runtime: externally linked programs don't detect deadlocks

[aarzilli]

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

$ go version
go version go1.11.4 linux/amd64

Does this issue reproduce with the latest release?

Yes (also master as of last week)

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

go env Output

$ go env
GOARCH="amd64"
GOBIN=""
GOCACHE="/home/a/.cache/go-build"
GOEXE=""
GOFLAGS="-mod=vendor"
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GOOS="linux"
GOPATH="/home/a/n/go/"
GOPROXY=""
GORACE=""
GOROOT="/usr/local/go"
GOTMPDIR=""
GOTOOLDIR="/usr/local/go/pkg/tool/linux_amd64"
GCCGO="gccgo"
CC="gcc"
CXX="g++"
CGO_ENABLED="1"
GOMOD=""
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-build536260035=/tmp/go-build -gno-record-gcc-switches"

What did you do?

Did go run -buildmode=pie with this:

package main

func main() {
	ch1 := make(chan string)
	ch2 := make(chan string)
	go func() {
		<-ch1
		ch2 <- "done"
	}()
	<-ch2
	ch1 <- "done"
}

What did you expect to see?

Process dies with the "all goroutines are asleep" message.

What did you see instead?

Not dead but forever sleeping.

[ianlancetaylor]

This is happening because using -buildmode=pie causes the final link to be done using the external linker, which causes the runtime to think that the program uses cgo. In a program that uses cgo the deadlock detector is, in effect, disabled, because it is possible that the C code has started a new thread.

The fix would require separating the use of the external linker from actually using cgo.

[cherrymui]

We now support internal linking for PIE (and use by default) on a few platforms, including Linux/AMD64, Linux/ARM64, macOS/AMD64, macOS/ARM64, Windows, and some Android platforms. And deadlock detection does work.

It still doesn't work with external linking (PIE or not).