wip hybrid mode (no work)

client/lib/cgutil/cgutil_linux.go

@@ -13,12 +13,28 @@ import (
 	lcc "github.com/opencontainers/runc/libcontainer/configs"
 )
 
+// In CI testing there are not yet runners available with v2 as the default, but
+// we can take advantage of hybrid mode and pretend like the system is in v2 mode
+// by setting NOMAD_CGROUP_V2_ROOT.
+//
+// The downside is we can no longer refer to IsCgroup2UnifiedMode anywhere in Nomad
+// code, and docker / other drivers will also need to be reconfigured.
+func init() {
+	if override := os.Getenv("NOMAD_CGROUP_V2_ROOT"); override != "" {
+		CgroupRoot = override
+		UseV2 = true
+	} else {
+		UseV2 = cgroups.IsCgroup2UnifiedMode()
+	}
+	fmt.Println("INIT, root:", CgroupRoot, "v2:", UseV2)
+}
+
 // UseV2 indicates whether only cgroups.v2 is enabled. If cgroups.v2 is not
 // enabled or is running in hybrid mode with cgroups.v1, Nomad will make use of
 // cgroups.v1
 //
 // This is a read-only value.
-var UseV2 = cgroups.IsCgroup2UnifiedMode()
+var UseV2 bool
 
 // GetCgroupParent returns the mount point under the root cgroup in which Nomad
 // will create cgroups. If parent is not set, an appropriate name for the version

client/lib/cgutil/cpuset_manager.go

@@ -11,11 +11,13 @@ import (
 	"github.com/hashicorp/nomad/nomad/structs"
 )
 
-const (
-	// CgroupRoot is hard-coded in the cgroups specification.
-	// It only applies to linux but helpers have references to it in driver(s).
-	CgroupRoot = "/sys/fs/cgroup"
-)
+// CgroupRoot is encoded in the cgroups specification.
+//
+// It only applies to Linux but there may be references to it in drivers.
+//
+// This value is read only, but may be modified by setting NOMAD_CGROUP_V2_ROOT
+// for testing purposes (i.e. undocumented feature).
+var CgroupRoot = "/sys/fs/cgroup"
 
 // CpusetManager is used to setup cpuset cgroups for each task.
 type CpusetManager interface {

client/lib/cgutil/cpuset_manager_v2.go

@@ -4,6 +4,7 @@ package cgutil
 
 import (
 	"context"
+	"fmt"
 	"os"
 	"path/filepath"
 	"strings"
@@ -280,12 +281,15 @@ func (c *cpusetManagerV2) write(id identity, set cpuset.CPUSet) {
 // ensureParentCgroup will create parent cgroup for the manager if it does not
 // exist yet. No PIDs are added to any cgroup yet.
 func (c *cpusetManagerV2) ensureParent() error {
+	fmt.Println("p parentAbs:", c.parentAbs)
 	mgr, err := fs2.NewManager(nil, c.parentAbs, rootless)
 	if err != nil {
+		fmt.Println("A err:", err)
 		return err
 	}
 
 	if err = mgr.Apply(CreationPID); err != nil {
+		fmt.Println("B err:", err)
 		return err
 	}