NewProcess does not return an error if process does not exist on Windows

[dhaavi]

Describe the bug

The docs for NewProcess state that An error will be returned if the process does not exist.
Running on Windows 10, NewProcess does not return an error if called with an invalid PID.
(Works fine on Linux 4.9)

To Reproduce

package main

import (
	"fmt"

	"github.com/shirou/gopsutil/process"
)

func main() {
	_, err := process.NewProcess(9999)
	if err == nil {
		fmt.Println("process exists!")
	}
}

Expected behavior
NewProcess should return an error.

Environment (please complete the following information):

• Windows: Microsoft Windows [Version 10.0.17763.615]
• Linux: Linux 74b9ced6499d 4.9.125-linuxkit #1 SMP Fri Sep 7 08:20:28 UTC 2018 x86_64 x86_64 x86_64 GNU/Linux
• Mac OS: [paste the result of sw_vers and uname -a
• FreeBSD: [paste the result of freebsd-version -k -r -u and uname -a]
• OpenBSD: [paste the result of uname -a]

Additional context
Library version:

  name = "github.com/shirou/gopsutil"
  revision = "4c8b404ee5c53b04b04f34b1744a26bf5d2910de"
  version = "v2.19.6"

Cross-compiling for Windows on Linux using GOOS=windows go build in a dir with only the file pasted above.

GOARCH="amd64"
GOBIN=""
GOCACHE="/home/user/.gocache"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GOOS="linux"
GOPATH="/home/user/go"
GOPROXY=""
GORACE=""
GOROOT="/usr/lib/go-1.12"
GOTMPDIR=""
GOTOOLDIR="/usr/lib/go-1.12/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-build008209370=/tmp/go-build -gno-record-gcc-switches"

Workaround

package main

import (
	"fmt"

	"github.com/shirou/gopsutil/process"
)

func main() {
	p, err := process.NewProcess(9999)
	if err == nil {
		_, err = p.Name() // edited
		if err == nil {
			fmt.Println("process exists!")
		}
	}
}

[Lomanic]

This check is only present on Linux currently (checking if /proc/$PID exists). Work in #713 will make it viable (without destroying performances as it would have before on some platforms, for example on Darwin) to check PID existence each time you call process.NewProcess.

[Lomanic]

Trying to call process.Status() on Windows is a bad idea to check if process exists on Windows as it is not implemented and returns a common.ErrNotImplementedError.

[dhaavi]

I see. Could you then update the documentation to reflect this behavior? The windows version of NewProcess is undocumented. (I must admit, I did not check it before raising the issue)

Ha! Good Catch! Thanks for that.
What would you recommend as a workaround until this can be fixed with #713?

[Lomanic]

The windows version of NewProcess is undocumented.

Documentation is only present on linux because it's godoc.org default GOOS (last time I checked there was no way to change GOOS via URL parameter on godoc.org) and because we have the same documentation for all the platforms.

You can check for process existence with process.PidExists(). This will be slow with current implementation of process.Pids() on darwin though (and on others that may call ps to list processes).

Could you then update the documentation to reflect this behavior?

This will be fixed with #713 being merged and the test for process existence being added to process.NewProcess() ; it's been like this for quite a long time, there is little value for me to modify the documentation when it will be fixed in the coming weeks.

[dhaavi]

there was no way to change GOOS via URL parameter on godoc.org

I know - that is why i checked the source code. I am really annoyed by this behavior of godoc.org.

there is little value for me to modify the documentation when it will be fixed in the coming weeks.

Perfect. Wasn't aware of that timeline.