os/exec: incorrect "fatal error: all goroutines are asleep - deadlock!"

[martinrode]

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

$ go version
go version go1.15 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="on"
GOARCH="amd64"
GOBIN="/Users/martin/go/bin"
GOCACHE="/Users/martin/Library/Caches/go-build"
GOENV="/Users/martin/Library/Application Support/go/env"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="darwin"
GOINSECURE=""
GOMODCACHE="/Users/martin/go/pkg/mod"
GONOPROXY=""
GONOSUMDB=""
GOOS="darwin"
GOPATH="/Users/martin/go"
GOPRIVATE=""
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/martin/go/src/github.com/programmfabrik/fylr/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/4f/l31kb4n521d6g89qxrwcx69h0000gp/T/go-build343164864=/tmp/go-build -gno-record-gcc-switches -fno-common"

What did you do?

Playground

package main

import (
	"context"
	"io"
	"os/exec"
	"time"
)

func main() {
	ctx, cancel := context.WithTimeout(context.Background(), 1*time.Second)
	defer cancel()
	cmd := exec.CommandContext(ctx, "sleep", "2")
	cmd.Stdin, _ = io.Pipe() // This prevents the "Timeout" to work correctly
	cmd.Run()                // Wrongly gets killed with "deadlock" detected
}

What did you expect to see?

I expect my program to not deadlock, but instead kill "sleep" and return from "Run" with an error. The unclosed STDIN should not prevent the program from returning.

Related or DUP?

Issue #23019 might be related to this?

What did you see instead?

The program deadlocks which is not what I expect.

[fatelei]

In golang, the deadlock is detected when there is only main goroutine running

[martinrode]

In golang, the deadlock is detected when there is only main goroutine running

Nope, comment the cmd.Stdin line, and see how the program works.

[andybons]

@bradfitz @ianlancetaylor

[ianlancetaylor]

This is an interesting case. It's related to #23019, certainly. Because you used io.Pipe rather than, say, os.Pipe, the Go runtime knows for sure that nothing is ever going to write to that pipe. So the os/exec package is waiting for something to write to the pipe, and the runtime knows that that will never happen. So the runtime is correctly reporting a deadlock.

I'm going to close this as a dup of #23019, because if we fix that problem, we will fix this one. And if we don't fix that problem, then I don't think there is anything we can do about this one.