Testing the values added in context in a request handler

I have a handler function that adds some key/values in the request context and wan to unittest it. My test is like this:

for _, tt := range tests {
	t.Run(tt.name, func(t *testing.T) {
		body := strings.NewReader(tt.payload)

		ctx := context.Background()
		ctx = context.WithValue(ctx, http.ContextUserKey, http.Account{ID: accID.String()})
		req, _ := nethttp.NewRequest("POST", "/api/groups/", body)
		req = req.WithContext(ctx)
		req.Header.Add("Content-Type", tt.contentType)

		rr := httptest.NewRecorder()

		router := mux.NewRouter()
		router.HandleFunc("/api/groups/", s.HandlePostGroups(kc, es, mySQL)).Methods("POST")
		router.ServeHTTP(rr, req)

		if rr.Code != tt.status {
			t.Errorf("wrong status code: got %v want %v", rr.Code, tt.status)
		}
		auditFields := req.Context().Value(http.AuditFields).(logrus.Fields)
        // Audit fields is what I want to assert. and are added in the request handler function
    })
}
 

Inside the handler function I update the context like this:

func (s *IDMServer) HandlePostGroups(kc keycloak.API, es auth.KeyCreator, mySQL store.IDMRelationalStore) http.HandlerFunc {
    return func(w http.ResponseWriter, r *http.Request) {
        ctx := r.Context()
        // Many things happening here but somewhere in code I do this
        if if err != nil {
			s.Log.WithField("error", err).Error("Unable to create key")
			auditFields["event.outcome"] = "failure"
			r = r.WithContext(context.WithValue(ctx, server.AuditFields, auditFields))
            return
        }
    }

}

So I set up a context to describe an event. But in my test I get a

panic: interface conversion: interface {} is nil, not logrus.Fields [recovered]
	panic: interface conversion: interface {} is nil, not logrus.Fields

because the key does not exist and the interface returned is nil. Why isn't the context propagating back to the test function. And how can you test your context in a request?

r = r.WithContext(...) changes r which is a pointer. After that line r points to a new request. The incoming request to ServeHTTP is unmodified by this statement.

This answers your question why it's not propagated upstream.

(I doubt propagating context changes upstream is something you should try. Just redesign your test.)