x/sync/errgroup: Group: document that Go may not be concurrent with Wait unless semaphore > 0 #70284
Description
(Edit: skip down to #70284 (comment); this is now a doc change request. --@adonovan)
Go version
go version go1.23.1 darwin/amd64
Output of go env in your module/workspace:
GO111MODULE=''
GOARCH='amd64'
GOBIN=''
GOCACHE='/Users/haaawk/Library/Caches/go-build'
GOENV='/Users/haaawk/Library/Application Support/go/env'
GOEXE=''
GOEXPERIMENT=''
GOFLAGS=''
GOHOSTARCH='amd64'
GOHOSTOS='darwin'
GOINSECURE=''
GOMODCACHE='/Users/haaawk/go/pkg/mod'
GONOPROXY=''
GONOSUMDB=''
GOOS='darwin'
GOPATH='/Users/haaawk/go'
GOPRIVATE=''
GOPROXY='https://proxy.golang.org,direct'
GOROOT='/usr/local/go'
GOSUMDB='sum.golang.org'
GOTMPDIR=''
GOTOOLCHAIN='auto'
GOTOOLDIR='/usr/local/go/pkg/tool/darwin_amd64'
GOVCS=''
GOVERSION='go1.23.1'
GODEBUG=''
GOTELEMETRY='local'
GOTELEMETRYDIR='/Users/haaawk/Library/Application Support/go/telemetry'
GCCGO='gccgo'
GOAMD64='v1'
AR='ar'
CC='clang'
CXX='clang++'
CGO_ENABLED='1'
GOMOD='/dev/null'
GOWORK=''
CGO_CFLAGS='-O2 -g'
CGO_CPPFLAGS=''
CGO_CXXFLAGS='-O2 -g'
CGO_FFLAGS='-O2 -g'
CGO_LDFLAGS='-O2 -g'
PKG_CONFIG='pkg-config'
GOGCCFLAGS='-fPIC -arch x86_64 -m64 -pthread -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -ffile-prefix-map=/var/folders/_y/nj5hcsh93l12tmsszxgx7ntr0000gn/T/go-build2396141927=/tmp/go-build -gno-record-gcc-switches -fno-common'What did you do?
When following code is run:
package main
import (
"runtime"
"golang.org/x/sync/errgroup"
)
func main() {
runtime.GOMAXPROCS(1)
g := &errgroup.Group{}
g.SetLimit(1)
ch := make(chan struct{})
wait := make(chan struct{}, 2)
g.Go(func() error {
<-ch
wait <- struct{}{}
return nil
})
go g.Go(func() error {
println("I'm not blocked")
wait <- struct{}{}
return nil
})
println("Ok let's play")
close(ch)
g.Wait()
println("It's over already?")
<-wait
<-wait
}
https://go.dev/play/p/xTIsT1iouTd
What did you see happen?
The program printed:
Ok let's play
It's over already?
I'm not blocked
What did you expect to see?
The program printing:
Ok let's play
I'm not blocked
It's over already?