cgroupManager: add/use AddPid

Commit df3fa11 adds a second call to cgroupManager.Apply() method in
order to add a second pid to the cgroup.

While I admit I don't understand the reason why it was added, it seems
that going through all the steps (configure cgroup parameters, figure
out paths to all controllers, create a systemd unit) twice is excessive
and should not be done.

Add and use AddPid method for adding the second PID to the cgroup.

Signed-off-by: Kir Kolyshkin <[email protected]>

libcontainer/cgroups/cgroups.go

@@ -12,6 +12,9 @@ type Manager interface {
 	// Applies cgroup configuration to the process with the specified pid
 	Apply(pid int) error
 
+	// AddPid adds another process to an already created and configured
+	AddPid(pid int) error
+
 	// Returns the PIDs inside the cgroup set
 	GetPids() ([]int, error)
 

libcontainer/cgroups/fs/apply_raw.go

@@ -192,6 +192,13 @@ func (m *manager) Apply(pid int) (err error) {
 	return nil
 }
 
+func (m *manager) AddPid(pid int) error {
+	if m.paths == nil {
+		return errors.New("cgroup is not yet configured")
+	}
+	return cgroups.EnterPid(m.paths, pid)
+}
+
 func (m *manager) Destroy() error {
 	if m.cgroups == nil || m.cgroups.Paths != nil {
 		return nil

libcontainer/cgroups/fs2/fs2.go

@@ -91,6 +91,13 @@ func (m *manager) Apply(pid int) error {
 	return nil
 }
 
+func (m *manager) AddPid(pid int) error {
+	if m.dirPath != "" {
+		return errors.New("cgroup is not yet configured")
+	}
+	return cgroups.WriteCgroupProc(m.dirPath, pid)
+}
+
 func (m *manager) GetPids() ([]int, error) {
 	return cgroups.GetPids(m.dirPath)
 }

libcontainer/cgroups/systemd/v1.go

@@ -226,6 +226,13 @@ func (m *legacyManager) Apply(pid int) error {
 	return nil
 }
 
+func (m *legacyManager) AddPid(pid int) error {
+	if m.paths == nil {
+		return errors.New("cgroup is not yet configured")
+	}
+	return cgroups.EnterPid(m.paths, pid)
+}
+
 func (m *legacyManager) Destroy() error {
 	if m.cgroups.Paths != nil {
 		return nil

libcontainer/cgroups/systemd/v2.go

@@ -178,6 +178,13 @@ func (m *unifiedManager) Apply(pid int) error {
 	return nil
 }
 
+func (m *unifiedManager) AddPid(pid int) error {
+	if m.path == "" {
+		return errors.New("cgroup is not yet configured")
+	}
+	return cgroups.WriteCgroupProc(m.path, pid)
+}
+
 func (m *unifiedManager) Destroy() error {
 	if m.cgroups.Paths != nil {
 		return nil

libcontainer/process_linux.go

@@ -348,7 +348,7 @@ func (p *initProcess) start() (retErr error) {
 	p.setExternalDescriptors(fds)
 	// Do this before syncing with child so that no children
 	// can escape the cgroup
-	if err := p.manager.Apply(childPid); err != nil {
+	if err := p.manager.AddPid(childPid); err != nil {
 		return newSystemErrorWithCause(err, "applying cgroup configuration for process")
 	}
 	if p.intelRdtManager != nil {