Inconsistent error reporting when using ReadOnlyFs and Windows

[jochil]

When using the ReadOnlyFs filter, the error returned when trying to write to the FS is different on Windows than on Linux or MacOSX

How to reproduce:

package afero_win

import (
	"errors"
	"os"
	"testing"

	pkgErrors "github.com/pkg/errors"
	"github.com/spf13/afero"
	"github.com/stretchr/testify/assert"
	"github.com/stretchr/testify/require"
)

func TestAferoPerm(t *testing.T) {
	fs := &afero.Afero{Fs: afero.NewReadOnlyFs(afero.NewMemMapFs())}

	err := fs.WriteFile("test.txt", []byte("foobar"), 0644);
	require.Error(t, err)

	assert.True(t, errors.Is(err, os.ErrPermission))
	assert.True(t, os.IsPermission(pkgErrors.Cause(err)))
	assert.True(t, pkgErrors.Is(err, os.ErrPermission))
}

while the test is running fine on Linux, on Windows it results in:

 --- FAIL: TestAferoPerm (0.00s)
    perm_test.go:22: 
        	Error Trace:	perm_test.go:22
        	Error:      	Should be true
        	Test:       	TestAferoPerm
    perm_test.go:23: 
        	Error Trace:	perm_test.go:23
        	Error:      	Should be true
        	Test:       	TestAferoPerm
    perm_test.go:24: 
        	Error Trace:	perm_test.go:24
        	Error:      	Should be true
        	Test:       	TestAferoPerm

Expected behavior

Under windows it already returns an error operation not permitted but it is not matching with os.ErrPermission I think it would be more consistent if this behaves identical on different operating systems

[jochil]

I created the linked PR, where the returned error is changed from syscall.EPERM to os.ErrPermission. I think this is more idiomatic to the std package This code creates a permission error under windows... this might helpful to verify

  os.Chmod(file, 0400)
  err := os.WriteFile(file, []byte("test"), 0400)
  assert.True(t, errors.Is(err, os.ErrPermission))