fix: cgroups: check for systemd init when needed

When `CanUseCgroups` is called, with systemd as the intended cgroups
manager, make sure that a systemd init is actually running.

This fixes flows where Singularity run nested inside another
container, that doesn't provide a systemd init or mount the host
`/run` directory into the container.

Fixes #3536

CHANGELOG.md

@@ -18,6 +18,8 @@
   native SIF, so environment sourcing does not fail.
 - Fix the Makefile generated by `mconfig -b` to work when the selected build
   directory is not a subdirectory of the source code.
+- Check for existence of `/run/systemd/system` when verifying cgroups can be
+  used via systemd manager.
 
 ### New Features & Functionality
 

internal/pkg/cgroups/manager_linux_test.go

@@ -19,6 +19,7 @@ import (
 
 	"github.com/sylabs/singularity/v4/internal/pkg/test"
 	"github.com/sylabs/singularity/v4/internal/pkg/test/tool/require"
+	"github.com/sylabs/singularity/v4/internal/pkg/util/fs"
 )
 
 // This file contains tests that will run under cgroups v1 & v2, and test utility functions.
@@ -55,6 +56,9 @@ func runCgroupfsTests(t *testing.T, tests CgroupTests) {
 
 func runSystemdTests(t *testing.T, tests CgroupTests) {
 	t.Run("systemd", func(t *testing.T) {
+		if !fs.IsDir("/run/systemd/system") {
+			t.Skip("systemd not running as init on this host")
+		}
 		for _, tt := range tests {
 			t.Run(tt.name, func(t *testing.T) {
 				tt.testFunc(t, true)

internal/pkg/cgroups/util.go

@@ -14,6 +14,7 @@ import (
 
 	"github.com/opencontainers/runc/libcontainer/cgroups"
 	lccgroups "github.com/opencontainers/runc/libcontainer/cgroups"
+	"github.com/sylabs/singularity/v4/internal/pkg/util/fs"
 	"github.com/sylabs/singularity/v4/internal/pkg/util/rootless"
 	"github.com/sylabs/singularity/v4/pkg/sylog"
 	"golang.org/x/sys/unix"
@@ -117,12 +118,25 @@ func HasXDGRuntimeDir() (bool, error) {
 }
 
 // CanUseCgroups checks whether it's possible to use the cgroups manager.
+// - Systemd cgroups management requires systemd running as init.
 // - Host root can always use cgroups.
 // - Rootless needs cgroups v2.
 // - Rootless needs systemd manager.
 // - Rootless needs DBUS_SESSION_BUS_ADDRESS and XDG_RUNTIME_DIR set properly.
 // warn controls whether configuration problems preventing use of cgroups will be logged as warnings, or debug messages.
 func CanUseCgroups(systemd bool, warn bool) bool {
+	if systemd {
+		systemdRunning := fs.IsDir("/run/systemd/system")
+		if !systemdRunning {
+			if warn {
+				sylog.Warningf("Cannot use systemd cgroups manager, systemd not running as init on this host.")
+			} else {
+				sylog.Warningf("Cannot use systemd cgroups manager, systemd not running as init on this host.")
+			}
+			return false
+		}
+	}
+
 	uid, err := rootless.Getuid()
 	if err != nil {
 		sylog.Errorf("cannot determine uid: %v", err)