testing: test process hangs beyond `-timeout` if a child process holds I/O streams open

[psanford]

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

go version go1.10 linux/amd64

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

$ go env
GOARCH="amd64"
GOBIN=""
GOCACHE="/home/psanford/.cache/go-build"
GOEXE=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GOOS="linux"
GOPATH="/home/psanford/projects/go"
GORACE=""
GOROOT="/usr/local/go"
GOTMPDIR=""
GOTOOLDIR="/usr/local/go/pkg/tool/linux_amd64"
GCCGO="gccgo"
CC="gcc"
CXX="g++"
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"
GOGCCFLAGS="-fPIC -m64 -pthread -fmessage-length=0 -fdebug-prefix-map=/tmp/go-build953969108=/tmp/go-build -gno-record-gcc-switches"

What did you do?

After upgrading to 1.10 we had one test that started to hang intermittently. The test in question starts a child process which it kills by canceling a context object at the end of the test method. It does not do an explicit cmd.Wait().

Here is a minimal test case that demonstrates the problem:

https://play.golang.org/p/8rq41A5Khsm

I can get this to hang consistently by running it in a bash while loop:

$ while true; do go test -timeout 5s -v -count 1 .; sleep 0.1; done
=== RUN   TestOSExecNoWait
start
done
--- PASS: TestOSExecNoWait (0.01s)
PASS
ok      _/tmp   0.012s
=== RUN   TestOSExecNoWait
start
done
--- PASS: TestOSExecNoWait (0.01s)
PASS
ok      _/tmp   0.012s
=== RUN   TestOSExecNoWait
start
done
--- PASS: TestOSExecNoWait (0.01s)
PASS
ok      _/tmp   0.012s
=== RUN   TestOSExecNoWait
start
done
--- PASS: TestOSExecNoWait (0.01s)
PASS

<hangs here indefinitely>

If I explicitly call cmd.Wait() the test does not hang. If I don't attach the child process' Stdout and Stderr to os.Std{out,err} the test does not hang.

On 1.9.4 the test does not hang.

Its also interesting that even though I specified -timeout 5s the test runner hangs forever.

[crvv]

the test runner hangs forever.

It only hangs 60 seconds on my machine.

go test hangs at

go/src/cmd/go/internal/test/test.go

Line 1271 in f6c6781

done <- cmd.Wait()

And cmd.Wait() is waiting at

go/src/os/exec/exec.go

Line 275 in f6c6781

_, err := io.Copy(w, pr)

This io.Copy() is reading a pipe reader. It won't return until the pipe writer is closed.
The pipe writer will be closed when the sleep 60 exits. So it hangs 60 seconds.

func TestOSExecNoWait(t *testing.T) {
...
	cancel()
}

After the cancel() returns, the main function will also return.
The child process may or may not be killed, so it hangs intermittently.

[psanford]

It only hangs 60 seconds on my machine.

Yes in the example provided it only hangs for 60 seconds because the child process exits.

In the test where I first found this issue the child process never exits so it hangs forever.

[crvv]

I don't think this issue is a regression introduced in Go 1.10.
I can use Go 1.9 to reproduce it but with a different condition.

This difference was introduced in bd95f88.
bd95f88#diff-acaf53a9cd478507ebbcf85037940b4dL1080

If the go command needs a bytes.Buffer to save the output, os/exec will open a pipe.
And go will hang until the pipe is closed.

The timeout doesn't work because it isn't handled by go.
There is a testKillTimeout in go, but it also hangs at cmd.Wait().

[gopherbot]

Change https://golang.org/cl/97497 mentions this issue: cmd/go/internal/test: don't wait for pending I/O if child process has gone

[ianlancetaylor]

See also #23019.