net: same tcp port listened without error on windows

[chengxuncc]

What version of Go are you using (go version)?

$ go version
go version go1.11.5 windows/amd64

Does this issue reproduce with the latest release?

Yes

What operating system and processor architecture are you using (go env)?

go env Output

$ go env
set GOARCH=amd64
set GOBIN=
set GOCACHE=C:\Users\cheng\AppData\Local\go-build
set GOEXE=.exe
set GOFLAGS=
set GOHOSTARCH=amd64
set GOHOSTOS=windows
set GOOS=windows
set GOPATH=C:\Users\cheng\go
set GOPROXY=
set GORACE=
set GOROOT=D:\Programs\Go
set GOTMPDIR=
set GOTOOLDIR=D:\Programs\Go\pkg\tool\windows_amd64
set GCCGO=gccgo
set CC=gcc
set CXX=g++
set CGO_ENABLED=1
set GOMOD=
set CGO_CFLAGS=-g -O2
set CGO_CPPFLAGS=
set CGO_CXXFLAGS=-g -O2
set CGO_FFLAGS=-g -O2
set CGO_LDFLAGS=-g -O2
set PKG_CONFIG=pkg-config
set GOGCCFLAGS=-m64 -mthreads -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -fdebug-prefix-map=C:\Users\cheng\AppData\Local\Temp\go-build360010358=/tmp/go-build -gno-record-gcc-switches

What did you do?

main.go

package main

import (
	"fmt"
	"log"
	"net"
)

func main()  {
	lister,e:=net.Listen("tcp","0.0.0.0:1080")
	if e!=nil{
		log.Panicln(e)
	}
	fmt.Println("Listen on 0.0.0.0:1080")
	for{
		conn,e:=lister.Accept()
		if e!=nil{
			log.Panicln("Accept Error",e)
		}
		go test_handle(conn)
	}
}
func test_handle(conn net.Conn){
	buf:=make([]byte,1024)
	for{
		n,e:=conn.Read(buf)
		if e!=nil{
			log.Println(e)
			return
		}
		fmt.Println(string(buf[:n]))
	}
}

main.py

import socket
server_address = ('0.0.0.0', 1080)
sock=socket.socket(socket.AF_INET,socket.SOCK_STREAM)
print('starting up on %s port %s' % server_address) 
sock.bind(server_address)
sock.listen(1)
while True:
    connection, client_address = sock.accept()
    try:
        data = connection.recv(1024)
        if data:
            print(client_address,data)
    finally:
        connection.close()

What did you expect to see?

Golang program should raise a port-had-been-used Error.

What did you see instead?

There are three conditions and which program listen on port first:

1. Golang Program vs Golang Program:

Second Golang Program raise a Error

2. Golang Program vs Python Program:

Golang first, Python second -> Both NO Error
Python first, Golang second -> Both NO Error

3. Golang Program vs C# or others Program:

Golang first, C# second -> C# raise a Error
C# first, Golang second -> Both NO Error

[chengxuncc]

#2307

[mikioh]

I think you perhaps might need to be informed that:

• the connection setup function Listen does a bit more than the pair of socket and bind system calls, it is able to return a dual-stack passive-open socket, so you have to use "tcp4" instead of "tcp" if you need an IPv4-only passive-open socket; see https://godoc.org/net#Listen,
• IIRC, unlike Unix variants, Windows has a funny characteristic that the kernel allows the coexistence of three types of sockets on the same transport-layer service port, for example, 127.0.0.1:8080, [::1]:8080 and [::ffff:0.0.0.0]:8080.

Also, the story of someone's adventure on the bind system call might be helpful: https://stackoverflow.com/questions/51071020/golang-net-listen-binds-to-port-thats-already-in-use

Can you please re-visit your test programs and circumstances with the point of view mentioned above?

[chengxuncc]

That's it, I should use "tcp4" instead of "tcp" if I want it raise a error, otherwise, a dual-stack passive-open socket will work. XD