How Can I Read from an io.Reader Multiple Times in Go?

Read Multiple Times from Single io.Reader

In Go, using the io.ReadCloser interface for reading data from a source can present a challenge when attempting to read it multiple times. As an example, consider a scenario where you want to use request.Body (type io.ReadCloser), which contains an image, for both writing directly to a file and decoding.

Problem:

Attempts to create multiple instances of the reader, as in the following code, result in the second call returning a nil value:

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))

}

Solution:

The io.Reader interface models a stream, meaning it cannot be read twice. To overcome this limitation, you can utilize the io.TeeReader function to create a duplicate 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)) 
}

This code demonstrates how to use io.TeeReader to create a duplicate stream from r and write the content to both tee and buf. By reading from tee first, you can ensure that the buffer is populated with the stream's content.

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