code review 6442080: os: test that IsExist and IsNotExist handle PathError a...

src/pkg/os/error_posix.go

 // Copyright 2009 The Go Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
 // +build darwin freebsd linux netbsd openbsd
 
 package os
 
 import "syscall"
 
 func isExist(err error) bool {
-        if pe, ok := err.(*PathError); ok {
+        switch pe := err.(type) {
+        case nil:
+                return false
+        case *PathError:
                 err = pe.Err
-        }
-        if pe, ok := err.(*LinkError); ok {
+        case *LinkError:
                 err = pe.Err
         }
         return err == syscall.EEXIST || err == ErrExist
 }
 
 func isNotExist(err error) bool {
-        if pe, ok := err.(*PathError); ok {
+        switch pe := err.(type) {
+        case nil:
+                return false
+        case *PathError:
                 err = pe.Err
-        }
-        if pe, ok := err.(*LinkError); ok {
+        case *LinkError:
                 err = pe.Err
         }
         return err == syscall.ENOENT || err == ErrNotExist
 }
 
 func isPermission(err error) bool {
-        if pe, ok := err.(*PathError); ok {
+        switch pe := err.(type) {
+        case nil:
+                return false
+        case *PathError:
                 err = pe.Err
-        }
-        if pe, ok := err.(*LinkError); ok {
+        case *LinkError:
                 err = pe.Err
         }
         return err == syscall.EACCES || err == syscall.EPERM || err == ErrPermission
 }