Why does an io.reader become empty after reading it?

func foo(buf *bytes.Buffer) {
    fmt.Println("0: ", len(buf.Bytes()))
    ioutil.ReadAll(buf)
    fmt.Println("1: ", len(buf.Bytes()))
}

The code shows the correct length the first time, but the second times it shows the length is zero.

Reading from a bytes.Buffer drains or consumes the bytes that were read. This means if you try to read again, those will not be returned.

Buffer.Bytes() returns the unread portion of the buffer, so it is the expected result for you to see 0 length after everything has been read (this is exactly what ioutil.ReadAll() does).

What if you just want to "peek" and not really "read" bytes?

There is no "peek" functionality in bytes.Buffer. The easiest would be to get the bytes of the buffer, and construct another bytes.Buffer from it and read from the new buffer.

It could look something like this:

func peek(buf *bytes.Buffer, b []byte) (int, error) {
    buf2 := bytes.NewBuffer(buf.Bytes())
    return buf2.Read(b)
}

peek() in action:

Error checks omitted for simplicity:

buf := &bytes.Buffer{}

buf.WriteString("Hello")
fmt.Printf("Len: %d, Content: %s\n", buf.Len(), buf)

fmt.Println("\nPeeking...")
data := make([]byte, 4)
peek(buf, data)
fmt.Printf("Peeked: %s\n", data)
fmt.Printf("Len: %d, Content: %s\n", buf.Len(), buf)

fmt.Println("\nReading...")
data = make([]byte, buf.Len())
buf.Read(data)
fmt.Printf("Read: %s\n", data)
fmt.Printf("Len: %d, Content: %s\n", buf.Len(), buf)

Output (try it on the Go Playground):

Len: 5, Content: Hello

Peeking...
Peeked: Hell
Len: 5, Content: Hello

Reading...
Read: Hello
Len: 0, Content: