From 0e991000c4a28b84e6798a195d3ad10a9178db24 Mon Sep 17 00:00:00 2001
From: Doug Fawley <dfawley@google.com>
Date: Wed, 7 Jun 2023 08:37:11 -0700
Subject: [PATCH] client: handle empty address lists correctly in
 addrConn.updateAddrs (#6354)

---
 clientconn.go        |   6 ++-
 test/subconn_test.go | 125 +++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 129 insertions(+), 2 deletions(-)
 create mode 100644 test/subconn_test.go

diff --git a/clientconn.go b/clientconn.go
index 314addcaa1cb..95a7459b02f6 100644
--- a/clientconn.go
+++ b/clientconn.go
@@ -1033,8 +1033,10 @@ func (ac *addrConn) updateAddrs(addrs []resolver.Address) {
 
 	// We have to defer here because GracefulClose => Close => onClose, which
 	// requires locking ac.mu.
-	defer ac.transport.GracefulClose()
-	ac.transport = nil
+	if ac.transport != nil {
+		defer ac.transport.GracefulClose()
+		ac.transport = nil
+	}
 
 	if len(addrs) == 0 {
 		ac.updateConnectivityState(connectivity.Idle, nil)
diff --git a/test/subconn_test.go b/test/subconn_test.go
new file mode 100644
index 000000000000..524acf9f70e3
--- /dev/null
+++ b/test/subconn_test.go
@@ -0,0 +1,125 @@
+/*
+ *
+ * Copyright 2023 gRPC authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ *
+ */
+
+package test
+
+import (
+	"context"
+	"errors"
+	"fmt"
+	"testing"
+	"time"
+
+	"google.golang.org/grpc"
+	"google.golang.org/grpc/balancer"
+	"google.golang.org/grpc/balancer/base"
+	"google.golang.org/grpc/connectivity"
+	"google.golang.org/grpc/internal/balancer/stub"
+	"google.golang.org/grpc/internal/stubserver"
+	testpb "google.golang.org/grpc/interop/grpc_testing"
+	"google.golang.org/grpc/resolver"
+)
+
+type tsccPicker struct {
+	sc balancer.SubConn
+}
+
+func (p *tsccPicker) Pick(info balancer.PickInfo) (balancer.PickResult, error) {
+	return balancer.PickResult{SubConn: p.sc}, nil
+}
+
+// TestSubConnEmpty tests that removing all addresses from a SubConn and then
+// re-adding them does not cause a panic and properly reconnects.
+func (s) TestSubConnEmpty(t *testing.T) {
+	ctx, cancel := context.WithTimeout(context.Background(), 5*time.Second)
+	defer cancel()
+
+	// sc is the one SubConn used throughout the test.  Created on demand and
+	// re-used on every update.
+	var sc balancer.SubConn
+
+	// Simple custom balancer that sets the address list to empty if the
+	// resolver produces no addresses.  Pickfirst, by default, will remove the
+	// SubConn in this case instead.
+	bal := stub.BalancerFuncs{
+		UpdateClientConnState: func(d *stub.BalancerData, ccs balancer.ClientConnState) error {
+			if sc == nil {
+				var err error
+				sc, err = d.ClientConn.NewSubConn(ccs.ResolverState.Addresses, balancer.NewSubConnOptions{})
+				if err != nil {
+					t.Errorf("error creating initial subconn: %v", err)
+				}
+			} else {
+				d.ClientConn.UpdateAddresses(sc, ccs.ResolverState.Addresses)
+			}
+			sc.Connect()
+
+			if len(ccs.ResolverState.Addresses) == 0 {
+				d.ClientConn.UpdateState(balancer.State{
+					ConnectivityState: connectivity.TransientFailure,
+					Picker:            base.NewErrPicker(errors.New("no addresses")),
+				})
+			} else {
+				d.ClientConn.UpdateState(balancer.State{
+					ConnectivityState: connectivity.Connecting,
+					Picker:            &tsccPicker{sc: sc},
+				})
+			}
+			return nil
+		},
+		UpdateSubConnState: func(d *stub.BalancerData, sc balancer.SubConn, scs balancer.SubConnState) {
+			switch scs.ConnectivityState {
+			case connectivity.Ready:
+				d.ClientConn.UpdateState(balancer.State{
+					ConnectivityState: connectivity.Ready,
+					Picker:            &tsccPicker{sc: sc},
+				})
+			case connectivity.TransientFailure:
+				d.ClientConn.UpdateState(balancer.State{
+					ConnectivityState: connectivity.TransientFailure,
+					Picker:            base.NewErrPicker(fmt.Errorf("error connecting: %v", scs.ConnectionError)),
+				})
+			}
+		},
+	}
+	stub.Register("tscc", bal)
+
+	// Start the stub server with our stub balancer.
+	ss := &stubserver.StubServer{
+		EmptyCallF: func(ctx context.Context, in *testpb.Empty) (*testpb.Empty, error) {
+			return &testpb.Empty{}, nil
+		},
+	}
+	if err := ss.Start(nil, grpc.WithDefaultServiceConfig(`{"loadBalancingConfig": [{"tscc":{}}]}`)); err != nil {
+		t.Fatalf("Error starting server: %v", err)
+	}
+	defer ss.Stop()
+	if _, err := ss.Client.EmptyCall(ctx, &testpb.Empty{}); err != nil {
+		t.Fatalf("EmptyCall failed: %v", err)
+	}
+
+	t.Log("Removing addresses from resolver and SubConn")
+	ss.R.UpdateState(resolver.State{Addresses: []resolver.Address{}})
+	awaitState(ctx, t, ss.CC, connectivity.TransientFailure)
+
+	t.Log("Re-adding addresses to resolver and SubConn")
+	ss.R.UpdateState(resolver.State{Addresses: []resolver.Address{{Addr: ss.Address}}})
+	if _, err := ss.Client.EmptyCall(ctx, &testpb.Empty{}); err != nil {
+		t.Fatalf("EmptyCall failed: %v", err)
+	}
+}