How to share mysql connection between http goroutines?

The database/sql package manages the connection pooling automatically for you.

sql.Open(..) returns a handle which represents a connection pool, not a single connection. The database/sql package automatically opens a new connection if all connections in the pool are busy.

Applied to your code this means, that you just need to share the db-handle and use it in the HTTP handlers:

package main

import (
    "database/sql"
    "fmt"
    "github.com/gorilla/mux"
    _ "github.com/go-sql-driver/mysql"
    "log"
    "net/http"
)

var db *sql.DB // global variable to share it between main and the HTTP handler

func main() {
    fmt.Println("starting up")

    var err error
    db, err = sql.Open("mysql", "root@unix(/tmp/mysql.sock)/mydb") // this does not really open a new connection
    if err != nil {
        log.Fatalf("Error on initializing database connection: %s", err.Error())
    }

    db.SetMaxIdleConns(100)

    err = db.Ping() // This DOES open a connection if necessary. This makes sure the database is accessible
    if err != nil {
        log.Fatalf("Error on opening database connection: %s", err.Error())
    }

    r := mux.NewRouter()
    r.HandleFunc("https://stackoverflow.com/", HomeHandler)

    http.Handle("https://stackoverflow.com/", r)
    http.ListenAndServe(":8080", nil)
}

func HomeHandler(w http.ResponseWriter, r *http.Request) {
    var msg string
    err := db.QueryRow("SELECT msg FROM hello WHERE page=?", "home").Scan(&msg)
    if err != nil {
        fmt.Fprintf(w, "Database Error!")
    } else {
        fmt.Fprintf(w, msg)
    }
}