net: Conn.Write() does not detect dropped TCP connection on the first attempt

[vbox]

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

go version go1.8.3 linux/amd64

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

GOARCH="amd64"
GOBIN=""
GOEXE=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GOOS="linux"
GOPATH="/home/kvs/go"
GORACE=""
GOROOT="/usr/local/go"
GOTOOLDIR="/usr/local/go/pkg/tool/linux_amd64"
GCCGO="gccgo"
CC="gcc"
GOGCCFLAGS="-fPIC -m64 -pthread -fmessage-length=0 -fdebug-prefix-map=/tmp/go-build565879858=/tmp/go-bui
CXX="g++"
CGO_ENABLED="1"
PKG_CONFIG="pkg-config"
CGO_CFLAGS="-g -O2"
CGO_CPPFLAGS=""
CGO_CXXFLAGS="-g -O2"
CGO_FFLAGS="-g -O2"
CGO_LDFLAGS="-g -O2"

What did you do?

A dropped TCP connection does not get detected on the first Write(), but detected on subsequent ones.

Steps to reproduce:

1. Compile Go client - https://gist.github.com/vbox/d6b8d3f431fd5195846929ee9659ed25

2. Open TCP socket with nc:
   nc -kl 5555

3. Start client binary, and immediately after client has established connection, kill nc. The client will attempt to write to a dropped connection, but no error is generated.

What did you expect to see?

$ ./client
connection established
Write to server failed: write tcp 127.0.0.1:48176->127.0.0.1:5555: write: broken pipe

What did you see instead?

$ ./client
connection established
writen to server = Halo
Write to server failed: write tcp 127.0.0.1:48176->127.0.0.1:5555: write: broken pipe

[davecheney]

This'll be Nagel

…

On Fri, 2 Jun 2017, 15:45 vbox ***@***.***> wrote: What version of Go are you using (go version)? go version go1.8.3 linux/amd64 What operating system and processor architecture are you using (go env)? GOARCH="amd64" GOBIN="" GOEXE="" GOHOSTARCH="amd64" GOHOSTOS="linux" GOOS="linux" GOPATH="/home/kvs/go" GORACE="" GOROOT="/usr/local/go" GOTOOLDIR="/usr/local/go/pkg/tool/linux_amd64" GCCGO="gccgo" CC="gcc" GOGCCFLAGS="-fPIC -m64 -pthread -fmessage-length=0 -fdebug-prefix-map=/tmp/go-build565879858=/tmp/go-bui CXX="g++" CGO_ENABLED="1" PKG_CONFIG="pkg-config" CGO_CFLAGS="-g -O2" CGO_CPPFLAGS="" CGO_CXXFLAGS="-g -O2" CGO_FFLAGS="-g -O2" CGO_LDFLAGS="-g -O2" What did you do? A dropped TCP connection does not get detected on the first Write(), but detected on subsequent ones. Steps to reproduce: 1. Compile Go client - https://gist.github.com/vbox/d6b8d3f431fd5195846929ee9659ed25 2. Open TCP socket with nc: nc -kl 5555 3. Start client binary, and immediately after client has established connection, kill nc. The client will attempt to write to a dropped connection, but no error is generated. What did you expect to see? $ ./client connection established Write to server failed: write tcp 127.0.0.1:48176->127.0.0.1:5555: write: broken pipe What did you see instead? $ ./client connection established writen to server = Halo Write to server failed: write tcp 127.0.0.1:48176->127.0.0.1:5555: write: broken pipe — You are receiving this because you are subscribed to this thread. Reply to this email directly, view it on GitHub <#20553>, or mute the thread <https://github.com/notifications/unsubscribe-auth/AAAcAzgesSxDQV-hDzefmX7tOcUX40Knks5r_6FwgaJpZM4Nt2vv> .

[mikioh]

Your test code doesn't guarantee that the first write call that invokes some functionality inside the kernel has got a closed state of the underlying connection before it returns to the test code. On linux net/ipv4/tcp.c and tcp_output.c describe how the write system call interacts with the TCP implementation.