How do I use the handles that I have connected to my mongodb and collections in another function?

I have a database that I've setup on mongo which is seeded with some data I need to query via a url parameter from an endpoint. In order to use the library, I had defined some handles and did the whole setup for the connection of the db in a separate setup() function, but I can't use the handles I require outside of it.

package main

import (
	"context"
	"encoding/json"
	"fmt"
	"log"
	"net/http"
	"time"

	"github.com/gorilla/mux"
	"go.mongodb.org/mongo-driver/mongo"
	"go.mongodb.org/mongo-driver/mongo/options"
	"go.mongodb.org/mongo-driver/mongo/readpref"
)

func setup() {
	clientOptions := options.Client().
		ApplyURI("mongodb+srv://<username>:<password>@cluster0.um5qb.mongodb.net/<db>?retryWrites=true&w=majority")
	ctx, cancel := context.WithTimeout(context.Background(), 10*time.Second)
	defer cancel()
	client, err := mongo.Connect(ctx, clientOptions)
	if err != nil {
		log.Fatal(err)
	}
	err = client.Ping(ctx, readpref.Primary())
	if err != nil {
		log.Fatal(err)
	}
	defer client.Disconnect(ctx)
	// DB := client.Database("cities-nighthack")
	// Cities := DB.Collection("city")

}

// model for user endpoint
type User struct {
	Email string `json:"email"`
}

// fake db to temp store users
var users []User

// checks if json is empty or not
func (u *User) IsEmpty() bool {
	return u.Email == ""
}

type App struct {
	Mongo *mongo.Client
}

func main() {
	setup()
	r := mux.NewRouter()
	r.HandleFunc("/user", createUser).Methods("POST")
	// r.HandleFunc("/suggest?city_name={city}", searchCity).Methods("GET")

	fmt.Println("Server running at port 8080")
	log.Fatal(http.ListenAndServe(":8080", r))

}

func createUser(w http.ResponseWriter, r *http.Request) {
	w.Header().Set("Content-Type", "application/json")
	if r.Body == nil {
		json.NewEncoder(w).Encode("Must send data")
	}

	var user User
	err := json.NewDecoder(r.Body).Decode(&user)
	if err != nil {
		log.Fatal(err)
	}
	if user.IsEmpty() {
		json.NewEncoder(w).Encode("Invalid! Enter user email.")
		return
	}
	users = append(users, user)
	json.NewEncoder(w).Encode(user)

}

func (a *App) searchCity(w http.ResponseWriter, r *http.Request) {

	vars := mux.Vars(r)
	city := vars["city"]
	
}

I figured I'd be able to simply pass the handler like so:

func searchCity(city *mongo.Collection) (w http.ResponseWriter, r *http.Request) {
...
}

However, gmux doesn't allow you to do that since it implicitly passes in http.ResponseWriter and a *http.Request. Therefore, any input can't be in the arguments. I tried declaring them globally but that didn't work and was recommended not to do so. I was told I could try using a closure or a struct to pass it in but I don't quite understand how I'd go about doing that either.

One way to do it is like this, first add a server type

type server struct {
router *mux.Router
cities *mongo.Collection
}

Add a routes wrapper to the server

func (s *server) routes() {
s.router.HandleFunc("/base", s.handleIndex()).Methods("GET")
}

The handler function

func (s *server) handleIndex() http.HandlerFunc {
return func(w http.ResponseWriter, r *http.Request) {
cities := s.cities.Find(...) // something like that
// write your response, etc
}
}

Then in main

func main() {
sr := &server{
router: mux.NewRouter(),
cities: getMongoDBCollection('cities') // implement this one :) should return a *mongo.Collection...
}
sr.routes()
...
}