*pool.RW implements io.Closer, so when one is used directly as an http.Request body the transport closes it after each attempt. A retry which then seeks and re-reads the RW would index the freed pages and panic with "index out of range" in readPage. Read, Write, ReadFrom, WriteTo and Seek now return ErrClosed (which wraps io/fs.ErrClosed) after Close, and Close is safe to call more than once.