Fix readdir and implement seek for directory

If n <= 0, Readdir should return the remaining files and
never return io.EOF.
If n > 0, Readdir should only return io.EOF if it returns
an empty slice, per the documentation of os.Readdir.
It should also not reset the offset to 0.

Implement seek for directories when offset is 0 and
whence is io.SeekStart, because os.Seek supports this.

virtual.go

@@ -203,24 +203,31 @@ func (vd *virtualDir) readdir(n int) (fi []os.FileInfo, err error) {
 	// Sort it by filename (lexical order)
 	sort.Sort(SortByName(files))
 
-	// Return all contents if that's what is requested
+	// Return all remainign contents if that's what is requested
 	if n <= 0 {
-		vd.offset = 0
-		return files, nil
+		offset := vd.offset
+		vd.offset = len(files)
+		return files[offset:], nil
 	}
 
+	end := vd.offset + n
 	// If user has requested past the end of our list
-	// return what we can and send an EOF
-	if vd.offset+n >= len(files) {
-		offset := vd.offset
-		vd.offset = 0
-		return files[offset:], io.EOF
+	// return what we can
+	if end >= len(files) {
+		end = len(files)
 	}
 
 	offset := vd.offset
-	vd.offset += n
-	return files[offset : offset+n], nil
+	vd.offset = end
+	fi = files[offset:end]
 
+	if len(fi) == 0 && n > 0 {
+		// Per File.Readdir, the slice must be non-empty or err
+		// must be non-nil if n > 0.
+		err = io.EOF
+	}
+
+	return fi, err
 }
 
 func (vd *virtualDir) read(bts []byte) (int, error) {
@@ -246,6 +253,11 @@ func (vd *virtualDir) seek(offset int64, whence int) (int64, error) {
 			Err:  errors.New("bad file descriptor"),
 		}
 	}
+	if whence == io.SeekStart && offset == 0 {
+		// special case for rewinddir
+		vd.offset = 0
+		return 0, nil
+	}
 	return 0, &os.PathError{
 		Op:   "seek",
 		Path: vd.Filename,