net/http: add Transport.GetProxyConnectHeader

[bradfitz]

net/http.Transport currently has:

https://golang.org/pkg/net/http/#Transport.ProxyConnectHeader

    // ProxyConnectHeader optionally specifies headers to send to
    // proxies during CONNECT requests.
    ProxyConnectHeader Header // Go 1.8

... but it's static for the life of the Transport.

Some authentication modes like NTLM/Kerberos/Negotiate require a dynamic header value, which currently requires making a new Transport for each request going through a proxy. That's heavy, as the Transport is the unit of connection reuse.

Proposal:

    // GetProxyConnectHeader is called to return headers to send to
    // proxuURL during a CONNECT request to the target ip:port. If it returns an error, the Transport
    // RoundTrip fails with that error. It can return (nil, nil) to not add headers.
    // If GetProxyConnectHeader is non-nil, ProxyConnectHeader is ignored.
    GetProxyConnectHeader func(ctx context.Context, proxyURL *url.URL, target string) (Header, error)

    // ProxyConnectHeader optionally specifies headers to send to
    // proxies during CONNECT requests. For dynamic use, see GetProxyConnectHeader.
    ProxyConnectHeader Header // Go 1.8

[bradfitz]

Potential implementation: tailscale@6db6825 (licensed+CLA'd for use by Go)

(Lacking tests for now.)

[rsc]

The Get is a bit non-standard but it does follow our pattern of having GetFoo func() T override Foo T in a struct when we realize too late that Foo T might not be initializable from the start. (For example, Request.GetBody vs Request.Body.)

Does anyone object to this?

[rsc]

Based on the discussion above, this seems like a likely accept.

[networkimprov]

cc @fraenkel @odeke-em for any input...

[rsc]

No change in consensus, so accepted.