libct/cg/sd: add dbus manager

[@kolyshkin: documentation nits]

Signed-off-by: Shiming Zhang <wzshiming@foxmail.com>
Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>

libcontainer/cgroups/systemd/common.go

@@ -2,7 +2,6 @@ package systemd
 
 import (
 	"bufio"
-	"context"
 	"fmt"
 	"math"
 	"os"
@@ -29,10 +28,6 @@ const (
 )
 
 var (
-	connOnce sync.Once
-	connDbus *systemdDbus.Conn
-	connErr  error
-
 	versionOnce sync.Once
 	version     int
 
@@ -292,19 +287,6 @@ func generateDeviceProperties(rules []*devices.Rule) ([]systemdDbus.Property, er
 	return properties, nil
 }
 
-// getDbusConnection lazy initializes systemd dbus connection
-// and returns it
-func getDbusConnection(rootless bool) (*systemdDbus.Conn, error) {
-	connOnce.Do(func() {
-		if rootless {
-			connDbus, connErr = NewUserSystemdDbus()
-		} else {
-			connDbus, connErr = systemdDbus.NewWithContext(context.TODO())
-		}
-	})
-	return connDbus, connErr
-}
-
 func newProp(name string, units interface{}) systemdDbus.Property {
 	return systemdDbus.Property{
 		Name:  name,

libcontainer/cgroups/systemd/dbus.go

@@ -0,0 +1,59 @@
+// +build linux
+
+package systemd
+
+import (
+	"context"
+	"sync"
+
+	systemdDbus "github.com/coreos/go-systemd/v22/dbus"
+)
+
+type dbusConnManager struct {
+	conn     *systemdDbus.Conn
+	rootless bool
+	sync.RWMutex
+}
+
+// newDbusConnManager initializes systemd dbus connection manager.
+func newDbusConnManager(rootless bool) *dbusConnManager {
+	return &dbusConnManager{
+		rootless: rootless,
+	}
+}
+
+// getConnection lazily initializes and returns systemd dbus connection.
+func (d *dbusConnManager) getConnection() (*systemdDbus.Conn, error) {
+	// In the case where d.conn != nil
+	// Use the read lock the first time to ensure
+	// that Conn can be acquired at the same time.
+	d.RLock()
+	if conn := d.conn; conn != nil {
+		d.RUnlock()
+		return conn, nil
+	}
+	d.RUnlock()
+
+	// In the case where d.conn == nil
+	// Use write lock to ensure that only one
+	// will be created
+	d.Lock()
+	defer d.Unlock()
+	if conn := d.conn; conn != nil {
+		return conn, nil
+	}
+
+	conn, err := d.newConnection()
+	if err != nil {
+		return nil, err
+	}
+	d.conn = conn
+	return conn, nil
+}
+
+func (d *dbusConnManager) newConnection() (*systemdDbus.Conn, error) {
+	if d.rootless {
+		return NewUserSystemdDbus()
+	}
+	return systemdDbus.NewWithContext(context.TODO())
+}

libcontainer/cgroups/systemd/v1.go

@@ -20,12 +20,14 @@ type legacyManager struct {
 	mu      sync.Mutex
 	cgroups *configs.Cgroup
 	paths   map[string]string
+	dbus    *dbusConnManager
 }
 
 func NewLegacyManager(cg *configs.Cgroup, paths map[string]string) cgroups.Manager {
 	return &legacyManager{
 		cgroups: cg,
 		paths:   paths,
+		dbus:    newDbusConnManager(false),
 	}
 }
 
@@ -164,7 +166,7 @@ func (m *legacyManager) Apply(pid int) error {
 	properties = append(properties,
 		newProp("DefaultDependencies", false))
 
-	dbusConnection, err := getDbusConnection(false)
+	dbusConnection, err := m.dbus.getConnection()
 	if err != nil {
 		return err
 	}
@@ -208,7 +210,7 @@ func (m *legacyManager) Destroy() error {
 	m.mu.Lock()
 	defer m.mu.Unlock()
 
-	dbusConnection, err := getDbusConnection(false)
+	dbusConnection, err := m.dbus.getConnection()
 	if err != nil {
 		return err
 	}
@@ -341,7 +343,7 @@ func (m *legacyManager) Set(container *configs.Config) error {
 	if container.Cgroups.Resources.Unified != nil {
 		return cgroups.ErrV1NoUnified
 	}
-	dbusConnection, err := getDbusConnection(false)
+	dbusConnection, err := m.dbus.getConnection()
 	if err != nil {
 		return err
 	}

libcontainer/cgroups/systemd/v2.go

@@ -26,6 +26,7 @@ type unifiedManager struct {
 	// path is like "/sys/fs/cgroup/user.slice/user-1001.slice/session-1.scope"
 	path     string
 	rootless bool
+	dbus     *dbusConnManager
 }
 
 func NewUnifiedManager(config *configs.Cgroup, path string, rootless bool) cgroups.Manager {
@@ -33,6 +34,7 @@ func NewUnifiedManager(config *configs.Cgroup, path string, rootless bool) cgrou
 		cgroups:  config,
 		path:     path,
 		rootless: rootless,
+		dbus:     newDbusConnManager(rootless),
 	}
 }
 
@@ -279,7 +281,7 @@ func (m *unifiedManager) Apply(pid int) error {
 	properties = append(properties,
 		newProp("DefaultDependencies", false))
 
-	dbusConnection, err := getDbusConnection(m.rootless)
+	dbusConnection, err := m.dbus.getConnection()
 	if err != nil {
 		return err
 	}
@@ -305,7 +307,7 @@ func (m *unifiedManager) Destroy() error {
 	m.mu.Lock()
 	defer m.mu.Unlock()
 
-	dbusConnection, err := getDbusConnection(m.rootless)
+	dbusConnection, err := m.dbus.getConnection()
 	if err != nil {
 		return err
 	}
@@ -344,7 +346,7 @@ func (m *unifiedManager) getSliceFull() (string, error) {
 	}
 
 	if m.rootless {
-		dbusConnection, err := getDbusConnection(m.rootless)
+		dbusConnection, err := m.dbus.getConnection()
 		if err != nil {
 			return "", err
 		}
@@ -427,7 +429,7 @@ func (m *unifiedManager) GetStats() (*cgroups.Stats, error) {
 }
 
 func (m *unifiedManager) Set(container *configs.Config) error {
-	dbusConnection, err := getDbusConnection(m.rootless)
+	dbusConnection, err := m.dbus.getConnection()
 	if err != nil {
 		return err
 	}