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/check.go

@@ -0,0 +1,51 @@
+package tun
+
+import (
+	"errors"
+	"fmt"
+	"os"
+	"syscall"
+)
+
+type Checker interface {
+	Check(path string) error
+}
+
+var (
+	ErrTUNNotAvailable = errors.New("TUN device is not available")
+	ErrTUNStat         = errors.New("cannot stat TUN file")
+	ErrTUNInfo         = errors.New("cannot get syscall stat info of TUN file")
+	ErrTUNBadRdev      = errors.New("TUN file has an unexpected rdev")
+	ErrTUNClose        = errors.New("cannot close TUN device")
+)
+
+// Check checks the tunnel device specified by path is present and accessible.
+func (t *Tun) Check(path string) error {
+	f, err := os.OpenFile(path, os.O_RDWR, 0)
+	if err != nil {
+		return fmt.Errorf("%w: %s", ErrTUNNotAvailable, err)
+	}
+	defer f.Close()
+
+	info, err := f.Stat()
+	if err != nil {
+		return fmt.Errorf("%w: %s", ErrTUNStat, err)
+	}
+
+	sys, ok := info.Sys().(*syscall.Stat_t)
+	if !ok {
+		return ErrTUNInfo
+	}
+
+	const expectedRdev = 2760 // corresponds to major 10 and minor 200
+	if sys.Rdev != expectedRdev {
+		return fmt.Errorf("%w: %d instead of expected %d",
+			ErrTUNBadRdev, sys.Rdev, expectedRdev)
+	}
+
+	if err := f.Close(); err != nil {
+		return fmt.Errorf("%w: %s", ErrTUNClose, err)
+	}
+
+	return nil
+}

internal/tun/create.go

@@ -0,0 +1,38 @@
+package tun
+
+import (
+	"errors"
+	"fmt"
+	"os"
+	"path/filepath"
+
+	"golang.org/x/sys/unix"
+)
+
+type Creator interface {
+	Create(path string) error
+}
+
+var (
+	ErrMknod = errors.New("cannot create TUN device file node")
+)
+
+// Create creates a TUN device at the path specified.
+func (t *Tun) Create(path string) error {
+	parentDir := filepath.Dir(path)
+	if err := os.MkdirAll(parentDir, 0751); err != nil { //nolint:gomnd
+		return err
+	}
+
+	const (
+		major = 10
+		minor = 200
+	)
+	dev := unix.Mkdev(major, minor)
+	err := t.mknod(path, unix.S_IFCHR, int(dev))
+	if err != nil {
+		return fmt.Errorf("%w: %s", ErrMknod, err)
+	}
+
+	return nil
+}

internal/tun/tun.go

@@ -0,0 +1,20 @@
+package tun
+
+import "golang.org/x/sys/unix"
+
+var _ Interface = (*Tun)(nil)
+
+type Interface interface {
+	Checker
+	Creator
+}
+
+type Tun struct {
+	mknod func(path string, mode uint32, dev int) (err error)
+}
+
+func New() *Tun {
+	return &Tun{
+		mknod: unix.Mknod,
+	}
+}

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
+}