Example of using https://github.com/peterbourgon/ctxdata as a middleware

context_middleware.go

package contextmiddleware

import (
	"net/http"
	"net/http/httptest"
	"testing"

	"github.com/peterbourgon/ctxdata/v2"
)

type controller struct {
	Error string
}

func (c *controller) logMiddleware(next http.Handler) http.Handler {
	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
		ctx, d := ctxdata.New(r.Context())
		defer func() {
			if msg, ok := d.Get("error"); ok {
				c.Error = msg
			}
		}()
		next.ServeHTTP(w, r.WithContext(ctx))
	})
}

var handler = http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
	d := ctxdata.From(r.Context())
	d.Set("error", "problem here")
})

func TestMiddleware(t *testing.T) {

	// Create the request and call the HTTP handler
	req, err := http.NewRequest("POST", "ABC", nil)
	if err != nil {
		t.Fatalf("err: %v", err)
	}

	resp := httptest.NewRecorder()

	c := &controller{}
	h := c.logMiddleware(handler)
	h.ServeHTTP(resp, req)

	if c.Error == "" {
		t.Error("Context does not hold error")
	}

	t.Log("error", c.Error)
}

Make sure to use .From() not .New(): https://play.golang.org/p/NyeM3HU_jf5

Using zap logger: https://play.golang.org/p/qreMsvqA-S0