fix: return a friendly error if the dialer is closed (#766)

This is a port of GoogleCloudPlatform/alloydb-go-connector#538

dialer.go

@@ -53,6 +53,9 @@ const (
 )
 
 var (
+	// ErrDialerClosed is used when a caller invokes Dial after closing the
+	// Dialer.
+	ErrDialerClosed = errors.New("cloudsqlconn: dialer is closed")
 	// versionString indicates the version of this library.
 	//go:embed version.txt
 	versionString string
@@ -91,8 +94,11 @@ type Dialer struct {
 	instances      map[instance.ConnName]connectionInfoCache
 	key            *rsa.PrivateKey
 	refreshTimeout time.Duration
-	sqladmin       *sqladmin.Service
-	logger         debug.Logger
+	// closed reports if the dialer has been closed.
+	closed chan struct{}
+
+	sqladmin *sqladmin.Service
+	logger   debug.Logger
 
 	// defaultDialConfig holds the constructor level DialOptions, so that it
 	// can be copied and mutated by the Dial function.
@@ -210,6 +216,7 @@ func NewDialer(ctx context.Context, opts ...Option) (*Dialer, error) {
 		return nil, err
 	}
 	d := &Dialer{
+		closed:            make(chan struct{}),
 		instances:         make(map[instance.ConnName]connectionInfoCache),
 		key:               cfg.rsaKey,
 		refreshTimeout:    cfg.refreshTimeout,
@@ -227,6 +234,11 @@ func NewDialer(ctx context.Context, opts ...Option) (*Dialer, error) {
 // icn argument must be the instance's connection name, which is in the format
 // "project-name:region:instance-name".
 func (d *Dialer) Dial(ctx context.Context, icn string, opts ...DialOption) (conn net.Conn, err error) {
+	select {
+	case <-d.closed:
+		return nil, ErrDialerClosed
+	default:
+	}
 	startTime := time.Now()
 	var endDial trace.EndSpanFunc
 	ctx, endDial = trace.StartSpan(ctx, "cloud.google.com/go/cloudsqlconn.Dial",
@@ -420,6 +432,13 @@ func (i *instrumentedConn) Close() error {
 // needed to connect. Additional dial operations may succeed until the information
 // expires.
 func (d *Dialer) Close() error {
+	// Check if Close has already been called.
+	select {
+	case <-d.closed:
+		return nil
+	default:
+	}
+	close(d.closed)
 	d.lock.Lock()
 	defer d.lock.Unlock()
 	for _, i := range d.instances {

dialer_test.go

@@ -795,3 +795,27 @@ func TestDialerSupportsOneOffDialFunction(t *testing.T) {
 		t.Fatal("one-off dial func was not called")
 	}
 }
+
+func TestDialerCloseReportsFriendlyError(t *testing.T) {
+	d, err := NewDialer(
+		context.Background(),
+		WithTokenSource(mock.EmptyTokenSource{}),
+	)
+	if err != nil {
+		t.Fatal(err)
+	}
+	_ = d.Close()
+
+	_, err = d.Dial(context.Background(), "p:r:i")
+	if !errors.Is(err, ErrDialerClosed) {
+		t.Fatalf("want = %v, got = %v", ErrDialerClosed, err)
+	}
+
+	// Ensure multiple calls to close don't panic
+	_ = d.Close()
+
+	_, err = d.Dial(context.Background(), "p:r:i")
+	if !errors.Is(err, ErrDialerClosed) {
+		t.Fatalf("want = %v, got = %v", ErrDialerClosed, err)
+	}
+}