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
+}