wintun: revise GetInterface()

- Make foreign interface found error numeric to ease condition
  detection.
- Update GetInterface() documentation.
- Make tun.CreateTUN() quit when foreign interface found before
  attempting to create a Wintun interface with a duplicate name.
  Creation is futile.

Signed-off-by: Simon Rozman <simon@rozman.si>

tun/tun_windows.go

@@ -71,6 +71,8 @@ func CreateTUN(ifname string) (TUNDevice, error) {
 		if err != nil {
 			return nil, fmt.Errorf("Unable to delete already existing Wintun interface: %v", err)
 		}
+	} else if err == windows.ERROR_ALREADY_EXISTS {
+		return nil, fmt.Errorf("Foreign network interface with the same name exists")
 	}
 	wt, _, err = wintun.CreateInterface("WireGuard Tunnel Adapter", 0)
 	if err != nil {

tun/wintun/wintun_windows.go

@@ -89,8 +89,9 @@ func makeWintun(deviceInfoSet setupapi.DevInfo, deviceInfoData *setupapi.DevInfo
 // optional and can be 0. If a specific top-level window is not required, set
 // hwndParent to 0.
 //
-// Function returns interface if found, or nil otherwise. If the interface is
+// Function returns interface if found, or windows.ERROR_OBJECT_NOT_FOUND
-// found but not Wintun-class, the function returns interface and an error.
+// otherwise. If the interface is found but not Wintun-class, the function
+// returns windows.ERROR_ALREADY_EXISTS.
 //
 func GetInterface(ifname string, hwndParent uintptr) (*Wintun, error) {
 	// Create a list of network devices.
@@ -164,7 +165,7 @@ func GetInterface(ifname string, hwndParent uintptr) (*Wintun, error) {
 			}
 
 			// This interface is not using Wintun driver.
-			return nil, errors.New("Foreign network interface with the same name exists")
+			return nil, windows.ERROR_ALREADY_EXISTS
 		}
 	}