proposal: net/http: set default headers in http.Client

[hebelsan]

In my application I use a struct with an http.client
At all points where a request is made with this client, I would like to set my own custom user agent.
But here is the problem, I don't see a nice way to set this user agent as default in my http.client.

My first idea is to manipulate the transport and the associated http.RoundTripper:

type AddHeaderTransport struct{
    T http.RoundTripper
}

func (adt *AddHeaderTransport) RoundTrip(req *http.Request) (*http.Response,error) {
    req.Header.Add("User-Agent", "go")
    return adt.T.RoundTrip(req)
}

However, The documentation for http.RoundTripper says: "RoundTrip should not modify the request, except for consuming and closing the Request's Body."

Alternatively, I can only see the possibility of manually setting the header in each request object created, but this can easily be forgotten and thus lead to inconsistencies.

It would be cool if you could specify default headers when creating the http.client, which would then be set automatically for each request.

[seankhliao]

see also #59266

[seankhliao]

also #13837 which was folded into #23707

[Jorropo]

I agree with #59266 on this subject, I put that on the RoundTripper implementation. Because there is no equivalent http.Client interface.
I don't think there is more value doing that in the client.