package main

import (
	"context"
	"fmt"
	"net/http"
)

func main() {
	http.Handle("/foo/", func(h http.Handler) http.Handler {
		return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
			h.ServeHTTP(w, r)
			// log user id
			fmt.Printf("UserID -> %#v\n", r.Context().Value("UserID"))
		})
	}(

		http.StripPrefix("/foo/", http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
			// set user id
			*r = *r.WithContext(context.WithValue(r.Context(), "UserID", "123"))
		})),

		//http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
		//	// set user id
		//	*r = *r.WithContext(context.WithValue(r.Context(), "UserID", "123"))
		//}),
	))
	go func() {
		if err := http.ListenAndServe(":8888", nil); err != nil {
			panic(err)
		}
	}()
	resp, err := http.Get("http://localhost:8888/foo/")
	if err != nil {
		panic(err)
	}
	defer resp.Body.Close()
}

UserID -> "123"

UserID -> <nil>

go version devel +d39649087b Thu Aug 31 03:30:43 2017 +0000 linux/amd64
GOARCH="amd64"
GOBIN=""
GOEXE=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GOOS="linux"
GOPATH="/home/reus/go"
GORACE=""
GOROOT="/home/reus/gotip"
GOTOOLDIR="/home/reus/gotip/pkg/tool/linux_amd64"
GCCGO="gccgo"
CC="gcc"
GOGCCFLAGS="-fPIC -m64 -pthread -fmessage-length=0 -fdebug-prefix-map=/tmp/go-build879880846=/tmp/go-build -gno-record-gcc-switches"
CXX="g++"
CGO_ENABLED="1"
CGO_CFLAGS="-g -O2"
CGO_CPPFLAGS=""
CGO_CXXFLAGS="-g -O2"
CGO_FFLAGS="-g -O2"
CGO_LDFLAGS="-g -O2"
PKG_CONFIG="pkg-config"
GOROOT/bin/go version: go version devel +d39649087b Thu Aug 31 03:30:43 2017 +0000 linux/amd64
GOROOT/bin/go tool compile -V: compile version devel +d39649087b Thu Aug 31 03:30:43 2017 +0000
uname -sr: Linux 4.12.8-2-ARCH
LSB Version:	1.4
Distributor ID:	Arch
Description:	Arch Linux
Release:	rolling
Codename:	n/a
/usr/lib/libc.so.6: GNU C Library (GNU libc) stable release version 2.25, by Roland McGrath et al.