go: runtime: close of a channel with pending sends incorrectly flagged by race detector

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

$ go version
go version go1.11.4 windows/amd64

Does this issue reproduce with the latest release?

Yes.

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

$ go env
set GOARCH=amd64
set GOBIN=
set GOCACHE=C:\Users\kjk\AppData\Local\go-build
set GOEXE=.exe
set GOFLAGS=
set GOHOSTARCH=amd64
set GOHOSTOS=windows
set GOOS=windows
set GOPATH=C:\Users\kjk\go
set GOPROXY=
set GORACE=
set GOROOT=C:\Go
set GOTMPDIR=
set GOTOOLDIR=C:\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\kjk\AppData\Local\Temp\go-build206244702=/tmp/go-build -gno-record-gcc-switches

What did you do?

Save this to e.g. t.go:

package main

import (
	"fmt"
	"time"
)

func main() {
	ch := make(chan string)
	for i := 0; i < 256; i++ {
		go func(n int) {
			ch <- fmt.Sprintf("string %d", n)
		}(i)
	}
	time.Sleep(time.Second)
	close(ch)
}

and run go -race t.go.

What did you see instead?

I see a data race reported:

> go run -race t.go
==================                                                                
WARNING: DATA RACE                                                                
Write at 0x00c00001a130 by main goroutine:                                        
  runtime.closechan()                                                             
      C:/Go/src/runtime/chan.go:327 +0x0                                          
  main.main()                                                                     
      C:/Users/kjk/src/t.go:16 +0xb5                                              
                                                                                  
Previous read at 0x00c00001a130 by goroutine 231:                                 
  runtime.chansend()                                                              
      C:/Go/src/runtime/chan.go:140 +0x0                                          
  main.main.func1()                                                               
      C:/Users/kjk/src/t.go:12 +0xcc                                              
                                                                                  
Goroutine 231 (running) created at:                                               
  main.main()                                                                     
      C:/Users/kjk/src/t.go:11 +0x83                                              
==================                                                                
panic: send on closed channel                                                     
                                                                                  
goroutine 201 [running]:                                                          
main.main.func1(0xc00001a120, 0xc3)                                               
        C:/Users/kjk/src/t.go:12 +0xcd                                            
created by main.main                                                              
        C:/Users/kjk/src/t.go:11 +0x84                                            
panic: send on closed channel                                                     
                                                                                  
goroutine 235 [running]:                                                          
main.main.func1(0xc00001a120, 0xe5)                                               
        C:/Users/kjk/src/t.go:12 +0xcd                                            
created by main.main                                                              
        C:/Users/kjk/src/t.go:11 +0x84                                            
exit status 2                                                                     

What did you expect to see?

I didn’t expect data race between close(ch) and pending ch <- v send.

The spec (https://golang.org/ref/spec#Channel_types) is curiously mum on the subject.

It says:

A single channel may be used in send statements, receive operations, and calls to the built-in functions cap and len by any number of goroutines without further synchronization.

By not mentioning close it implies that one has to synchronize close and sends.

But I wouldn’t know how to synchronize those. My sends have already been dispatched and closing a channel seems like a valid (and only) way to ensure those sending goroutines don’t stick forever (assumingI recover the resulting panic).

The data race seems harmless, as I’m done with this channel, but I run my tests with -race detection enabled and this makes the test fail.