clientv3: don't hold client lock while dialing

Causes async reconnect to block while the client is dialing.

This was also causing problems with the Close error message, so
now Close() will return the last dial error (if any) instead of
clearing it out with a cancel().

Fixes #5416

clientv3/integration/kv_test.go

@@ -131,6 +131,13 @@ func TestKVPutWithRequireLeader(t *testing.T) {
 	if err != rpctypes.ErrNoLeader {
 		t.Fatal(err)
 	}
+
+	// clients may give timeout errors since the members are stopped; take
+	// the clients so that terminating the cluster won't complain
+	clus.Client(1).Close()
+	clus.Client(2).Close()
+	clus.TakeClient(1)
+	clus.TakeClient(2)
 }
 
 func TestKVRange(t *testing.T) {
@@ -633,13 +640,22 @@ func TestKVPutStoppedServerAndClose(t *testing.T) {
 	defer testutil.AfterTest(t)
 	clus := integration.NewClusterV3(t, &integration.ClusterConfig{Size: 1})
 	defer clus.Terminate(t)
+	cli := clus.Client(0)
 	clus.Members[0].Stop(t)
 	// this Put fails and triggers an asynchronous connection retry
-	_, err := clus.Client(0).Put(context.TODO(), "abc", "123")
+	_, err := cli.Put(context.TODO(), "abc", "123")
 	if err == nil ||
 		(!strings.Contains(err.Error(), "connection is closing") &&
 			!strings.Contains(err.Error(), "transport is closing")) {
 		t.Fatal(err)
 	}
-	// cluster will terminate and close the client with the retry in-flight
+
+	// wait some so the client closes with the retry in-flight
+	time.Sleep(time.Second)
+
+	// get the timeout
+	clus.TakeClient(0)
+	if err := cli.Close(); err == nil || !strings.Contains(err.Error(), "timed out") {
+		t.Fatal(err)
+	}
 }