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

I would avoid stuffing ‘application scoped’ dependencies (e.g. a DB connection pool) into a request context. Your two ‘easiest’ options are:

1. Make it a global. This is OK for smaller projects, and *sql.DB is thread-safe.
2. Pass it explicitly in a closure so that the return type satisfies gin.HandlerFunc

e.g.

// SomeHandler returns a `func(*gin.Context)` to satisfy Gin's router methods
// db could turn into an 'Env' struct that encapsulates all of your
// app dependencies - e.g. DB, logger, env vars, etc.
func SomeHandler(db *sql.DB) gin.HandlerFunc {
    fn := func(c *gin.Context) {
        // Your handler code goes in here - e.g.
        rows, err := db.Query(...)

        c.String(200, results)
    }

    return gin.HandlerFunc(fn)
}

func main() {
    db, err := sql.Open(...)
    // handle the error

    router := gin.Default()
    router.GET("/test", SomeHandler(db))
    router.Run(":8080")
}