Maint: tun package to handle tun device operations

- Moved from openvpn package to tun package
- TUN check verifies Rdev value
- TUN create
- Inject as interface to main function
- Add integration test
- Clearer log message for end users if tun device does not exist
- Remove unix package (unneeded for tests)
- Remove tun file opening at the end of tun file creation
- Do not mock unix.Mkdev (no OS operation)
- Remove Tun operations from OpenVPN configurator

internal/tun/tun_test.go

@@ -0,0 +1,68 @@
+package tun
+
+import (
+	"os"
+	"testing"
+
+	"github.com/stretchr/testify/require"
+)
+
+func Test_Tun(t *testing.T) {
+	t.Parallel()
+
+	path := getTempPath(t)
+
+	tun := New()
+
+	defer func() {
+		err := os.RemoveAll(path)
+		require.NoError(t, err)
+	}()
+
+	// No file check fail
+	err := tun.Check(path)
+	require.Error(t, err)
+	expectedMessage := "TUN device is not available: open " + path + ": no such file or directory"
+	require.Equal(t, expectedMessage, err.Error())
+
+	// Create simple file
+	file, err := os.Create(path)
+	require.NoError(t, err)
+	err = file.Close()
+	require.NoError(t, err)
+
+	// Simple file check fail
+	err = tun.Check(path)
+	require.Error(t, err)
+	expectedMessage = "TUN file has an unexpected rdev: 0 instead of expected 2760"
+	require.Equal(t, expectedMessage, err.Error())
+
+	// Create TUN device fail as file exists
+	err = tun.Create(path)
+	require.Error(t, err)
+	require.Equal(t, "cannot create TUN device file node: file exists", err.Error())
+
+	// Remove simple file
+	err = os.Remove(path)
+	require.NoError(t, err)
+
+	// Create TUN device success
+	err = tun.Create(path)
+	require.NoError(t, err)
+
+	// Check TUN device success
+	err = tun.Check(path)
+	require.NoError(t, err)
+}
+
+func getTempPath(t *testing.T) (path string) {
+	t.Helper()
+	file, err := os.CreateTemp("", "")
+	require.NoError(t, err)
+	path = file.Name()
+	err = file.Close()
+	require.NoError(t, err)
+	err = os.Remove(path)
+	require.NoError(t, err)
+	return path
+}