How to pass arguments to router handlers in Golang using Gin web framework?

I'm using Gin, https://gin-gonic.github.io/gin/, to build a simple RESTful JSON API with Golang.

The routes are setup with something like this:

func testRouteHandler(c *gin.Context) {
    // do smth
}

func main() {
    router := gin.Default()
    router.GET("/test", testRouteHandler)
    router.Run(":8080")
}

My question is how can I pass down an argument to the testRouteHandler function? For example a common database connection could be something that one wants to reuse among routes.

Is the best way to have this in a global variable? Or is there some way in Go to pass along an extra variable to the testRouteHandler function? Are there optional arguments for functions in Go?

PS. I'm just getting started in learning Go, so could be something obvious that I'm missing :)

Using the link i posted on comments, I have created a simple example.

package main

import (
    "log"

    "github.com/gin-gonic/gin"
    "github.com/jinzhu/gorm"
    _ "github.com/mattn/go-sqlite3"
)

// ApiMiddleware will add the db connection to the context
func ApiMiddleware(db gorm.DB) gin.HandlerFunc {
    return func(c *gin.Context) {
        c.Set("databaseConn", db)
        c.Next()
    }
}

func main() {
    r := gin.New()

    // In this example, I'll open the db connection here...
    // In your code you would probably do it somewhere else
    db, err := gorm.Open("sqlite3", "./example.db")
    if err != nil {
        log.Fatal(err)
    }

    r.Use(ApiMiddleware(db))

    r.GET("/api", func(c *gin.Context) {
        // Don't forget type assertion when getting the connection from context.
        dbConn, ok := c.MustGet("databaseConn").(gorm.DB)
        if !ok {
            // handle error here...
        }

        // do your thing here...
    })

    r.Run(":8080")
}

This is just a simple POC. But i believe it's a start. Hope it helps.