feat: allow for configuring the Dial func (#57)

Fixes #56.

dialer.go

@@ -81,6 +81,10 @@ type Dialer struct {
 	// *only* when a client has configured OpenCensus exporters.
 	dialerID string
 
+	// dialFunc is the function used to connect to the address on the named
+	// network. By default it is golang.org/x/net/proxy#Dial.
+	dialFunc func(cxt context.Context, network, addr string) (net.Conn, error)
+
 	// iamTokenSource supplies the OAuth2 token used for IAM DB Authn. If IAM DB
 	// Authn is not enabled, iamTokenSource will be nil.
 	iamTokenSource oauth2.TokenSource
@@ -95,6 +99,7 @@ func NewDialer(ctx context.Context, opts ...DialerOption) (*Dialer, error) {
 	cfg := &dialerConfig{
 		refreshTimeout: 30 * time.Second,
 		sqladminOpts:   []option.ClientOption{option.WithUserAgent(userAgent)},
+		dialFunc:       proxy.Dial,
 	}
 	for _, opt := range opts {
 		opt(cfg)
@@ -153,6 +158,7 @@ func NewDialer(ctx context.Context, opts ...DialerOption) (*Dialer, error) {
 		defaultDialCfg: dialCfg,
 		dialerID:       uuid.New().String(),
 		iamTokenSource: cfg.tokenSource,
+		dialFunc:       cfg.dialFunc,
 	}
 	return d, nil
 }
@@ -190,7 +196,7 @@ func (d *Dialer) Dial(ctx context.Context, instance string, opts ...DialOption)
 	ctx, connectEnd = trace.StartSpan(ctx, "cloud.google.com/go/cloudsqlconn/internal.Connect")
 	defer func() { connectEnd(err) }()
 	addr = net.JoinHostPort(addr, serverProxyPort)
-	conn, err = proxy.Dial(ctx, "tcp", addr)
+	conn, err = d.dialFunc(ctx, "tcp", addr)
 	if err != nil {
 		// refresh the instance info in case it caused the connection failure
 		i.ForceRefresh()

dialer_test.go

@@ -18,6 +18,8 @@ import (
 	"context"
 	"errors"
 	"io/ioutil"
+	"net"
+	"strings"
 	"testing"
 	"time"
 
@@ -189,3 +191,32 @@ func TestIAMAuthn(t *testing.T) {
 		}
 	}
 }
+
+func TestDialerWithCustomDialFunc(t *testing.T) {
+	inst := mock.NewFakeCSQLInstance("my-project", "my-region", "my-instance")
+	svc, cleanup, err := mock.NewSQLAdminService(
+		context.Background(),
+		mock.InstanceGetSuccess(inst, 1),
+		mock.CreateEphemeralSuccess(inst, 1),
+	)
+	d, err := NewDialer(context.Background(),
+		WithTokenSource(mock.EmptyTokenSource{}),
+		WithDialFunc(func(ctx context.Context, network, addr string) (net.Conn, error) {
+			return nil, errors.New("sentinel error")
+		}),
+	)
+	if err != nil {
+		t.Fatalf("expected NewDialer to succeed, but got error: %v", err)
+	}
+	d.sqladmin = svc
+	defer func() {
+		if err := cleanup(); err != nil {
+			t.Fatalf("%v", err)
+		}
+	}()
+
+	_, err = d.Dial(context.Background(), "my-project:my-region:my-instance")
+	if !strings.Contains(err.Error(), "sentinel error") {
+		t.Fatalf("want = sentinel error, got = %v", err)
+	}
+}

options.go

@@ -18,6 +18,7 @@ import (
 	"context"
 	"crypto/rsa"
 	"io/ioutil"
+	"net"
 	"net/http"
 	"time"
 
@@ -36,6 +37,7 @@ type dialerConfig struct {
 	rsaKey         *rsa.PrivateKey
 	sqladminOpts   []apiopt.ClientOption
 	dialOpts       []DialOption
+	dialFunc       func(ctx context.Context, network, addr string) (net.Conn, error)
 	refreshTimeout time.Duration
 	useIAMAuthN    bool
 	tokenSource    oauth2.TokenSource
@@ -120,6 +122,15 @@ func WithHTTPClient(client *http.Client) DialerOption {
 	}
 }
 
+// WithDialFunc configures the function used to connect to the address on the
+// named network. This option is generally unnecessary except for advanced
+// use-cases.
+func WithDialFunc(dial func(ctx context.Context, network, addr string) (net.Conn, error)) DialerOption {
+	return func(d *dialerConfig) {
+		d.dialFunc = dial
+	}
+}
+
 // WithIAMAuthN enables automatic IAM Authentication. If no token source has
 // been configured (such as with WithTokenSource, WithCredentialsFile, etc), the
 // dialer will use the default token source as defined by