How to get the size of an io.reader object

I would like to get the size of an io.reader to return the length of a picture for a bse64 concerns.

The main problem is to get the size of dec to return a content-length header

Here is my code :

<!-- language: lang-golang -->

package main
import (
"fmt"
"net/http"
"time"
"strconv"
"io"
base64 "encoding/base64"
"bytes"
)
func pix(w http.ResponseWriter, r *http.Request) {
fmt.Fprintf(w, "Hi there, I love %s!", r.URL.Path[1:])
cookie, _ := r.Cookie("csrftoken")
fmt.Printf(cookie.Value)
}
func img(w http.ResponseWriter, r *http.Request) {
expiration := time.Now().Add(365 * 24 * time.Hour)
cookie    :=    http.Cookie{Name: "csrftoken",Value:"abcd",Expires:expiration,HttpOnly: false}
http.SetCookie(w, &cookie)
img := bytes.NewBufferString("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")
dec := base64.NewDecoder(base64.StdEncoding, img)
w.Header().Set("Content-Type", "image/jpeg")
// the problem is here i would like to get the size to return the length of the picture  (lend(dec))
w.Header().Set("Content-Length", strconv.Itoa(len(dec)))
io.Copy(w, dec)
}
func main() {
http.HandleFunc("/pix/", pix)
http.HandleFunc("/img/", img)
http.HandleFunc("/red/", img)
http.HandleFunc("/", img)
http.ListenAndServe(":8080", nil)
}

regards and thanks

Since io.Reader interface doesn't know anything about size or length of underlying data,
there are 2 options:
1- Use buffer and read or copy all data:

<!-- language: lang-golang -->

buf := &bytes.Buffer{}
nRead, err := io.Copy(buf, dec) 
if err != nil {
fmt.Println(err)
}
w.Header().Set("Content-Length", strconv.FormatInt(nRead, 10)) //len(dec)

Like this working sample code:

<!-- language: lang-golang -->

package main
import (
"bytes"
base64 "encoding/base64"
"fmt"
"io"
"net/http"
"strconv"
"time"
)
func pix(w http.ResponseWriter, r *http.Request) {
fmt.Fprintf(w, "Hi there, I love %s!", r.URL.Path[1:])
cookie, _ := r.Cookie("csrftoken")
fmt.Printf(cookie.Value)
}
func img(w http.ResponseWriter, r *http.Request) {
expiration := time.Now().Add(365 * 24 * time.Hour)
cookie := http.Cookie{Name: "csrftoken", Value: "abcd", Expires: expiration, HttpOnly: false}
http.SetCookie(w, &cookie)
img := bytes.NewBufferString("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")
dec := base64.NewDecoder(base64.StdEncoding, img)
w.Header().Set("Content-Type", "image/jpeg")
buf := &bytes.Buffer{}
nRead, err := io.Copy(buf, dec) 
if err != nil {
fmt.Println(err)
}
w.Header().Set("Content-Length", strconv.FormatInt(nRead, 10)) //len(dec)
io.Copy(w, buf)
}
func main() {
http.HandleFunc("/pix/", pix)
http.HandleFunc("/img/", img)
http.HandleFunc("/red/", img)
http.HandleFunc("/", img)
http.ListenAndServe(":8080", nil)
}

2- Don't set the header size, like this working sample code:

<!-- language: lang-golang -->

package main
import (
"bytes"
base64 "encoding/base64"
"fmt"
"io"
"net/http"
"time"
)
func pix(w http.ResponseWriter, r *http.Request) {
fmt.Fprintf(w, "Hi there, I love %s!", r.URL.Path[1:])
cookie, _ := r.Cookie("csrftoken")
fmt.Printf(cookie.Value)
}
func img(w http.ResponseWriter, r *http.Request) {
expiration := time.Now().Add(365 * 24 * time.Hour)
cookie := http.Cookie{Name: "csrftoken", Value: "abcd", Expires: expiration, HttpOnly: false}
http.SetCookie(w, &cookie)
img := bytes.NewBufferString("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")
dec := base64.NewDecoder(base64.StdEncoding, img)
w.Header().Set("Content-Type", "image/jpeg")
io.Copy(w, dec)
}
func main() {
http.HandleFunc("/pix/", pix)
http.HandleFunc("/img/", img)
http.HandleFunc("/red/", img)
http.HandleFunc("/", img)
http.ListenAndServe(":8080", nil)
}

func getSize(stream io.Reader) int {
buf := new(bytes.Buffer)
buf.ReadFrom(stream)
return buf.Len()
}

The application does not need to set the content length. If the application does not specify the content length, then net/http server uses chunked encoding for HTTP/1.1 clients. For HTTP/1.0 clients, the net/http server closes the connection after writing the response. There are some edge cases that are not important for this discussion. What is important is that the net/http server can write the response without knowing the content length.

Use the following code to write the response:

func img(w http.ResponseWriter, r *http.Request) {
expiration := time.Now().Add(365 * 24 * time.Hour)
cookie := http.Cookie{Name: "csrftoken",Value:"abcd",Expires:expiration,HttpOnly: false}
http.SetCookie(w, &cookie)
img := bytes.NewBufferString("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")
dec := base64.NewDecoder(base64.StdEncoding, img)
w.Header().Set("Content-Type", "image/jpeg")
io.Copy(w, dec)
}

You can reduce memory allocations by using strings.NewReader instead of bytes.NewBufferString:

img := strings.NewReader("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")

If the application must avoid chunked responses, then the application must set the Content-Length header. Decode the image to a buffer, get the length of that buffer and set the header before writing the body:

func img(w http.ResponseWriter, r *http.Request) {
expiration := time.Now().Add(365 * 24 * time.Hour)
cookie := http.Cookie{Name: "csrftoken",Value:"abcd",Expires:expiration,HttpOnly: false}
http.SetCookie(w, &cookie)
p, err := base64.StdEncoding.DecodeString("iVBORw0KGgoAAAANSUhEUgAAADIAAAAyCAIAAACRXR/mAAADMUlEQVRYw+2YTUgUYRjHZzOJIoNA+rrUyYNIRQgRHaLo4qFDBEGeunSxS9TFU0QEnhIh6IvokrUzO2uamRmbG6XmR/mVaKZpZVbYvvO143zszsxOz+yahNm+785sITEP72F3Z+adH8/zf5+PpagwtxKXj+Vj+Vg+lo/lY+W+WI4KpddKwWIQFUSF97nNLcLGZt75SiOHchEXfskDVmYjlowpiEoei3UT2ljcFJOpOd169C1Z2SuvgsdpB7cgzB16EV/byGM2xDIVPxQujKmBDF/2m2l0vFvmEin7N2v8kiiPiOeGlGHRvP1RdxA9eYtGR7pk2Pf6lI7RCoP2RaWkZWe3fsFc18hvesAHPGEFUc24ltnx3kyiCJwfRMs6dTXLdSIjO9Osal18qzKfE5V9coDxhlU7qS3uOyiaB55JDtkS2TKoLCLaOLPS4b02pQdCHiUfRKf653/d2kjZN6f10jYxI2EnrGk5H+2WsVi6ZZ8fVSmGQKaYyyFuR6ugmUtVrJo2C7HokeGq8447sYpOPBbo3XFzKC95626sZlz905sUM9XLGbXvtKtTOhZrQDApkhNNkiAOPo/viojh2YSZsj1aF2eQ5n2stuomNQjiiGQanrFufdCXP8gu8tbhjridJ6saVPKExXJrwlwfb3pnAg2Ut0tEBZFI8gza81Tik15DCDIoINQ7aQdBo90RMfrdwNaWLFY9opJGkBQrhCA/HXspQ8W1XHkN6vfWFiGH9ouwhdpJUFuy2JX3eg6uyqENpNHZYcUd02jcLMI2WO67UwZVv1G1HLMq3L83KuEbLPdY7IL2L42p0MMQiuzkq/ncwucOi6qPbWkWoPfCUsENpweUnP1EmE4XGhgagT72RyXolkSCHBbTU3By3fgJj8VyJW3CmSHl8oTWMJuYUUizVvtcsuyJ6J4J663CMLevXar/lJgnKNSgbphzKjriTn5i0F8eX9ODXnEzf6JHvjGtv+aNGdWCOEKnJRmpr5oFVQV8WTWglIKHMlPhv5uqQ1xGYfB5fRMPo+n2VmFbi7ChiS9oWBhZvXrI01TNLg7yPxt51v9rxMfysXwsH8vH+g+wfgDUr+5LcyNV4AAAAABJRU5ErkJggg==")
if err != nil {
http.Error(w, "internal error", 500)
return
}
w.Header().Set("Content-Type", "image/jpeg")
w.Header().Set("Content-Length", strconv.Itoa(len(p)))
w.Write(p)
}

If your io.Reader is also io.Seeker (for example when you want to serve a file back to the user) you can do the following and avoid caching the file into memory:

if s, ok := reader.(io.Seeker); ok {
    if size, err := s.Seek(0, io.SeekEnd); err == nil {
        // seek back to the start of file so that it can be served properly
        if _, err = s.Seek(0, io.SeekStart) {
            w.Header().Set("Content-Length", strconv.Itoa(size))
        }
        // error handeling
    }
    // error handeling
}
// continue serving the file