MemMapFs clears out non chmod mode bits when writing file

[sridgeway7]

I recently update a project from v1.3.3 to v1.8.2 and it looks like file mode is not preserved when creating files using MemMapFs.

I'm trying to write a file with os.ModeSocket to a MemMapFs instance in a test. After writing the file, I can see that os.ModeSocket is not set. It looks like this is cause by this line which mutate perm at the top of MemMapFs.OpenFile:

afero/memmap.go

Lines 223 to 224 in 45ef346

	func (m *MemMapFs) OpenFile(name string, flag int, perm os.FileMode) (File, error) {
	perm &= chmodBits

Which was changed here: b598fbb

This effectively zeroes out any perm bits that are not the chmodBits which make it impossible to create irregular files. It looks like the handling for preserving previous bits while only honoring chmodBits is already in MemMapFs.Chmod. So maybe a better approach is to not modify perm, set the mode as passed on creation, and then call Chmod if chmod is true?:

	if os.IsNotExist(err) && (flag&os.O_CREATE > 0) {
		file, err = m.Create(name)
		if err == nil {
			err = file.setFileMode(perm & ^chmodBits)
		}

		chmod = true
	}

	/* ... */

	if chmod {
		return file, m.Chmod(name, perm)
	}