Why do the client and server cause a data race?

I'm trying to create a TCP relay using channels as a basic PubSub. My goal is to relay one TCP stream to many clients (one to many). I haven't been able to fix a data race between the client and server connections. I would be grateful for any insight as to why the data race occurs between the client and server connections?

I think the pubsub part is OK. It was adapted from the following blog:

https://eli.thegreenplace.net/2020/pubsub-using-channels-in-go/

According to the data race warning, the race occurs in the main function code block below. I put some comments on the lines that cause the data race. I thought it would be possible to run a server and client concurrently, am I mistaken?

package main
import (
"flag"
"net"
"os"
"sync"
)
var (
laddr = flag.String("l", "", "listen address (:port)")
raddr = flag.String("r", "", "remote address (host:port)")
)
type Sub struct {
topic string
id    int64
}
type Pubsub struct {
mu      sync.RWMutex
subs    map[Sub]chan []byte
closed  bool
counter int64
}
func NewPubsub() *Pubsub {
ps := &Pubsub{}
ps.subs = make(map[Sub]chan []byte)
ps.closed = false
return ps
}
func (ps *Pubsub) Close() {
ps.mu.Lock()
defer ps.mu.Unlock()
if !ps.closed {
ps.closed = true
for _, sub := range ps.subs {
close(sub)
}
}
}
func (ps *Pubsub) Subscribe(topic string) (<-chan []byte, Sub) {
ps.mu.Lock()
defer ps.mu.Unlock()
// initialze the subscription
sub := Sub{topic: topic, id: ps.counter}
// Add the subscription to the map
ch := make(chan []byte, 1)
ps.subs[sub] = ch
// Increment the counter
ps.counter++
return ch, sub
}
func (ps *Pubsub) Unsubscribe(s Sub) {
ps.mu.Lock()
defer ps.mu.Unlock()
delete(ps.subs, s)
}
func (ps *Pubsub) Publish(topic string, msg []byte) {
ps.mu.RLock()
defer ps.mu.RUnlock()
for sub, ch := range ps.subs {
if sub.topic == topic {
ch <- msg
}
}
}
func main() {
flag.Parse()
if *laddr == "" || *raddr == "" {
flag.PrintDefaults()
os.Exit(1)
}
ps := NewPubsub()
publisher := func(topic string) {
remote, err := net.Dial("tcp", *raddr)
if err != nil {
return
}
buf := make([]byte, 2048)
for {
n, _ := remote.Read(buf)  // *** RACE HERE ***
ps.Publish(topic, buf[:n])
}
}
go publisher("relay")
subscriber := func(conn net.Conn, ch <-chan []byte) {
for i := range ch {
conn.Write([]byte(i))  // *** RACE HERE ***
}
}
ln, err := net.Listen("tcp", *laddr)
if err != nil {
return
}
for {
conn, err := ln.Accept()
if err != nil {
continue
}
ch, _ := ps.Subscribe("relay")
go subscriber(conn, ch)
}
}

The data race output when using "go run -race pubsub.go" is shown below.

The data race warning does not occur until the first client connects to the listening server's port.

I have not seen any other types of data races while this program runs. But when I relay binary data, bytes are infrequently either corrupted or missing, suggesting that their may be other issues with my naïve implementation.

==================
WARNING: DATA RACE
Write at 0x00c0000f8000 by goroutine 7:
internal/race.WriteRange()
/usr/local/go/src/internal/race/race.go:49 +0xaa
syscall.Read()
/usr/local/go/src/syscall/syscall_unix.go:190 +0x89
internal/poll.ignoringEINTRIO()
/usr/local/go/src/internal/poll/fd_unix.go:581 +0x1c8
internal/poll.(*FD).Read()
/usr/local/go/src/internal/poll/fd_unix.go:162 +0x17c
net.(*netFD).Read()
/usr/local/go/src/net/fd_posix.go:55 +0x68
net.(*conn).Read()
/usr/local/go/src/net/net.go:183 +0xeb
net.(*TCPConn).Read()
<autogenerated>:1 +0x69
main.main.func1()
/pubsub/pubsub.go:101 +0x154
Previous read at 0x00c0000f8000 by goroutine 9:
internal/race.ReadRange()
/usr/local/go/src/internal/race/race.go:45 +0xb0
syscall.Write()
/usr/local/go/src/syscall/syscall_unix.go:215 +0x94
internal/poll.ignoringEINTRIO()
/usr/local/go/src/internal/poll/fd_unix.go:581 +0x16e
internal/poll.(*FD).Write()
/usr/local/go/src/internal/poll/fd_unix.go:274 +0x294
net.(*netFD).Write()
/usr/local/go/src/net/fd_posix.go:73 +0x68
net.(*conn).Write()
/usr/local/go/src/net/net.go:195 +0xeb
net.(*TCPConn).Write()
<autogenerated>:1 +0x69
main.main.func2()
/pubsub/pubsub.go:110 +0x84
Goroutine 7 (running) created at:
main.main()
/pubsub/pubsub.go:106 +0x288
Goroutine 9 (running) created at:
main.main()
/pubsub/pubsub.go:125 +0x38f
==================

Quick fix:

    // buf := make([]byte, 2048) // <- move this ...
for {
buf := make([]byte, 2048) // <- ... to here
n, _ := remote.Read(buf)
ps.Publish(topic, buf[:n])
}

Why is this broken? Since a single (constant) buf is passed via a channel to multiple subscribers (readers) - when the next for iteration occurs - those readers will get corrupt racy data.

Creating a unique buffer per iteration will ensure no new writes will corrupt old messages sent & still being processed by subscribers.