Share context between handleFunc and modifyResponse in Go reverse proxy

Question

I'm trying to learn Go and figured a nice little project would be an A/B testing proxy to put in front of a web server. Little did I know Go essentially offers a reverse proxy out of the box, so the setup was easy. I've got it to the point where I'm proxying traffic, but here's the thing, I have trouble implementing the actual functionality because wherever I have access to the response, I don't have access to assigned A/B test variations:

• In the handleFunc I'm assigning variations of each test to the request, so the upstream server can also be aware of it and use if for implementations in it's backend.
• I'm setting a cookie with all tests and variations, both on the request that's being proxied to the upstream and on the response that's returned to the client.
• Tests that consist of a find/replace pair will do mutations on the response body after the response comes back from the upstream server.
• I'm trying to use the modifyResponse function of httputil.ReverseProxy to do the response mutation.

The problem is that I can't figure out how to share the assigned variations between the handleFunc and modifyResponse, without changing the upstream server. I'd like to be able to share this context (basically a map[string]string somehow.

Code sample:

Here's a distilled version of my code, where my question basically is, how can modifyRequest know about random assignments that happened in handleFunc?

package main

import (
    config2 "ab-proxy/config"
    "bytes"
    "fmt"
    "io/ioutil"
    "net/http"
    "net/http/httputil"
    "net/url"
    "strconv"
    "strings"
)

var config config2.ProxyConfig
var reverseProxy *httputil.ReverseProxy
var tests config2.Tests

func overwriteCookie(req *http.Request, cookie *http.Cookie) {
    // omitted for brevity, will replace a cookie header, instead of adding a second value
}

func parseRequestCookiesToAssignedTests(req *http.Request) map[string]string {
    // omitted for brevity, builds a map where the key is the identifier of the test, the value the assigned variant
}

func renderCookieForAssignedTests(assignedTests map[string]string) string {
    // omitted for brevity, builds a cookie string
}

func main () {
    var err error

    if  config, err = config2.LoadConfig(); err != nil {
        fmt.Println(err)

        return
    }

    if tests, err = config2.LoadTests(); err != nil {
        fmt.Println(err)

        return
    }

    upstreamUrl, _ := url.Parse("0.0.0.0:80")

    reverseProxy = httputil.NewSingleHostReverseProxy(upstreamUrl)
    reverseProxy.ModifyResponse = modifyResponse

    http.HandleFunc("/", handleRequest)

    if err := http.ListenAndServe("0.0.0.0:80", nil); err != nil {
        fmt.Println("Could not start proxy")
    }
}

func handleRequest(res http.ResponseWriter, req *http.Request) {
    assigned := parseRequestCookiesToAssignedTests(req)

    newCookies := make(map[string]string)

    for _, test := range tests.Entries {
        val, ok := assigned[test.Identifier]

        if ok {
            newCookies[test.Identifier] = val
        } else {
            newCookies[test.Identifier] = "not-assigned-yet" // this will be replaced by random variation assignment
        }
    }

    testCookie := http.Cookie{Name: config.Cookie.Name, Value: renderCookieForAssignedTests(newCookies)}

    // Add cookie to request to be sent to upstream
    overwriteCookie(req, &testCookie)

    // Add cookie to response to be returned to client
    http.SetCookie(res, &testCookie)

    reverseProxy.ServeHTTP(res, req)
}

func modifyResponse (response *http.Response) error {
    body, err := ioutil.ReadAll(response.Body)

    if err != nil {
        return  err
    }

    err = response.Body.Close()

    if err != nil {
        return err
    }

    response.Body = ioutil.NopCloser(bytes.NewReader(body))
    response.ContentLength = int64(len(body))
    response.Header.Set("Content-Length", strconv.Itoa(len(body)))

    return nil
}

Answer 1

Use a standard context.Context. This is accessible in your handler via the *http.Request. And the request is also accessible via the *http.Response argument to modifyResponse.

In your handler:

ctx := req.Context()
// Set values, deadlines, etc.
req = req.WithContext(ctx)
reverseProxy.ServeHTTP(res, req)

Then in modifyResponse:

ctx := response.Request.Context()
// fetch values, check for cancellation, etc