Draft: Add WrappedFS interface

basepath.go

@@ -220,3 +220,7 @@ func (b *BasePathFs) ReadlinkIfPossible(name string) (string, error) {
 	}
 	return "", &os.PathError{Op: "readlink", Path: name, Err: ErrNoReadlink}
 }
+
+func (b *BasePathFs) ParentFS() Fs {
+	return b.source
+}

cacheOnReadFs.go

@@ -313,3 +313,7 @@ func (u *CacheOnReadFs) Create(name string) (File, error) {
 	}
 	return &UnionFile{Base: bfh, Layer: lfh}, nil
 }
+
+func (u *CacheOnReadFs) ParentFS() Fs {
+	return u.base
+}

readonlyfs.go

@@ -94,3 +94,7 @@ func (r *ReadOnlyFs) MkdirAll(n string, p os.FileMode) error {
 func (r *ReadOnlyFs) Create(n string) (File, error) {
 	return nil, syscall.EPERM
 }
+
+func (r *ReadOnlyFs) ParentFS() Fs {
+	return r.source
+}

regexpfs.go

@@ -153,6 +153,10 @@ func (r *RegexpFs) Create(name string) (File, error) {
 	return r.source.Create(name)
 }
 
+func (r *RegexpFs) ParentFS() Fs {
+	return r.source
+}
+
 func (f *RegexpFile) Close() error {
 	return f.f.Close()
 }

wrappedfs.go

@@ -0,0 +1,24 @@
+// Copyright © 2018 Steve Francia <[email protected]>.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package afero
+
+// WrappedFs is an optional interface in Afero. It is only implemented by the
+// filesystems saying so.
+// Filesystems which wrap another filesystem can provide access to their parent
+// filesystem by implementing this interface.
+// If this interface returns nil, then the filesystem either has no parents or
+// is unable to provide a parent.
+type WrappedFs interface {
+	ParentFS() Fs
+}