runtime/race: Data race in append() missed by the race detector

[Escapist25]

Please answer these questions before submitting your issue. Thanks!

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

go version go1.9.2 darwin/amd64

Does this issue reproduce with the latest release?

yes

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

GOARCH="amd64"
GOBIN="/Users/escapist/gocode/bin"
GOEXE=""
GOHOSTARCH="amd64"
GOHOSTOS="darwin"
GOOS="darwin"
GOPATH="/Users/escapist/gocode/"
GORACE=""
GOROOT="/usr/local/Cellar/go/1.9.2/libexec"
GOTOOLDIR="/usr/local/Cellar/go/1.9.2/libexec/pkg/tool/darwin_amd64"
GCCGO="gccgo"
CC="clang"
GOGCCFLAGS="-fPIC -m64 -pthread -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -fdebug-prefix-map=/var/folders/_l/pbv9psw11872p3zrn8dptkq40000gn/T/go-build881315384=/tmp/go-build -gno-record-gcc-switches -fno-common"
CXX="clang++"
CGO_ENABLED="1"
CGO_CFLAGS="-g -O2"
CGO_CPPFLAGS=""
CGO_CXXFLAGS="-g -O2"
CGO_FFLAGS="-g -O2"
CGO_LDFLAGS="-g -O2"
PKG_CONFIG="pkg-config"

What did you do?

I run the following sample code with -race option:
code is also available at https://play.golang.org/p/L9SMvPzfcu

package main

import (
	"time"
)

var safeSlice = []int{1}
var testS = make([]int,0,1)
func main() {
	for i := 0;i < 5;i++ {
		go func(index int) {
			for {
				s1 := append(safeSlice, index)
				if s1[1] != index {
					panic("")
				}
				s2 := append(testS, index)   // <-   data race here
				if s2[0] != index {          // <-  panic here
					panic("")
				}
			}
		}(i)
	}
	time.Sleep(time.Second*5)
}

If possible, provide a recipe for reproducing the error.
A complete runnable program is good.
A link on play.golang.org is best.

What did you expect to see?

Append to a fixed slice with capacity larger than size is not thread-safe because every goroutine is trying to write data to same memory location in the underlying array.
I expect the data race to be detected by the race detector.

What did you see instead?

The race detector missed the data race and every goroutine could see a slice with an unexpected value.

[artyom]

Interestingly for me the example outputs race warnings in a similar environment (100 runs out of 100):

ex ¶ go run -race ex.go
==================
WARNING: DATA RACE
Write at 0x00c420078000 by goroutine 6:
  main.main.func1()
      /tmp/ex/ex.go:17 +0x132

Previous write at 0x00c420078000 by goroutine 5:
  main.main.func1()
      /tmp/ex/ex.go:17 +0x132

Goroutine 6 (running) created at:
  main.main()
      /tmp/ex/ex.go:11 +0x50

Goroutine 5 (running) created at:
  main.main()
      /tmp/ex/ex.go:11 +0x50
==================
==================
WARNING: DATA RACE
Write at 0x00c420078000 by goroutine 7:
  main.main.func1()
      /tmp/ex/ex.go:17 +0x132

Previous write at 0x00c420078000 by goroutine 5:
  main.main.func1()
      /tmp/ex/ex.go:17 +0x132

Goroutine 7 (running) created at:
  main.main()
      /tmp/ex/ex.go:11 +0x50

Goroutine 5 (running) created at:
  main.main()
      /tmp/ex/ex.go:11 +0x50
==================
panic: 

goroutine 19 [running]:
main.main.func1(0x1)
	/tmp/ex/ex.go:19 +0x214
created by main.main
	/tmp/ex/ex.go:11 +0x51
exit status 2

This is an official build — go version go1.9.2 darwin/amd64 from go1.9.2.darwin-amd64.tar.gz file:

GOARCH="amd64"
GOBIN=""
GOEXE=""
GOHOSTARCH="amd64"
GOHOSTOS="darwin"
GOOS="darwin"
GOPATH="/tmp/go:/Users/artyom/go"
GORACE=""
GOROOT="/Users/artyom/Library/go"
GOTOOLDIR="/Users/artyom/Library/go/pkg/tool/darwin_amd64"
GCCGO="gccgo"
CC="clang"
GOGCCFLAGS="-fPIC -m64 -pthread -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -fdebug-prefix-map=/var/folders/lb/3rk8rqs53czgb4v35w_342xc0000gn/T/go-build997454935=/tmp/go-build -gno-record-gcc-switches -fno-common"
CXX="clang++"
CGO_ENABLED="1"
CGO_CFLAGS="-g -O2"
CGO_CPPFLAGS=""
CGO_CXXFLAGS="-g -O2"
CGO_FFLAGS="-g -O2"
CGO_LDFLAGS="-g -O2"
PKG_CONFIG="pkg-config"

[ianlancetaylor]

I also see a data race warning.

Please show us the exact commands that you are executing and exactly what you see. Thanks.

[Escapist25]

I ran this with go run -race
I tried again with go build -race and saw the expected data race warning.

[ianlancetaylor]

Odd. I see the race with both go build -race and go run -race.

Can you show us the complete output of go run -race FILE.go?

[Escapist25]

go run tx.go ‹system›
panic:

goroutine 4 [running]:
main.main.func1(0x0)
/Users/escapist/gocode/src/github.com/ysxue/pipe/tx.go:19 +0x143
created by main.main
/Users/escapist/gocode/src/github.com/ysxue/pipe/tx.go:11 +0x43
panic:

goroutine 8 [running]:
main.main.func1(0x4)
/Users/escapist/gocode/src/github.com/ysxue/pipe/tx.go:19 +0x143
created by main.main
/Users/escapist/gocode/src/github.com/ysxue/pipe/tx.go:11 +0x43
exit status 2

go run -race tx.go ‹system›

WARNING: DATA RACE
Write at 0x00c420016080 by goroutine 6:
main.main.func1()
/Users/escapist/gocode/src/github.com/ysxue/pipe/tx.go:17 +0x132

Previous write at 0x00c420016080 by goroutine 5:
main.main.func1()
/Users/escapist/gocode/src/github.com/ysxue/pipe/tx.go:17 +0x132

Goroutine 6 (running) created at:
main.main()
/Users/escapist/gocode/src/github.com/ysxue/pipe/tx.go:11 +0x50

Goroutine 5 (running) created at:
main.main()
/Users/escapist/gocode/src/github.com/ysxue/pipe/tx.go:11 +0x50

panic:

goroutine 5 [running]:
main.main.func1(0x1)
/Users/escapist/gocode/src/github.com/ysxue/pipe/tx.go:19 +0x214
created by main.main
/Users/escapist/gocode/src/github.com/ysxue/pipe/tx.go:11 +0x51
exit status 2

It looks like I misplaced the -race option.
The race detector works fine.