How Can I Reuse an HTTP Request Body in Go Middleware Handlers?

Reusing HTTP Request Body in Go Middleware Handlers

In Go's net/http package, middleware handlers provide a convenient way to process and modify incoming HTTP requests before handling by the actual application code. However, a common challenge arises when a middleware handler needs to reuse the request body that has already been read by a preceding handler.

Consider the following code snippet:

func Registration(w http.ResponseWriter, r *http.Request) {
    b, err := ioutil.ReadAll(r.Body) // if you delete this line, the user will be created
    // ...other code
    // if all good then create new user
    user.Create(w, r)
}

...

func Create(w http.ResponseWriter, r *http.Request) {
    b, err := ioutil.ReadAll(r.Body)
    // ...other code
    // ... there I get the problem with parse JSON from &b
}

In this scenario, the Registration handler reads the request body into the variable b and passes the request r to the user.Create handler, which attempts to read the body again. However, this results in an error because the body has already been consumed by the Registration handler.

The solution to this issue is straightforward: restore the request body in the outer handler after it has been read. This can be achieved using the bytes.NewReader() and io.NopCloser functions:

func Registration(w http.ResponseWriter, r *http.Request) {
    b, err := io.ReadAll(r.Body)
    // ...other code
    r.Body = io.NopCloser(bytes.NewReader(b))
    user.Create(w, r)
}

The bytes.NewReader() function creates an io.Reader from a byte slice, and io.NopCloser converts an io.Reader to the required io.ReadCloser type for r.Body. By restoring the body, subsequent handlers can access the original request data.

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