chore(all): return concrete types, accept interfaces

- Remove exported interfaces unused locally
- Define interfaces to accept arguments
- Return concrete types, not interfaces

internal/tun/check.go

@@ -7,10 +7,6 @@ import (
 	"syscall"
 )
 
-type Checker interface {
-	Check(path string) error
-}
-
 var (
 	ErrTUNInfo    = errors.New("cannot get syscall stat info of TUN file")
 	ErrTUNBadRdev = errors.New("TUN file has an unexpected rdev")

internal/tun/create.go

@@ -8,10 +8,6 @@ import (
 	"golang.org/x/sys/unix"
 )
 
-type Creator interface {
-	Create(path string) error
-}
-
 // Create creates a TUN device at the path specified.
 func (t *Tun) Create(path string) error {
 	parentDir := filepath.Dir(path)

internal/tun/tun.go

@@ -2,13 +2,6 @@ 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)
 }