fix: support MySQL driver’s conn check.

dialer.go

@@ -20,11 +20,13 @@ import (
 	"crypto/rsa"
 	"crypto/tls"
 	_ "embed"
+	"errors"
 	"fmt"
 	"net"
 	"strings"
 	"sync"
 	"sync/atomic"
+	"syscall"
 	"time"
 
 	"cloud.google.com/go/cloudsqlconn/errtype"
@@ -230,7 +232,7 @@ func (d *Dialer) Dial(ctx context.Context, instance string, opts ...DialOption)
 		trace.RecordDialLatency(ctx, instance, d.dialerID, latency)
 	}()
 
-	return newInstrumentedConn(tlsConn, func() {
+	return newInstrumentedConn(conn, tlsConn, func() {
 		n := atomic.AddUint64(&i.OpenConns, ^uint64(0))
 		trace.RecordOpenConnections(context.Background(), int64(n), d.dialerID, i.String())
 	}), nil
@@ -264,9 +266,10 @@ func (d *Dialer) Warmup(ctx context.Context, instance string, opts ...DialOption
 
 // newInstrumentedConn initializes an instrumentedConn that on closing will
 // decrement the number of open connects and record the result.
-func newInstrumentedConn(conn net.Conn, closeFunc func()) *instrumentedConn {
+func newInstrumentedConn(rawConn, conn net.Conn, closeFunc func()) *instrumentedConn {
 	return &instrumentedConn{
 		Conn:      conn,
+		rawConn:   rawConn,
 		closeFunc: closeFunc,
 	}
 }
@@ -275,9 +278,21 @@ func newInstrumentedConn(conn net.Conn, closeFunc func()) *instrumentedConn {
 // is closed.
 type instrumentedConn struct {
 	net.Conn
+	// rawConn is the underlying net.Conn without TLS
+	rawConn   net.Conn
 	closeFunc func()
 }
 
+// SyscallConn supports a connection check in the MySQL driver by delegating to
+// the underlying non-TLS net.Conn.
+func (i *instrumentedConn) SyscallConn() (syscall.RawConn, error) {
+	sconn, ok := i.rawConn.(syscall.Conn)
+	if !ok {
+		return nil, errors.New("connection is not a syscall.Conn")
+	}
+	return sconn.SyscallConn()
+}
+
 // Close delegates to the underylying net.Conn interface and reports the close
 // to the provided closeFunc only when Close returns no error.
 func (i *instrumentedConn) Close() error {

dialer_test.go

@@ -22,6 +22,7 @@ import (
 	"os"
 	"runtime"
 	"strings"
+	"syscall"
 	"testing"
 	"time"
 
@@ -75,6 +76,48 @@ func TestDialerCanConnectToInstance(t *testing.T) {
 	testSuccessfulDial(t, d, context.Background(), "my-project:my-region:my-instance", WithPublicIP())
 }
 
+func TestDialerConnectionSupportsSyscalls(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),
+	)
+	if err != nil {
+		t.Fatalf("failed to init SQLAdminService: %v", err)
+	}
+	stop := mock.StartServerProxy(t, inst)
+	defer func() {
+		stop()
+		if err := cleanup(); err != nil {
+			t.Fatalf("%v", err)
+		}
+	}()
+
+	d, err := NewDialer(context.Background(),
+		WithDefaultDialOptions(WithPublicIP()),
+		WithTokenSource(mock.EmptyTokenSource{}),
+	)
+	if err != nil {
+		t.Fatalf("expected NewDialer to succeed, but got error: %v", err)
+	}
+	d.sqladmin = svc
+
+	conn, err := d.Dial(context.Background(), "my-project:my-region:my-instance")
+	if err != nil {
+		t.Fatalf("expected Dial to succeed, but got error: %v", err)
+	}
+	defer conn.Close()
+	sconn, ok := conn.(syscall.Conn)
+	if !ok {
+		t.Fatalf("expected conn to be a syscall.Conn, but it was not")
+	}
+	_, err = sconn.SyscallConn()
+	if err != nil {
+		t.Fatalf("expected syscall.RawConn, got error: %v", err)
+	}
+}
+
 func TestDialWithAdminAPIErrors(t *testing.T) {
 	inst := mock.NewFakeCSQLInstance("my-project", "my-region", "my-instance")
 	svc, cleanup, err := mock.NewSQLAdminService(context.Background())