runtime/race: false positive with closed channels in default select (go1.14beta1) #36714
Description
What version of Go are you using (go version)?
go1.14beta1
$ go version go version devel +a5bfd9da1d Tue Dec 17 14:59:30 2019 +0000 linux/amd64
Does this issue reproduce with the latest release?
No, go1.13.6 does not have the problem.
What operating system and processor architecture are you using (go env)?
go env Output
$ go env GO111MODULE="on" GOARCH="amd64" GOBIN="" GOCACHE="/home/jeff/.cache/go-build" GOENV="/home/jeff/.config/go/env" GOEXE="" GOFLAGS="" GOHOSTARCH="amd64" GOHOSTOS="linux" GOINSECURE="" GONOPROXY="" GONOSUMDB="" GOOS="linux" GOPATH="/home/jeff/go" GOPRIVATE="" GOPROXY="https://proxy.golang.org" GOROOT="/home/jeff/gotip" GOSUMDB="sum.golang.org" GOTMPDIR="" GOTOOLDIR="/home/jeff/gotip/pkg/tool/linux_amd64" GCCGO="gccgo" AR="ar" CC="gcc" CXX="g++" CGO_ENABLED="1" GOMOD="/home/jeff/tmp/raceissue/go.mod" 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 -fmessage-length=0 -fdebug-prefix-map=/tmp/go-build491500605=/tmp/go-build -gno-record-gcc-switches"
What did you do?
https://play.golang.org/p/aJkZKenyXHu
package main
func main() {
for {
var loc int
var write = make(chan struct{})
var read = make(chan struct{})
go func() {
select {
case <-write:
_ = loc
default:
}
close(read)
}()
go func() {
loc = 1
close(write)
}()
<-read
}
}I ran the above code with GORACE="halt_on_error=1" go run -race *.go
What did you expect to see?
No error.
What did you see instead?
==================
WARNING: DATA RACE
Read at 0x00c000112000 by goroutine 6:
main.main.func1()
/home/jeff/tmp/raceissue/race.go:12 +0x73
Previous write at 0x00c000112000 by goroutine 7:
main.main.func2()
/home/jeff/tmp/raceissue/race.go:19 +0x38
Goroutine 6 (running) created at:
main.main()
/home/jeff/tmp/raceissue/race.go:9 +0xd4
Goroutine 7 (finished) created at:
main.main()
/home/jeff/tmp/raceissue/race.go:18 +0x100
==================
exit status 66
I did a git bisect between go1.13.6 and go1.14beta1 and it pointed at e1446d9 which seemed to be a great likely candidate. After finding that, I found that the issue does not present itself if the write channel in the example is sent on instead of closed, or if there is not a default case in the select. Additionally, running on go1.14beta1 with that commit reverted also causes the example to succeed.