Add tests for GetTLSConnectionState method in SMTP client

This commit introduces four test cases for the GetTLSConnectionState method. These tests cover scenarios with a valid TLS connection, no connection, a non-TLS connection, and a non-TLS connection with the TLS flag set on the client. This ensures comprehensive validation of the method's behavior across different states.

smtp/smtp_test.go

@@ -3396,6 +3396,122 @@ func TestClient_UpdateDeadline(t *testing.T) {
 	})
 }
 
+func TestClient_GetTLSConnectionState(t *testing.T) {
+	t.Run("get state on sane client connection", func(t *testing.T) {
+		ctx, cancel := context.WithCancel(context.Background())
+		defer cancel()
+		PortAdder.Add(1)
+		serverPort := int(TestServerPortBase + PortAdder.Load())
+		featureSet := "250-DSN\r\n250 STARTTLS"
+		go func() {
+			if err := simpleSMTPServer(ctx, t, &serverProps{
+				FeatureSet: featureSet,
+				ListenPort: serverPort,
+			},
+			); err != nil {
+				t.Errorf("failed to start test server: %s", err)
+				return
+			}
+		}()
+		time.Sleep(time.Millisecond * 30)
+		client, err := Dial(fmt.Sprintf("%s:%d", TestServerAddr, serverPort))
+		if err != nil {
+			t.Fatalf("failed to dial to test server: %s", err)
+		}
+		t.Cleanup(func() {
+			if err = client.Close(); err != nil {
+				t.Errorf("failed to close client: %s", err)
+			}
+		})
+		tlsConfig := getTLSConfig(t)
+		tlsConfig.MinVersion = tls.VersionTLS12
+		tlsConfig.MaxVersion = tls.VersionTLS12
+		if err = client.StartTLS(tlsConfig); err != nil {
+			t.Fatalf("failed to start TLS on client: %s", err)
+		}
+		state, err := client.GetTLSConnectionState()
+		if err != nil {
+			t.Fatalf("failed to get TLS connection state: %s", err)
+		}
+		if state == nil {
+			t.Error("expected TLS connection state to be non-nil")
+		}
+		if state.Version != tls.VersionTLS12 {
+			t.Errorf("expected TLS connection state version to be %d, got: %d", tls.VersionTLS12, state.Version)
+		}
+	})
+	t.Run("get state on no connection", func(t *testing.T) {
+		client := &Client{}
+		_, err := client.GetTLSConnectionState()
+		if err == nil {
+			t.Fatal("expected client to have no tls connection state")
+		}
+	})
+	t.Run("get state on non-tls client connection", func(t *testing.T) {
+		ctx, cancel := context.WithCancel(context.Background())
+		defer cancel()
+		PortAdder.Add(1)
+		serverPort := int(TestServerPortBase + PortAdder.Load())
+		featureSet := "250 DSN"
+		go func() {
+			if err := simpleSMTPServer(ctx, t, &serverProps{
+				FeatureSet: featureSet,
+				ListenPort: serverPort,
+			},
+			); err != nil {
+				t.Errorf("failed to start test server: %s", err)
+				return
+			}
+		}()
+		time.Sleep(time.Millisecond * 30)
+		client, err := Dial(fmt.Sprintf("%s:%d", TestServerAddr, serverPort))
+		if err != nil {
+			t.Fatalf("failed to dial to test server: %s", err)
+		}
+		t.Cleanup(func() {
+			if err = client.Close(); err != nil {
+				t.Errorf("failed to close client: %s", err)
+			}
+		})
+		_, err = client.GetTLSConnectionState()
+		if err == nil {
+			t.Error("expected client to have no tls connection state")
+		}
+	})
+	t.Run("fail to get state on non-tls connection with tls flag set", func(t *testing.T) {
+		ctx, cancel := context.WithCancel(context.Background())
+		defer cancel()
+		PortAdder.Add(1)
+		serverPort := int(TestServerPortBase + PortAdder.Load())
+		featureSet := "250 DSN"
+		go func() {
+			if err := simpleSMTPServer(ctx, t, &serverProps{
+				FeatureSet: featureSet,
+				ListenPort: serverPort,
+			},
+			); err != nil {
+				t.Errorf("failed to start test server: %s", err)
+				return
+			}
+		}()
+		time.Sleep(time.Millisecond * 30)
+		client, err := Dial(fmt.Sprintf("%s:%d", TestServerAddr, serverPort))
+		if err != nil {
+			t.Fatalf("failed to dial to test server: %s", err)
+		}
+		client.tls = true
+		t.Cleanup(func() {
+			if err = client.Close(); err != nil {
+				t.Errorf("failed to close client: %s", err)
+			}
+		})
+		_, err = client.GetTLSConnectionState()
+		if err == nil {
+			t.Error("expected client to have no tls connection state")
+		}
+	})
+}
+
 // faker is a struct embedding io.ReadWriter to simulate network connections for testing purposes.
 type faker struct {
 	io.ReadWriter