Fix error message on podman stats on cgroups v1 rootless environments

podman stats does not work in rootless environments with cgroups V1.
Fix error message and document this fact.

Signed-off-by: Daniel J Walsh <dwalsh@redhat.com>

cmd/podman/stats.go

@@ -134,9 +134,13 @@ func statsCmd(c *cliconfig.StatsValues) error {
 		initialStats, err := ctr.GetContainerStats(&libpod.ContainerStats{})
 		if err != nil {
 			// when doing "all", dont worry about containers that are not running
-			if c.All && errors.Cause(err) == define.ErrCtrRemoved || errors.Cause(err) == define.ErrNoSuchCtr || errors.Cause(err) == define.ErrCtrStateInvalid {
+			cause := errors.Cause(err)
+			if c.All && (cause == define.ErrCtrRemoved || cause == define.ErrNoSuchCtr || cause == define.ErrCtrStateInvalid) {
 				continue
 			}
+			if cause == cgroups.ErrCgroupV1Rootless {
+				err = cause
+			}
 			return err
 		}
 		containerStats[ctr.ID()] = initialStats

docs/podman-stats.1.md

@@ -9,6 +9,10 @@ podman\-stats - Display a live stream of 1 or more containers' resource usage st
 ## DESCRIPTION
 Display a live stream of one or more containers' resource usage statistics
 
+Note:  Podman stats will not work in rootless environments that use CGroups V1.
+Podman stats relies on CGroup information for statistics, and CGroup v1 is not
+supported for rootless use cases.
+
 ## OPTIONS
 
 **--all**, **-a**
@@ -69,14 +73,14 @@ a9f807ffaacd   frosty_hodgkin   --      3.092MB / 16.7GB    0.02%   -- / --   --
 # podman stats --no-stream --format=json a9f80
 [
     {
-        "id": "a9f807ffaacd",
+	"id": "a9f807ffaacd",
-        "name": "frosty_hodgkin",
+	"name": "frosty_hodgkin",
-        "cpu_percent": "--",
+	"cpu_percent": "--",
-        "mem_usage": "3.092MB / 16.7GB",
+	"mem_usage": "3.092MB / 16.7GB",
-        "mem_percent": "0.02%",
+	"mem_percent": "0.02%",
-        "netio": "-- / --",
+	"netio": "-- / --",
-        "blocki": "-- / --",
+	"blocki": "-- / --",
-        "pids": "2"
+	"pids": "2"
     }
 ]
 ```

libpod/runtime_pod_linux.go

@@ -204,7 +204,7 @@ func (r *Runtime) removePod(ctx context.Context, p *Pod, removeCtrs, force bool)
 		// Get the conmon CGroup
 		conmonCgroupPath := filepath.Join(p.state.CgroupPath, "conmon")
 		conmonCgroup, err := cgroups.Load(conmonCgroupPath)
-		if err != nil && err != cgroups.ErrCgroupDeleted {
+		if err != nil && err != cgroups.ErrCgroupDeleted && err != cgroups.ErrCgroupV1Rootless {
 			removalErr = errors.Wrapf(err, "error retrieving pod %s conmon cgroup %s", p.ID(), conmonCgroupPath)
 		}
 
@@ -266,7 +266,7 @@ func (r *Runtime) removePod(ctx context.Context, p *Pod, removeCtrs, force bool)
 			// hard - instead, just log errors.
 			conmonCgroupPath := filepath.Join(p.state.CgroupPath, "conmon")
 			conmonCgroup, err := cgroups.Load(conmonCgroupPath)
-			if err != nil && err != cgroups.ErrCgroupDeleted {
+			if err != nil && err != cgroups.ErrCgroupDeleted && err != cgroups.ErrCgroupV1Rootless {
 				if removalErr == nil {
 					removalErr = errors.Wrapf(err, "error retrieving pod %s conmon cgroup", p.ID())
 				} else {
@@ -283,7 +283,7 @@ func (r *Runtime) removePod(ctx context.Context, p *Pod, removeCtrs, force bool)
 				}
 			}
 			cgroup, err := cgroups.Load(p.state.CgroupPath)
-			if err != nil && err != cgroups.ErrCgroupDeleted {
+			if err != nil && err != cgroups.ErrCgroupDeleted && err != cgroups.ErrCgroupV1Rootless {
 				if removalErr == nil {
 					removalErr = errors.Wrapf(err, "error retrieving pod %s cgroup", p.ID())
 				} else {

pkg/cgroups/cgroups.go

@@ -10,6 +10,7 @@ import (
 	"strconv"
 	"strings"
 
+	"github.com/containers/libpod/pkg/rootless"
 	systemdDbus "github.com/coreos/go-systemd/dbus"
 	"github.com/godbus/dbus"
 	spec "github.com/opencontainers/runtime-spec/specs-go"
@@ -19,7 +20,9 @@ import (
 
 var (
 	// ErrCgroupDeleted means the cgroup was deleted
-	ErrCgroupDeleted = errors.New("cgroups: cgroup deleted")
+	ErrCgroupDeleted = errors.New("cgroup deleted")
+	// ErrCgroupV1Rootless means the cgroup v1 were attempted to be used in rootless environmen
+	ErrCgroupV1Rootless = errors.New("no support for CGroups V1 in rootless environments")
 )
 
 // CgroupControl controls a cgroup hierarchy
@@ -339,6 +342,9 @@ func Load(path string) (*CgroupControl, error) {
 			p := control.getCgroupv1Path(name)
 			if _, err := os.Stat(p); err != nil {
 				if os.IsNotExist(err) {
+					if rootless.IsRootless() {
+						return nil, ErrCgroupV1Rootless
+					}
 					// compatible with the error code
 					// used by containerd/cgroups
 					return nil, ErrCgroupDeleted