Example how to parse /proc/{pid}/task/{tid}/children recursively (see 'man 2 proc')

proc_task_children.go

package main

import (
  "fmt"
  "io/ioutil"
  "os"
  "strings"
)

func GetAllChildren(proc string, pid string) ([]string, error) {
  children, err := GetChildren(proc, pid)
  if err != nil {
    return nil, err
  }
  if len(children) == 0 {
    return nil, nil
  }
  for _, childPid := range children {
    grandChildren, err := GetAllChildren(proc, childPid)
    if err != nil {
      if os.IsNotExist(err) {
        continue
      }
      return nil, err
    }
    if grandChildren != nil {
      children = append(children, grandChildren...)
    }
  }
  return children, nil
}

func GetChildren(proc string, pid string) ([]string, error) {
  p := fmt.Sprintf("%s/%s/task/%s/children", proc, pid, pid)
  data, err := ioutil.ReadFile(p)
  if err != nil {
    return nil, err
  }
  println(len(data))
  if len(data) == 0 {
    return nil, nil
  }
 return strings.Split(strings.TrimSpace(string(data)), " "), nil
}

proc_task_children_test.go

package main

import (
  "github.com/stretchr/testify/require"
  "io/ioutil"
  "os"
  "path/filepath"
  "testing"
)

func TestGetDescendants(t *testing.T) {
  procValues := map[string]string{
    "1/task/1/children": "2 3 4",
    "2/task/2/children": "5 6 7",
    "3/task/3/children": "8 9 10",
    "4/task/4/children": "",
    "5/task/5/children": "",
    "6/task/6/children": "",
    "7/task/7/children": "",
    "8/task/8/children": "",
    "9/task/9/children": "11 12 13",
    "10/task/10/children": "",
    "11/task/11/children": "",
  }
  dir, err := ioutil.TempDir("", "crio-lxc-proc")
  require.NoError(t, err)
  defer os.RemoveAll(dir)
  for fileName, pids := range procValues {
    fp := filepath.Join(dir, fileName)
    os.MkdirAll(filepath.Dir(fp), 0755)
    err := ioutil.WriteFile(fp, []byte(pids), 0660)
    require.NoError(t, err)
  }

  c1, err := GetAllChildren(dir, "1")
  require.NoError(t, err)
  c1e := []string{"2", "3", "4", "5", "6", "7", "8", "9", "10", "11", "12", "13"}
  require.EqualValues(t, c1e,  c1)

  c2, err := GetAllChildren(dir, "2")
  require.NoError(t, err)
  c2e := []string{"5", "6", "7"}
  require.EqualValues(t, c2, c2e)

  c3, err := GetAllChildren(dir, "6")
  require.Empty(t, c3)
  require.NoError(t, err)

  c4, err := GetAllChildren(dir, "12")
  require.Empty(t, c4)
  require.Error(t, os.ErrExist, err)
}