Go Tidbit: Peek a Snippet From io.Reader

You have an io.Reader, and you want to extract a small snippet from the beginning of the io.Reader and then put it back.

Ideally, io.ReadSeeker would let you do that. But not all io.Readers can seek.

And, like toothpaste from its tube, you cannot put something back once you read it from an io.Reader. But you can do the next best thing:

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// PeekSnippet takes r and returns the first n bytes from it and another
// io.Reader with the entire data.
func PeekSnippet(r io.Reader, n int) ([]byte, io.Reader, error) {
	lr := io.LimitReader(r, n)
	b, err := io.ReadAll(lr) // Read first n bytes from r through lr.
	if err != nil {
		return nil, nil, err
	}
	r = io.MultiReader(bytes.NewReader(b), r) // Make a new reader combining the bytes just read and the remaining data in r.
	return b, r, nil
}

As you can see, you can use io.LimitReader to read the first n bytes. Then use io.MultiReader to combine those bytes with the remainder of data in r.

This post is 14th of my #100DaysToOffload challenge. Want to get involved? Find out more at 100daystooffload.com.