How to read multiple times from same io.Reader

Question

I want to use request.Body(type io.ReadCloser) which is containing a image.

I dont want to use ioutil.ReadAll() as i want to write this body directly to the file as well as want to decode it, so i only want to use the reference to the content to pass to further function calls,

I tried creating multiple instances of reader for example shown below

package main

import (
    "io/ioutil"
    "log"
    "strings"
)


func main() {
    r := strings.NewReader("some io.Reader stream to be read\n")
    a := &r
    b := &r
    log.Println(ioutil.ReadAll(*a))
    log.Println(ioutil.ReadAll(*b))

}

but in second call it always results into nil.

Please help me how can i pass multiple separate reference for the same reader?

Answer 1

io.Reader is treated like a stream. Because of this you cannot read it twice. Imagine the an incoming TCP connection. You cannot rewind the whats coming in.

But you can use the io.TeeReader to duplicate the stream:

package main

import (
    "bytes"
    "io"
    "io/ioutil"
    "log"
    "strings"
)

func main() {
    r := strings.NewReader("some io.Reader stream to be read\n")
    var buf bytes.Buffer
    tee := io.TeeReader(r, &buf)

    log.Println(ioutil.ReadAll(tee))
    log.Println(ioutil.ReadAll(&buf)) 
}

Example on Go Playground

Edit: As @mrclx pointed out: You need to read from the TeeReader first, otherwise the buffer will be empty.