Fix check pid existence when running in different process namespace (container)

process/process_posix.go

@@ -12,6 +12,7 @@ import (
 	"strings"
 	"syscall"
 
+	"github.com/shirou/gopsutil/internal/common"
 	"golang.org/x/sys/unix"
 )
 
@@ -78,6 +79,16 @@ func PidExistsWithContext(ctx context.Context, pid int32) (bool, error) {
 	if err != nil {
 		return false, err
 	}
+
+	if _, err := os.Stat(common.HostProc()); err == nil { //Means that proc filesystem exist
+		// Checking PID existence based on existence of /<HOST_PROC>/proc/<PID> folder
+		// This covers the case when running inside container with a different process namespace (by default)
+		_, err := os.Stat(common.HostProc(strconv.Itoa(int(pid))))
+		return err == nil, err
+	}
+
+	//'/proc' filesystem is not exist, checking of PID existence is done via signalling the process
+	//Make sense only if we run in the same process namespace
 	err = proc.Signal(syscall.Signal(0))
 	if err == nil {
 		return true, nil
@@ -95,6 +106,7 @@ func PidExistsWithContext(ctx context.Context, pid int32) (bool, error) {
 	case syscall.EPERM:
 		return true, nil
 	}
+
 	return false, err
 }