Return os consistent error in ReadOnlyFs filter

fixes #350: Use `os.ErrPermission` instead of `syscall.EPerm` to make sure
that the error is consistent over different operating systems and can be
checked with `os.isPermission(err)`

README.md

@@ -290,7 +290,7 @@ A thin wrapper around the source Fs providing a read only view.
 ```go
 fs := afero.NewReadOnlyFs(afero.NewOsFs())
 _, err := fs.Create("/file.txt")
-// err = syscall.EPERM
+// err = os.ErrPermission 
 ```
 
 # RegexpFs

readonlyfs.go

@@ -2,7 +2,6 @@ package afero
 
 import (
 	"os"
-	"syscall"
 	"time"
 )
 
@@ -21,15 +20,15 @@ func (r *ReadOnlyFs) ReadDir(name string) ([]os.FileInfo, error) {
 }
 
 func (r *ReadOnlyFs) Chtimes(n string, a, m time.Time) error {
-	return syscall.EPERM
+	return os.ErrPermission
 }
 
 func (r *ReadOnlyFs) Chmod(n string, m os.FileMode) error {
-	return syscall.EPERM
+	return os.ErrPermission
 }
 
 func (r *ReadOnlyFs) Chown(n string, uid, gid int) error {
-	return syscall.EPERM
+	return os.ErrPermission
 }
 
 func (r *ReadOnlyFs) Name() string {
@@ -61,20 +60,20 @@ func (r *ReadOnlyFs) ReadlinkIfPossible(name string) (string, error) {
 }
 
 func (r *ReadOnlyFs) Rename(o, n string) error {
-	return syscall.EPERM
+	return os.ErrPermission
 }
 
 func (r *ReadOnlyFs) RemoveAll(p string) error {
-	return syscall.EPERM
+	return os.ErrPermission
 }
 
 func (r *ReadOnlyFs) Remove(n string) error {
-	return syscall.EPERM
+	return os.ErrPermission
 }
 
 func (r *ReadOnlyFs) OpenFile(name string, flag int, perm os.FileMode) (File, error) {
-	if flag&(os.O_WRONLY|syscall.O_RDWR|os.O_APPEND|os.O_CREATE|os.O_TRUNC) != 0 {
-		return nil, syscall.EPERM
+	if flag&(os.O_WRONLY|os.O_RDWR|os.O_APPEND|os.O_CREATE|os.O_TRUNC) != 0 {
+		return nil, os.ErrPermission
 	}
 	return r.source.OpenFile(name, flag, perm)
 }
@@ -84,13 +83,13 @@ func (r *ReadOnlyFs) Open(n string) (File, error) {
 }
 
 func (r *ReadOnlyFs) Mkdir(n string, p os.FileMode) error {
-	return syscall.EPERM
+	return os.ErrPermission
 }
 
 func (r *ReadOnlyFs) MkdirAll(n string, p os.FileMode) error {
-	return syscall.EPERM
+	return os.ErrPermission
 }
 
 func (r *ReadOnlyFs) Create(n string) (File, error) {
-	return nil, syscall.EPERM
+	return nil, os.ErrPermission
 }

readonlyfs_test.go

@@ -0,0 +1,34 @@
+package afero
+
+import (
+	"os"
+	"testing"
+	"time"
+)
+
+func checkForErrPermission(t *testing.T, err error) {
+	t.Helper()
+	if err == nil || !os.IsPermission(err) {
+		t.Errorf("Expected err !=nil && err == ErrPermission, got %[1]T (%[1]v)", err)
+	}
+}
+
+// Make sure that the ReadOnlyFs filter returns errors that can be
+// checked with os.IsPermission
+func TestReadOnlyFsErrPermission(t *testing.T) {
+	fs := NewReadOnlyFs(NewMemMapFs())
+
+	_, err := fs.Create("test")
+	checkForErrPermission(t, err)
+	checkForErrPermission(t, fs.Chtimes("test", time.Now(), time.Now()))
+	checkForErrPermission(t, fs.Chmod("test", os.ModePerm))
+	checkForErrPermission(t, fs.Chown("test", 0, 0))
+	checkForErrPermission(t, fs.Mkdir("test", os.ModePerm))
+	checkForErrPermission(t, fs.MkdirAll("test", os.ModePerm))
+	_, err = fs.OpenFile("test", os.O_CREATE, os.ModePerm)
+	checkForErrPermission(t, err)
+	checkForErrPermission(t, fs.Remove("test"))
+	checkForErrPermission(t, fs.RemoveAll("test"))
+	checkForErrPermission(t, fs.Rename("test", "test"))
+
+}