Golang fatal error: concurrent map read and map write

I'm writing minecraft server in Go, when server is being stressed by 2000+ connections I get this crash:

<blockquote>
fatal error: concurrent map read and map write/root/work/src/github.com/user/imoobler/limbo.go:78 +0x351
created by main.main /root/work/src/github.com/user/imoobler/limbo.go:33 +0x368
</blockquote>

My code:

package main
import (
"log"
"net"
"bufio"
"time"
"math/rand"
"fmt"
)
var (
connCounter = 0
)
func main() {
InitConfig()
InitPackets()
port := int(config["port"].(float64))
ln, err := net.Listen("tcp", fmt.Sprintf(":%d", port))
if err != nil {
log.Fatal(err)
}
log.Println("Server launched on port", port)
go KeepAlive()
for {
conn, err := ln.Accept()
if err != nil {
log.Print(err)
} else {
connCounter+=1
go HandleConnection(conn, connCounter)
}
}
}
func KeepAlive() {
r := rand.New(rand.NewSource(15768735131534))
keepalive := &PacketPlayKeepAlive{
id: 0,
}
for {
for _, player := range players {
if player.state == PLAY {
id := int(r.Uint32())
keepalive.id = id
player.keepalive = id
player.WritePacket(keepalive)
}
}
time.Sleep(20000000000)
}
}
func HandleConnection(conn net.Conn, id int) {
log.Printf("%s connected.", conn.RemoteAddr().String())
player := &Player {
id: id,
conn: conn,
state: HANDSHAKING,
protocol: V1_10,
io: &ConnReadWrite{
rdr: bufio.NewReader(conn),
wtr: bufio.NewWriter(conn),
},
inaddr: InAddr{
"",
0,
},
name: "",
uuid: "d979912c-bb24-4f23-a6ac-c32985a1e5d3",
keepalive: 0,
}
for {
packet, err := player.ReadPacket()
if err != nil {
break
}
CallEvent("packetReceived", packet)
}
player.unregister()
conn.Close()
log.Printf("%s disconnected.", conn.RemoteAddr().String())
}

For now server is only "limbo".

Generally speaking (without having access to the code where the error occurs) you have a few options. Here are two of them:

sync.RWMutex

Control access to the map with sync.RWMutex{}. Use this option if you have single reads and writes, not loops over the map. See RWMutex

Here a sample with access control to someMap via someMapMutex:

var (
someMap      = map[string]string{}
someMapMutex = sync.RWMutex{}
)
go func() {
someMapMutex.Lock()
someMap["key"] = "value"
someMapMutex.Unlock()
}()
someMapMutex.RLock()
v, ok := someMap["key"]
someMapMutex.RUnlock()
if !ok {
fmt.Println("key missing")
return
}
fmt.Println(v)

syncmap.Map

Use a syncmap.Map{} instead of a normal map. This map is already taking care of race issues but may be slower depending on your usage. syncmap.Map{}s main advantage lies with for loops. See syncmap

var (
someMap = syncmap.Map{}
)
go func() {
someMap.Store("key", "value")
}()
v, ok := someMap.Load("key")
if !ok {
fmt.Println("key missing")
return
}
fmt.Println(v)
// with syncmap, looping over all keys is simple without locking the whole map for the entire loop
someMap.Range(func(key, value interface{}) bool {
// cast value to correct format
val, ok := value.(string)
if !ok {
// this will break iteration
return false
}
// do something with key/value
fmt.Println(key, val)
// this will continue iterating
return true
})

General Advice

You should test your server with -race option and then eliminate all the race conditions it throws. That way you can easier eliminate such errors before they occur.

go run -race server.go

See golang race detector