net: conn.WriteToUDP([]byte("not ok"), addr)

[gertcuykens]

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

$ go version
go version devel +ab00f89c27 Mon Oct 7 20:58:25 2019 +0000 darwin/amd64

Does this issue reproduce with the latest release?

Don't know. Using master see go version

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

OSX Mojave 10.14.6

go env Output

$ go env
O111MODULE=""
GOARCH="amd64"
GOBIN="/Users/gert/bin"
GOCACHE="/Users/gert/Library/Caches/go-build"
GOENV="/Users/gert/Library/Application Support/go/env"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="darwin"
GONOPROXY=""
GONOSUMDB=""
GOOS="darwin"
GOPATH="/Users/gert/go"
GOPRIVATE=""
GOPROXY="https://proxy.golang.org,direct"
GOROOT="/usr/local/go"
GOSUMDB="sum.golang.org"
GOTMPDIR=""
GOTOOLDIR="/usr/local/go/pkg/tool/darwin_amd64"
GCCGO="gccgo"
AR="ar"
CC="clang"
CXX="clang++"
CGO_ENABLED="1"
GOMOD=""
CGO_CFLAGS="-g -O2"
CGO_CPPFLAGS=""
CGO_CXXFLAGS="-g -O2"
CGO_FFLAGS="-g -O2"
CGO_LDFLAGS="-g -O2"
PKG_CONFIG="pkg-config"
GOGCCFLAGS="-fPIC -m64 -pthread -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -fdebug-prefix-map=/var/folders/dv/8tlwvjr91zjdyq4rk14lkkfm0000gn/T/go-build945555905=/tmp/go-build -gno-record-gcc-switches -fno-common"

What did you do?

go build; ./example

What did you expect to see?

go build; ./udp 
listen address: 127.0.0.1:5127 
dial from: 127.0.0.1:50136 
dial to: 127.0.0.1:5127 
dial send: bytes=2 to=127.0.0.1:5127
listen received: bytes=2 from=127.0.0.1:50136
listen send: bytes=2 to=127.0.0.1:50136
dial received: bytes=2 from=127.0.0.1:5127

I expected using n, err := conn.WriteToUDP([]byte("not ok"), addr)
to have the same result as using n, err := conn.Write([]byte("ok"))

What did you see instead?

n, err := conn.WriteToUDP([]byte("not ok"), addr) get stuck trying to connect

go build; ./udp 
listen address: 127.0.0.1:5127 
dial from: 127.0.0.1:56423 
dial to: 127.0.0.1:5127 
dial send: bytes=0 to=127.0.0.1:5127

package main

import (
	"fmt"
	"net"
	"time"
)

func Listen() error {
	addr := &net.UDPAddr{Port: 5127, IP: net.ParseIP("127.0.0.1")}
	conn, err := net.ListenUDP("udp", addr)
	if err != nil {
		return err
	}
	defer conn.Close()

	if conn != nil {
		fmt.Printf("listen address: %s \n", conn.LocalAddr().String())
	}

	buffer := make([]byte, 1024)

	n, addr, err := conn.ReadFromUDP(buffer)
	fmt.Printf("listen received: bytes=%d from=%s\n", n, addr.String())

	n, err = conn.WriteToUDP([]byte("ok"), addr)
	fmt.Printf("listen send: bytes=%d to=%s\n", n, addr.String())

	return err
}

func Dial() error {
	addr := &net.UDPAddr{Port: 5127, IP: net.ParseIP("127.0.0.1")}
	conn, err := net.DialUDP("udp", nil, addr)
	if err != nil {
		return err
	}
	defer conn.Close()

	if conn != nil {
		fmt.Printf("dial from: %s \n", conn.LocalAddr().String())
		fmt.Printf("dial to: %s \n", conn.RemoteAddr().String())
	}

	n, err := conn.WriteToUDP([]byte("not ok"), addr)
	// n, err := conn.Write([]byte("ok"))
	fmt.Printf("dial send: bytes=%d to=%s\n", n, addr.String())

	buffer := make([]byte, 1024)
	n, addr, err = conn.ReadFromUDP(buffer)
	fmt.Printf("dial received: bytes=%d from=%s\n", n, addr.String())

	return err
}

func main() {
	go Listen()
	time.Sleep(1 * time.Second)
	err := Dial()
	if err != nil {
		panic(err)
	}
}

https://play.golang.org/p/8Fq1XyYTFs1

[bradfitz]

This isn't a great bug report.

You pasted a lot of code and said "all this code doesn't work".

You will likely get more attention on this if you use the recommended bug template (which you deleted) and tell us what you expected to see and what happened instead, and details about your system (which macOS version?), etc.

[bradfitz]

Here's almost certainly your problem, though:

	go Listen()
	err := Dial()

You have no coordination between starting to Listen and Dialing.

Your Dial might happen before you even start to Listen.

[gertcuykens]

Removed the first example above and added only the short version and a timeout between listen and Dial. So basically if you copy paste the code above and run it you can compare the result between

	n, err := conn.WriteToUDP([]byte("not ok get stuck"), addr) 
	// n, err := conn.Write([]byte("ok works perfect"))

Will add the rest of the template but in my opinion itsn't related to the issue but could be wrong. If I can make a guess I believe addr get changed under the hood and isn't trivial? If it had to do with coordination I still expected identical behavior between the two lines of code, not the second line always working and the first line always failing to connect.

https://play.golang.org/p/8Fq1XyYTFs1

[gertcuykens]

Thx Andrey Mirtchovski

err was write udp 127.0.0.1:59864->127.0.0.1:5127: use of WriteTo with pre-connected connection

closing