convert from `bufio.Reader` to `io.ReadWriteCloser`

I have an io.ReadWriteCloser in which I want to peek into it without advancing the reader,

so I am using

bi := bufio.NewReader(i)
bi.Peek(1)

So far so good, but later when I want to reuse the original io.ReadWriteCloser (i) it has only EOF.

So my question is how to convert back from bufio.Reader back to io.ReadWriteCloser

Because the bufio.Reader buffers data from the underlying reader, the application must read from the bufio.Reader after the call to Peek.

To get an io.ReadWriteCloser that does this, wrap the bufio.Reader and the original io.ReadWriteCloser:

// BufferedReadWriteCloser has all of the methods
// from *bufio.Reader and io.ReadWriteCloser.
type BufferedReadWriteCloser struct {
	*bufio.Reader
	io.ReadWriteCloser
}

func (rw *BufferedReadWriteCloser) Read(p []byte) (int, error) {
	return rw.Reader.Read(p)
}

Here's how to use it:

rw := &BufferedReadWriteCloser{bufio.NewReader(i), i}
p, err := rw.Peek(1)

The value of rw satisfies the io.ReadWriteCloser interface.

There is no requirement or assumption that the io.ReadWriteCloser has a Seek method.

As mentioned in my comment above, you need access to the original reader's Seek method. This means that passing the reader around as an io.ReadWriteCloser is insufficient. Having said that, the following helper function may be a workaround:

func peek(r io.Reader, n int) ([]byte, error) {
	bi := bufio.NewReader(r)
	peeked, err := bi.Peek(n)
	if err != nil {
		return nil, err
	}
    // Use type assertion to check if r implements the 
    // io.Seeker interface. If it does, then use it to
    // reset the offset.
	if seeker, ok := r.(io.Seeker); ok {
		seeker.Seek(0, 0)
	}
	return peeked, nil
}

Now you can pass the io.ReadWriteCloser to this peek function. The peek function checks if the reader happens to implement the Seek method. If the Seek method is implemented, then peek will call it.