os: Getpid() returns parent's PID even called from sub process

[keakon]

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

$ go version
go version go1.20.2 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=""
GOCACHE="/Users/keakon/Library/Caches/go-build"
GOENV="/Users/keakon/Library/Application Support/go/env"
GOEXE=""
GOEXPERIMENT=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="darwin"
GOINSECURE=""
GOMODCACHE="/Users/keakon/go/pkg/mod"
GONOPROXY=""
GONOSUMDB=""
GOOS="darwin"
GOPATH="/Users/keakon/go"
GOPRIVATE=""
GOPROXY="https://goproxy.cn,direct"
GOROOT="/usr/local/Cellar/go/1.20.2/libexec"
GOSUMDB="sum.golang.org"
GOTMPDIR=""
GOTOOLDIR="/usr/local/Cellar/go/1.20.2/libexec/pkg/tool/darwin_amd64"
GOVCS=""
GOVERSION="go1.20.2"
GCCGO="gccgo"
GOAMD64="v1"
AR="ar"
CC="cc"
CXX="c++"
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 -fdebug-prefix-map=/var/folders/jw/d0g3224x6tx5dwqrvkl8pp440000gn/T/go-build1704497747=/tmp/go-build -gno-record-gcc-switches -fno-common"

What did you do?

Run this code:

package main

import (
	"fmt"
	"os"
	"syscall"
)

func main() {
	r1, r2, err := syscall.Syscall(syscall.SYS_FORK, 0, 0, 0)
	if err != 0 {
		panic(err)
	}
	fmt.Println("Pid of sub process is", r1)
	if r2 == 0 { // parent
		fmt.Println("os.Getpid() of parent process is", os.Getpid())
	} else { // sub
		fmt.Println("os.Getpid() of sub process is", os.Getpid())
	}
}

What did you expect to see?

Pid of sub process is 8048
os.Getpid() of parent process is 8047
Pid of sub process is 8048
os.Getpid() of sub process is 8048

What did you see instead?

Pid of sub process is 8048
os.Getpid() of parent process is 8047
Pid of sub process is 8048
os.Getpid() of sub process is 8047

[rittneje]

It looks like on macos, Go hooks into libc's getpid implementation. I am seeing some references to libc attempting to cache the result instead of making the syscall each time, but I cannot find anything authoritative for macos. My guess is since you are directly issuing the SYS_FORK syscall, this cache is not being invalidated, hence os.Getpid returns the wrong thing. Note that if you directly do syscall.Syscall(syscall.SYS_GETPID, 0, 0, 0) it gives the right answer.

[bcmills]

When it comes to SYS_FORK, if you break it, you get to keep all the pieces. Use syscall.ForkExec or (better still) os/exec instead.

(https://www.microsoft.com/en-us/research/publication/a-fork-in-the-road/ is a good read; search in particular for “Fork isn't thread-safe.”)