Table of Contents

Overview

http.Client struct is used to make HTTP requests in golang.

https://golang.org/src/net/http/client.go

http.Client lets you specify a policy of how redirects can be handled. Below is the structure of http.Client struct

type Client struct {
	Transport RoundTripper
	CheckRedirect func(req *Request, via []*Request) error
	Jar CookieJar
	Timeout time.Duration
}

The CheckRedirect field is a function actually which lets you specify a way of how redirects can be handled. This function is called before following any future redirects. So basically whenever the server responds with a redirect the client will first call the CheckRedirect function to check if redirect should be followed or not. Below is the signature of the CheckRedirect function

func(req *Request, via []*Request) error

The first parameter req is the upcoming request

The second parameter via is the requests already made. That is why it is a slice. The first element is the oldest request.

There are two cases to consider

CheckDirect is nil

You don’t specify the CheckRedirect field during the initialization of http.Client struct. In this case, default behavior kicks in, and the HTTP client will follow 10 redirects, and then it will return an error. Below is the defaultCheckRedirect function which quits after 10 retries.

func defaultCheckRedirect(req *Request, via []*Request) error {
	if len(via) >= 10 {
		return errors.New("stopped after 10 redirects")
	}
	return nil
}

CheckDirect is not-nil

You specify your own CheckRedirect function during the initialization of the http.Client struct. When you specify this function, then this function will be called before following any future redirect. When you write your own CheckRedirect function you need to know the below things in mind
When the CheckRedirect function returns an error, then the HTTP client will return two things

The previous response along with body closed

The actual error (wrapped in a url.Error)

There is also a special case when CheckRedirect function returns ErrUseLastResponse error. In this case, the client will return

Most recent response along with body unclosed

Non-nil error

So basically when you don’t want HTTP Client to follow the redirect, then please specify your own CheckRedirect function to control the behavior the way you want. For example below is one sample CheckRedirect function that will inform HTTP client to not follow any redirect and also not return any error, just return the last response along with body unclosed.

func MyCheckRedirect(req *Request, via []*Request) error {
	return http.ErrUseLastResponse
}

Here is the definition of ErrUseLastResponse in the http package

var ErrUseLastResponse = errors.New("net/http: use last response")

Create http.Client as below specifying the CheckRedirect function

client := &http.Client{
    CheckRedirect: func(req *Request, via []*Request) error {
	return http.ErrUseLastResponse
    },
}

Here is another sample CheckRedirect function which will only follow redirect two times and then return the error

func MyCheckRedirect(req *Request, via []*Request) error {
	if len(via) >= 2 {
		return errors.New("stopped after 10 redirects")
	}
	return nil
}

redirect