Refactor systemd connection initialization

1. Use the simpler IsRunningSystemd for checking
if systemd is running.
2. Lazy initialization the systemd dbus connection
in the systemd package.

Signed-off-by: Mrunal Patel <mrunalp@gmail.com>

libcontainer/cgroups/systemd/apply_systemd.go

@@ -71,8 +71,9 @@ const (
 )
 
 var (
-	connLock sync.Mutex
-	theConn  *systemdDbus.Conn
+	connOnce sync.Once
+	connDbus *systemdDbus.Conn
+	connErr  error
 )
 
 func newProp(name string, units interface{}) systemdDbus.Property {
@@ -97,22 +98,13 @@ func IsRunningSystemd() bool {
 	return fi.IsDir()
 }
 
-func UseSystemd() bool {
-	if !IsRunningSystemd() {
-		return false
-	}
-
-	connLock.Lock()
-	defer connLock.Unlock()
-
-	if theConn == nil {
-		var err error
-		theConn, err = systemdDbus.New()
-		if err != nil {
-			return false
-		}
-	}
-	return true
+// getDbusConnection lazy initializes systemd dbus connection
+// and returns it
+func getDbusConnection() (*systemdDbus.Conn, error) {
+	connOnce.Do(func() {
+		connDbus, connErr = systemdDbus.New()
+	})
+	return connDbus, connErr
 }
 
 func NewSystemdCgroupsManager() (func(config *configs.Cgroup, paths map[string]string) cgroups.Manager, error) {
@@ -245,8 +237,13 @@ func (m *LegacyManager) Apply(pid int) error {
 		}
 	}
 
+	dbusConnection, err := getDbusConnection()
+	if err != nil {
+		return err
+	}
+
 	statusChan := make(chan string, 1)
-	if _, err := theConn.StartTransientUnit(unitName, "replace", properties, statusChan); err == nil {
+	if _, err := dbusConnection.StartTransientUnit(unitName, "replace", properties, statusChan); err == nil {
 		select {
 		case <-statusChan:
 		case <-time.After(time.Second):
@@ -280,9 +277,13 @@ func (m *LegacyManager) Destroy() error {
 	if m.Cgroups.Paths != nil {
 		return nil
 	}
+	dbusConnection, err := getDbusConnection()
+	if err != nil {
+		return err
+	}
 	m.mu.Lock()
 	defer m.mu.Unlock()
-	theConn.StopUnit(getUnitName(m.Cgroups), "replace", nil)
+	dbusConnection.StopUnit(getUnitName(m.Cgroups), "replace", nil)
 	if err := cgroups.RemovePaths(m.Paths); err != nil {
 		return err
 	}

libcontainer/cgroups/systemd/unified_hierarchy.go

@@ -136,8 +136,13 @@ func (m *UnifiedManager) Apply(pid int) error {
 		}
 	}
 
+	dbusConnection, err := getDbusConnection()
+	if err != nil {
+		return err
+	}
+
 	statusChan := make(chan string, 1)
-	if _, err := theConn.StartTransientUnit(unitName, "replace", properties, statusChan); err == nil {
+	if _, err := dbusConnection.StartTransientUnit(unitName, "replace", properties, statusChan); err == nil {
 		select {
 		case <-statusChan:
 		case <-time.After(time.Second):
@@ -171,9 +176,13 @@ func (m *UnifiedManager) Destroy() error {
 	if m.Cgroups.Paths != nil {
 		return nil
 	}
+	dbusConnection, err := getDbusConnection()
+	if err != nil {
+		return err
+	}
 	m.mu.Lock()
 	defer m.mu.Unlock()
-	theConn.StopUnit(getUnitName(m.Cgroups), "replace", nil)
+	dbusConnection.StopUnit(getUnitName(m.Cgroups), "replace", nil)
 	if err := cgroups.RemovePaths(m.Paths); err != nil {
 		return err
 	}

libcontainer/integration/exec_test.go

@@ -506,7 +506,7 @@ func TestFreeze(t *testing.T) {
 }
 
 func TestSystemdFreeze(t *testing.T) {
-	if !systemd.UseSystemd() {
+	if !systemd.IsRunningSystemd() {
 		t.Skip("Systemd is unsupported")
 	}
 	testFreeze(t, true)
@@ -563,7 +563,7 @@ func TestCpuShares(t *testing.T) {
 }
 
 func TestCpuSharesSystemd(t *testing.T) {
-	if !systemd.UseSystemd() {
+	if !systemd.IsRunningSystemd() {
 		t.Skip("Systemd is unsupported")
 	}
 	testCpuShares(t, true)
@@ -598,7 +598,7 @@ func TestPids(t *testing.T) {
 }
 
 func TestPidsSystemd(t *testing.T) {
-	if !systemd.UseSystemd() {
+	if !systemd.IsRunningSystemd() {
 		t.Skip("Systemd is unsupported")
 	}
 	testPids(t, true)
@@ -684,7 +684,7 @@ func TestRunWithKernelMemory(t *testing.T) {
 }
 
 func TestRunWithKernelMemorySystemd(t *testing.T) {
-	if !systemd.UseSystemd() {
+	if !systemd.IsRunningSystemd() {
 		t.Skip("Systemd is unsupported")
 	}
 	testRunWithKernelMemory(t, true)

utils_linux.go

@@ -47,7 +47,7 @@ func loadFactory(context *cli.Context) (libcontainer.Factory, error) {
 		cgroupManager = libcontainer.RootlessCgroupfs
 	}
 	if context.GlobalBool("systemd-cgroup") {
-		if systemd.UseSystemd() {
+		if systemd.IsRunningSystemd() {
 			cgroupManager = libcontainer.SystemdCgroups
 		} else {
 			return nil, fmt.Errorf("systemd cgroup flag passed, but systemd support for managing cgroups is not available")