n2, err := w.ReadFrom(r)
	if n2 != 4 || err != nil {
		t.Fatalf("ReadFrom returned (%v, %v), want (4, nil)", n2, err)
	}
	w.Flush()
	if got, want := buf.String(), "0123abcd"; got != want {
		t.Fatalf("buf.Bytes() returned %q, want %q", got, want)
	}
}

func TestWriterReadFromErrNoProgress(t *testing.T) {
	buf := new(bytes.Buffer)
	w := NewWriterSize(buf, 5)