Testing golang middleware that modifies the request

Question

I have some middleware that adds a context with a request id to a request.

func AddContextWithRequestID(next http.Handler) http.Handler {
return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
    var ctx context.Context
    ctx = NewContextWithRequestID(ctx, r)
    next.ServeHTTP(w, r.WithContext(ctx))
})}

How do I write a test for this ?

Answer 1

To test that, you need to run that handler passing in a request, and using a custom next handler that checks that the request was indeed modified.

You can create that handler as follows:

(I am assuming your NewContextWithRequestID adds a "reqId" key to the request with a "1234" value, you should of course modify the assertions as needed)

// create a handler to use as "next" which will verify the request
nextHandler := http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
    val := r.Context().Value("reqId")
    if val == nil {
        t.Error("reqId not present")
    }
    valStr, ok := val.(string)
    if !ok {
        t.Error("not string")
    }
    if valStr != "1234" {
        t.Error("wrong reqId")
    }
})

You can then use that handler as your next one:

// create the handler to test, using our custom "next" handler
handlerToTest := AddContextWithRequestID(nextHandler)

And then invoke that handler:

// create a mock request to use
req := httptest.NewRequest("GET", "http://testing", nil)
// call the handler using a mock response recorder (we'll not use that anyway)
handlerToTest.ServeHTTP(httptest.NewRecorder(), req)

Putting everything together as a working test, that'd be the code below.

Note: I fixed a small bug in your original "AddContextWithRequestID", as the ctx value started with a nil value when you just declared it with no initialization.

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

func NewContextWithRequestID(ctx context.Context, r *http.Request) context.Context {
    return context.WithValue(ctx, "reqId", "1234")
}

func AddContextWithRequestID(next http.Handler) http.Handler {
    return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
        var ctx = context.Background()
        ctx = NewContextWithRequestID(ctx, r)
        next.ServeHTTP(w, r.WithContext(ctx))
    })
}

func TestIt(t *testing.T) {

    // create a handler to use as "next" which will verify the request
    nextHandler := http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
        val := r.Context().Value("reqId")
        if val == nil {
            t.Error("reqId not present")
        }
        valStr, ok := val.(string)
        if !ok {
            t.Error("not string")
        }
        if valStr != "1234" {
            t.Error("wrong reqId")
        }
    })

    // create the handler to test, using our custom "next" handler
    handlerToTest := AddContextWithRequestID(nextHandler)

    // create a mock request to use
    req := httptest.NewRequest("GET", "http://testing", nil)

    // call the handler using a mock response recorder (we'll not use that anyway)
    handlerToTest.ServeHTTP(httptest.NewRecorder(), req)
}