Implement AddPid method for cgroup managers

To this day, cgroup managers did not have an ability to add a process
to an existing cgroup. One might use cgroups.WriteCgroupProc, but
this is cgroupfs operation and does not take into account systemd.

Let's introduce AddPid, which will add a process, identified by PID,
to the cgroup (or, optionally, a sub cgroup) of the manager.

This will allow runtimes like runc to ask systemd to move the process
into a proper unit, instead of using cgroupfs directly.

Implementation for systemd requires [1] and systemd >= v238.

[1]: coreos/go-systemd#458
Signed-off-by: Kir Kolyshkin <[email protected]>

Author: kolyshkin

cgroups.go

@@ -29,6 +29,11 @@ type Manager interface {
 	// can be used to merely create a cgroup.
 	Apply(pid int) error
 
+	// AddPid adds a process with a given pid to an existing cgroup.
+	// The subcgroup argument is either empty, or a path relative to
+	// a cgroup under under the manager's cgroup.
+	AddPid(subcgroup string, pid int) error
+
 	// GetPids returns the PIDs of all processes inside the cgroup.
 	GetPids() ([]int, error)
 

fs/fs.go

@@ -4,6 +4,8 @@ import (
 	"errors"
 	"fmt"
 	"os"
+	"path"
+	"strings"
 	"sync"
 
 	"golang.org/x/sys/unix"
@@ -139,6 +141,33 @@ func (m *Manager) Apply(pid int) (retErr error) {
 	return retErr
 }
 
+// AddPid adds a process with a given pid to an existing cgroup.
+// The subcgroup argument is either empty, or a path relative to
+// a cgroup under under the manager's cgroup.
+func (m *Manager) AddPid(subcgroup string, pid int) (retErr error) {
+	m.mu.Lock()
+	defer m.mu.Unlock()
+
+	c := m.cgroups
+
+	for _, dir := range m.paths {
+		path := path.Join(dir, subcgroup)
+		if !strings.HasPrefix(path, dir) {
+			return fmt.Errorf("bad sub cgroup path: %s", subcgroup)
+		}
+
+		if err := cgroups.WriteCgroupProc(path, pid); err != nil {
+			if isIgnorableError(c.Rootless, err) && c.Path == "" {
+				retErr = cgroups.ErrRootless
+				continue
+			}
+			return err
+		}
+	}
+
+	return retErr
+}
+
 func (m *Manager) Destroy() error {
 	m.mu.Lock()
 	defer m.mu.Unlock()

fs2/fs2.go

@@ -4,6 +4,7 @@ import (
 	"errors"
 	"fmt"
 	"os"
+	"path/filepath"
 	"strings"
 
 	"github.com/opencontainers/cgroups"
@@ -83,6 +84,18 @@ func (m *Manager) Apply(pid int) error {
 	return nil
 }
 
+// AddPid adds a process with a given pid to an existing cgroup.
+// The subcgroup argument is either empty, or a path relative to
+// a cgroup under under the manager's cgroup.
+func (m *Manager) AddPid(subcgroup string, pid int) error {
+	path := filepath.Join(m.dirPath, subcgroup)
+	if !strings.HasPrefix(path, m.dirPath) {
+		return fmt.Errorf("bad sub cgroup path: %s", subcgroup)
+	}
+
+	return cgroups.WriteCgroupProc(path, pid)
+}
+
 func (m *Manager) GetPids() ([]int, error) {
 	return cgroups.GetPids(m.dirPath)
 }

systemd/common.go

@@ -6,6 +6,7 @@ import (
 	"fmt"
 	"math"
 	"os"
+	"path"
 	"strconv"
 	"strings"
 	"sync"
@@ -208,6 +209,20 @@ func stopUnit(cm *dbusConnManager, unitName string) error {
 	return nil
 }
 
+func addPid(cm *dbusConnManager, unitName, subcgroup string, pid int) error {
+	absSubcgroup := subcgroup
+	if !path.IsAbs(absSubcgroup) {
+		absSubcgroup = "/" + subcgroup
+	}
+	if absSubcgroup != path.Clean(absSubcgroup) {
+		return fmt.Errorf("bad sub cgroup path: %s", subcgroup)
+	}
+
+	return cm.retryOnDisconnect(func(c *systemdDbus.Conn) error {
+		return c.AttachProcessesToUnit(context.TODO(), unitName, absSubcgroup, []uint32{uint32(pid)})
+	})
+}
+
 func resetFailedUnit(cm *dbusConnManager, name string) error {
 	return cm.retryOnDisconnect(func(c *systemdDbus.Conn) error {
 		return c.ResetFailedUnitContext(context.TODO(), name)

systemd/v1.go

@@ -215,6 +215,26 @@ func (m *LegacyManager) Apply(pid int) error {
 	return nil
 }
 
+// AddPid adds a process with a given pid to an existing cgroup.
+// The subcgroup argument is either empty, or a path relative to
+// a cgroup under under the manager's cgroup.
+func (m *LegacyManager) AddPid(subcgroup string, pid int) error {
+	m.mu.Lock()
+	defer m.mu.Unlock()
+
+	if err := addPid(m.dbus, getUnitName(m.cgroups), subcgroup, pid); err != nil {
+		return err
+	}
+	// systemd only joins controllers it knows; use cgroupfs for the rest.
+	fsMgr, err := fs.NewManager(m.cgroups, m.paths)
+	if err != nil {
+		return err
+	}
+	fsMgr.AddPid(subcgroup, pid)
+
+	return nil
+}
+
 func (m *LegacyManager) Destroy() error {
 	m.mu.Lock()
 	defer m.mu.Unlock()

systemd/v2.go

@@ -383,6 +383,16 @@ func cgroupFilesToChown() ([]string, error) {
 	return filesToChown, nil
 }
 
+// AddPid adds a process with a given pid to an existing cgroup.
+// The subcgroup argument is either empty, or a path relative to
+// a cgroup under under the manager's cgroup.
+func (m *UnifiedManager) AddPid(subcgroup string, pid int) error {
+	m.mu.Lock()
+	defer m.mu.Unlock()
+
+	return addPid(m.dbus, getUnitName(m.cgroups), subcgroup, pid)
+}
+
 func (m *UnifiedManager) Destroy() error {
 	m.mu.Lock()
 	defer m.mu.Unlock()