Test failures on Ubuntu containers (`no cgroup mount found in mountinfo`)

[LocutusOfBorg]

Hello, some tests are failing when building new version under Ubuntu.
I can presume that systemd is not installed into the build environment, but maybe it is shown as running because it runs on the host?
https://launchpadlibrarian.net/827407202/buildlog_ubuntu-resolute-amd64.golang-github-opencontainers-cgroups_0.0.4-2_BUILDING.txt.gz

=== RUN TestInvalidCgroupPath
paths_test.go:19: couldn't get cgroup root: no cgroup mount found in mountinfo
--- FAIL: TestInvalidCgroupPath (0.00s)
=== RUN TestTryDefaultCgroupRoot
paths_test.go:102: tryDefaultCgroupRoot: want "/sys/fs/cgroup", got ""
--- FAIL: TestTryDefaultCgroupRoot (0.00s)
=== RUN TestPidsSetMax
--- PASS: TestPidsSetMax (0.00s)
=== RUN TestPidsSetUnlimited
--- PASS: TestPidsSetUnlimited (0.00s)
=== RUN TestPidsStats
--- PASS: TestPidsStats (0.00s)
=== RUN TestPidsStatsUnlimited
--- PASS: TestPidsStatsUnlimited (0.00s)
FAIL
FAIL github.com/opencontainers/cgroups/fs 0.075s
=== RUN TestParseCgroupFromReader
--- PASS: TestParseCgroupFromReader (0.00s)
=== RUN TestDefaultDirPath
defaultpath_test.go:52: need cgroupv2

? github.com/opencontainers/cgroups/internal/path [no test files]
=== RUN TestNilResources
manager_test.go:40: no cgroup mount found in mountinfo
--- FAIL: TestNilResources (0.00s)
=== RUN TestNilResourcesSystemd
manager_test.go:24: requires systemd
--- SKIP: TestNilResourcesSystemd (0.00s)
FAIL
FAIL github.com/opencontainers/cgroups/manager 0.004s
=== RUN TestRangeToBits

[LocutusOfBorg]

Looks like it fails also in Debian under pbuilder environment

golang-github-opencontainers-cgroups_0.0.4-2_amd64.build.txt

[siretart]

sounds like this function is not detecting the Ubuntu/Debian build environment correctly:

cgroups/utils.go

Lines 35 to 51 in e0c56cb

	func IsCgroup2UnifiedMode() bool {
	isUnifiedOnce.Do(func() {
	var st unix.Statfs_t
	err := unix.Statfs(unifiedMountpoint, &st)
	if err != nil {
	level := logrus.WarnLevel
	if os.IsNotExist(err) && userns.RunningInUserNS() {
	// For rootless containers, sweep it under the rug.
	level = logrus.DebugLevel
	}
	logrus.StandardLogger().Logf(level,
	"statfs %s: %v; assuming cgroup v1", unifiedMountpoint, err)
	}
	isUnified = st.Type == unix.CGROUP2_SUPER_MAGIC
	})
	return isUnified
	}