
to exit idle.

internal/balancergroup/balancergroup.go

@@ -506,3 +506,13 @@ func (bg *BalancerGroup) ExitIdle() {
 	}
 	bg.outgoingMu.Unlock()
 }
+
+// ExitIdleOne instructs the sub-balancer `id` to exit IDLE state, if
+// appropriate and possible.
+func (bg *BalancerGroup) ExitIdleOne(id string) {
+	bg.outgoingMu.Lock()
+	if config := bg.idToBalancerConfig[id]; config != nil {
+		config.exitIdle()
+	}
+	bg.outgoingMu.Unlock()
+}

internal/balancergroup/balancergroup_test.go

@@ -509,3 +509,29 @@ func (s) TestBalancerGroupBuildOptions(t *testing.T) {
 		t.Fatal(err)
 	}
 }
+
+func (s) TestBalancerExitIdleOne(t *testing.T) {
+	const balancerName = "stub-balancer-test-balancergroup-exit-idle-one"
+	exitIdleCh := make(chan struct{}, 1)
+	stub.Register(balancerName, stub.BalancerFuncs{
+		ExitIdle: func(*stub.BalancerData) {
+			exitIdleCh <- struct{}{}
+		},
+	})
+	cc := testutils.NewTestClientConn(t)
+	bg := New(cc, balancer.BuildOptions{}, nil, nil)
+	bg.Start()
+	defer bg.Close()
+
+	// Add the stub balancer build above as a child policy.
+	builder := balancer.Get(balancerName)
+	bg.Add(testBalancerIDs[0], builder)
+
+	// Call ExitIdle on the child policy.
+	bg.ExitIdleOne(testBalancerIDs[0])
+	select {
+	case <-time.After(time.Second):
+		t.Fatal("Timeout when waiting for ExitIdle to be invoked on child policy")
+	case <-exitIdleCh:
+	}
+}