Adding support for WriteAt

fs.go

@@ -164,6 +164,8 @@ type File interface {
 // Name returns the name of the file as presented to Open.
 Name() string
 io.Writer
+// TODO: Add io.WriterAt for v6 
+// io.WriterAt
 io.Reader
 io.ReaderAt
 io.Seeker

memfs/memory.go

@@ -272,6 +272,10 @@ func (f *file) Seek(offset int64, whence int) (int64, error) {
 }
 
 func (f *file) Write(p []byte) (int, error) {
+return f.WriteAt(p, f.position)
+}
+
+func (f *file) WriteAt(p []byte, off int64) (int, error) {
 if f.isClosed {
 return 0, os.ErrClosed
 }
@@ -280,8 +284,8 @@ func (f *file) Write(p []byte) (int, error) {
 return 0, errors.New("write not supported")
 }
 
-n, err := f.content.WriteAt(p, f.position)
-f.position += int64(n)
+n, err := f.content.WriteAt(p, off)
+f.position = off + int64(n)
 
 return n, err
 }

test/mock.go

@@ -115,6 +115,10 @@ func (*FileMock) ReadAt(b []byte, off int64) (int, error) {
 return 0, nil
 }
 
+func (*FileMock) WriteAt(b []byte, off int64) (int, error) {
+return 0, nil
+}
+
 func (*FileMock) Seek(offset int64, whence int) (int64, error) {
 return 0, nil
 }