httprouter.ParamsWithContext(r.Context()) is returning []

[samirprakash]

Using alice with httprouter to chain middleware and wrapping params before passing it to the middleware function.

func (s *Server) wrap(next http.Handler) httprouter.Handle {
	return func(w http.ResponseWriter, r *http.Request, ps httprouter.Params) {
		ctx := context.WithValue(r.Context(), params, ps)
		next.ServeHTTP(w, r.WithContext(ctx))
	}
}

/ routes configure server API endpoints
func (s *Server) routes() http.Handler {
	r := httprouter.New()
	secure := alice.New(s.authenticateWithIDMS)

	// unsecured end points
	r.HandlerFunc(http.MethodGet, "/api/v2/projects/:id", s.getProject)

	// secured end points
	r.GET("/api/v2/projects/:id", s.wrap(secure.ThenFunc(s.getProject)))

	return s.enableCORS(r)
}

func (s *Server) enableCORS(next http.Handler) http.Handler {
	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
		w.Header().Set("Access-Control-Allow-Origin", "*")
		w.Header().Set("Access-Control-Allow-Methods", "GET,POST,DELETE,PUT,PATCH")
		w.Header().Set("Access-Control-Allow-Headers", "Content-Type,Authorization")

		next.ServeHTTP(w, r)
	})
}

// getProject returns all the projects as an API response
func (s *Server) getProject(w http.ResponseWriter, r *http.Request) {
	params := httprouter.ParamsFromContext(r.Context())
	log.Println(params)

	id, err := strconv.Atoi(params.ByName("id"))
	if err != nil {
		s.logger.Println(errors.New("invalid movie id : "), err)
		utils.ErrorJSON(w, err)
		return
	}

	utils.WriteJSON(w, http.StatusOK, id, "data")
}

Inside getProjects(), r.Context().Value("params") returns [{id 1}] but httprouter.ParamsFromContext(r.Context()) returns [] when I use the s.wrap() call, but if I do not use it then the params are passed properly.

Any pointers?

[cerpmath123]

What exactly is params in your wrap() function? Please provide a minimal working example to replicate the issue next time.

The issue you are facing is due to the wrong key you are passing as params in the wrap() function. httprouter.ParamsFromContext() expects to find the parameters with the key httprouter.ParamsKey. Here is a working version of your wrap() function:

func (s *Server) wrap(next http.Handler) httprouter.Handle {
	return func(w http.ResponseWriter, r *http.Request, ps httprouter.Params) {
		ctx := context.WithValue(r.Context(), httprouter.ParamsKey, ps)
		next.ServeHTTP(w, r.WithContext(ctx))
	}
}

and here is a minimal working example

package main

import (
	"context"
	"errors"
	"fmt"
	"github.com/julienschmidt/httprouter"
	"github.com/justinas/alice"
	"log"
	"net/http"
	"strconv"
)

type Server struct {
}

func (s *Server) routes() http.Handler {
	r := httprouter.New()
	secure := alice.New()

	// unsecured end points
	r.HandlerFunc(http.MethodGet, "/api/v2/projects/:id", s.getProject)

	// secured end points
	r.GET("/api/v2/secure/projects/:id", s.wrap(secure.ThenFunc(s.getProject)))

	return alice.New().Then(r)
}

func (s *Server) getProject(w http.ResponseWriter, r *http.Request) {
	params := httprouter.ParamsFromContext(r.Context())
	fmt.Printf("Parameters: %s\n", params)

	id, err := strconv.Atoi(params.ByName("id"))
	if err != nil {
		log.Println(errors.New("invalid movie id : "), err)
		return
	}

	fmt.Printf("Id: %d\n", id)
}

func (s *Server) wrap(next http.Handler) httprouter.Handle {
	return func(w http.ResponseWriter, r *http.Request, ps httprouter.Params) {
		ctx := context.WithValue(r.Context(), httprouter.ParamsKey, ps)
		next.ServeHTTP(w, r.WithContext(ctx))
	}
}

func main() {
	s := Server{}
	srv := &http.Server{
		Addr:    "localhost:8080",
		Handler: s.routes(),
	}

	err := srv.ListenAndServe()
	log.Fatal(err)
}