Concurrent calls to errors.As results in a data race

[yarikbratashchuk]

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

$ go version
go version go1.14.1 darwin/amd64

Does this issue reproduce with the latest release?

yes

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

go env Output

$ go env
GO111MODULE=""
GOARCH="amd64"
GOBIN=""
GOCACHE="/Users/yarik/Library/Caches/go-build"
GOENV="/Users/yarik/Library/Application Support/go/env"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="darwin"
GOINSECURE=""
GONOPROXY="github.com/uploadcare/common-go"
GONOSUMDB="github.com/uploadcare/common-go"
GOOS="darwin"
GOPATH="/Users/yarik/go"
GOPRIVATE="github.com/uploadcare/common-go"
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="/Users/yarik/work/uploadcare/webhook/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 -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -fdebug-prefix-map=/var/folders/m_/z4_ls1zd2bdd3z1846w1l8sw0000gn/T/go-build030379024=/tmp/go-build -gno-record-gcc-switches -fno-common"

What did you do?

Concurrently calling errors.As:
https://play.golang.org/p/7FQegBPdSkq

What did you expect to see?

No data race

What did you see instead?

Data race:

==================
WARNING: DATA RACE
Write at 0x00000135ba10 by goroutine 10:
  reflect.typedmemmove()
      /usr/local/go/src/runtime/mbarrier.go:177 +0x0
  internal/reflectlite.typedmemmove()
      /usr/local/go/src/runtime/mbarrier.go:191 +0x3e
  errors.As()
      /usr/local/go/src/errors/wrap.go:92 +0x38f
  github.com/yarikbratashchuk/datarace.fireDataRace()
      /Users/yarik/Desktop/main.go:20 +0x6e

Previous write at 0x00000135ba10 by goroutine 8:
  reflect.typedmemmove()
      /usr/local/go/src/runtime/mbarrier.go:177 +0x0
  internal/reflectlite.typedmemmove()
      /usr/local/go/src/runtime/mbarrier.go:191 +0x3e
  errors.As()
      /usr/local/go/src/errors/wrap.go:92 +0x38f
  github.com/yarikbratashchuk/datarace.fireDataRace()
      /Users/yarik/Desktop/main.go:20 +0x6e

Goroutine 10 (running) created at:
  github.com/yarikbratashchuk/datarace.test()
      /Users/yarik/Desktop/main.go:15 +0x4b
  github.com/yarikbratashchuk/datarace.TestMain()
      /Users/yarik/Desktop/main_test.go:8 +0x2f
  testing.tRunner()
      /usr/local/go/src/testing/testing.go:992 +0x1eb

Goroutine 8 (finished) created at:
  github.com/yarikbratashchuk/datarace.test()
      /Users/yarik/Desktop/main.go:15 +0x4b
  github.com/yarikbratashchuk/datarace.TestMain()
      /Users/yarik/Desktop/main_test.go:8 +0x2f
  testing.tRunner()
      /usr/local/go/src/testing/testing.go:992 +0x1eb
==================
    TestMain: testing.go:906: race detected during execution of test
--- FAIL: TestMain (0.01s)

[cespare]

That's expected. The second argument to errors.As, target, is mutated by the function. You wouldn't call it with a global value like context.DeadlineExceeded; you'd call it on a local error value of a particular type. An example from the errors documentation is:

var perr *os.PathError
if errors.As(err, &perr) {
	fmt.Println(perr.Path)
}

If you want to check whether you have an error that matches context.DeadlineExceeded, you'd use errors.Is, not errors.As:

if errors.Is(err, context.DeadlineExceeded) {

playground

[yarikbratashchuk]

Got it,
much thanks