CreateSession：没有可访问的服务器-mgo

I'm trying to connect to MongoDB Atlas free cluster using mgo.

Golang Code -

package main

import (
     "fmt"
     "gopkg.in/mgo.v2"
     "time"
     "log"
)

const (
    AuthDatabase = "mydatabase"
    AuthUserName = "databaseadmin"
    AuthPassword = "databasepassword"
    ReplicaSetName = "myproject-shard-0"
)

func main(){

MongoDBHosts := []string{
    "myproject-shard-00-00-w4vds.mongodb.net:27017",
    "myproject-shard-00-01-w4vds.mongodb.net:27017",
    "myproject-shard-00-02-w4vds.mongodb.net:27017",
}

mongoDBDialInfo := &mgo.DialInfo{
    Addrs:    MongoDBHosts,
    Timeout:  60 * time.Second,
    Database: AuthDatabase,
    Username: AuthUserName,
    Password: AuthPassword,
    ReplicaSetName: ReplicaSetName,
}

mongoSession, err := mgo.DialWithInfo(mongoDBDialInfo)
if err != nil {
    log.Fatalf("CreateSession: %s
", err)
}

defer  mongoSession.Close()
fmt.Printf("Connected to replica set %v!
", mongoSession.LiveServers())
}

Error Message -

CreateSession: no reachable servers

Environment

I'm using mongodb free cluster with Google App Engine GO SDK

To connect to MongoDB Atlas, you need SSL

Prerequisites

TLS/SSL

    Clients must have support for TLS/SSL to connect to an Atlas cluster.

    Clients must have support for the SNI TLS extension to connect to an Atlas M0 Free Tier cluster.


Whitelist

    To access a cluster, you must connect from an IP address on the Atlas group’s IP whitelist. If you need to add an IP address to the whitelist, you can do so in the Connect dialog. You can also add the IP address from the Security tab.

Thus I had to change the following piece of code

mongoDBDialInfo := &mgo.DialInfo{
    Addrs:    MongoDBHosts,
    Timeout:  60 * time.Second,
    Database: AuthDatabase,
    Username: AuthUserName,
    Password: AuthPassword,
    ReplicaSetName: ReplicaSetName,
}

mongoDBDialInfo.DialServer = func(addr *mgo.ServerAddr) (net.Conn, error) {
        conn, err := tls.Dial("tcp", addr.String(), tlsConfig)
        return conn, err
    }

I had to import the following too

"crypto/tls" 
"net"