Warn if cgroups-v1

Podman v5 will not support cgroups-v1. This commit will print a warning
if it detects a cgroups-v1 system.

This warning is patched out for RHEL 9 builds as cgroups-v1 will still
be supported on RHEL 9 systems.

Resolves: https://issues.redhat.com/browse/RUN-1957

[NO NEW TESTS NEEDED]

Co-authored-by: Giuseppe Scrivano <gscrivan@redhat.com>
Signed-off-by: Lokesh Mandvekar <lsm5@redhat.com>

cmd/podman/main.go

@@ -7,6 +7,7 @@ import (
 	"strconv"
 	"strings"
 
+	"github.com/containers/common/pkg/cgroups"
 	_ "github.com/containers/podman/v4/cmd/podman/completion"
 	_ "github.com/containers/podman/v4/cmd/podman/farm"
 	_ "github.com/containers/podman/v4/cmd/podman/generate"
@@ -40,6 +41,11 @@ func main() {
 		return
 	}
 
+	cgroupv2, _ := cgroups.IsCgroup2UnifiedMode()
+	if !cgroupv2 {
+		logrus.Warnf("WARNING! You are using cgroups-v1 which is not supported with Podman v5 and will be removed in a future version. Consider switching to cgroups-v2.")
+	}
+
 	if filepath.Base(os.Args[0]) == registry.PodmanSh ||
 		(len(os.Args[0]) > 0 && filepath.Base(os.Args[0][1:]) == registry.PodmanSh) {
 		shell := strings.TrimPrefix(os.Args[0], "-")

rpm/podman.spec

@@ -211,6 +211,11 @@ when `%{_bindir}/%{name}sh` is set as a login shell or set as os.Args[0].
 %autosetup -Sgit -n %{name}-%{version_no_tilde}
 sed -i 's;@@PODMAN@@\;$(BINDIR);@@PODMAN@@\;%{_bindir};' Makefile
 
+# cgroups-v1 is supported on rhel9
+%if 0%{?rhel} == 9
+sed -i '/WARNING! You are using cgroups-v1/d' cmd/podman/main.go
+%endif
+
 # These changes are only meant for copr builds
 %if %{defined copr_build}
 # podman --version should show short sha