cgroups: premount cgroups on cgroup2-only systems

Fixes: #3183
Cc: Thomas Moschny <thomas.moschny@gmx.de>
Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>

src/lxc/cgroups/cgfsng.c

@@ -1826,11 +1826,24 @@ __cgfsng_ops static bool cgfsng_mount(struct cgroup_ops *ops,
 		wants_force_mount = true;
 	}
 
-	if (!wants_force_mount){
+	if (!wants_force_mount) {
 		if (!lxc_list_empty(&handler->conf->keepcaps))
 			wants_force_mount = !in_caplist(CAP_SYS_ADMIN, &handler->conf->keepcaps);
 		else
 			wants_force_mount = in_caplist(CAP_SYS_ADMIN, &handler->conf->caps);
+
+		/*
+		 * Most recent distro versions currently have init system that
+		 * do support cgroup2 but do not mount it by default unless
+		 * explicitly told so even if the host is cgroup2 only. That
+		 * means they often will fail to boot. Fix this by pre-mounting
+		 * cgroup2 by default. We will likely need to be doing this a
+		 * few years until all distros have switched over to cgroup2 at
+		 * which point we can safely assume that their init systems
+		 * will mount it themselves.
+		 */
+		if (pure_unified_layout(ops))
+			wants_force_mount = true;
 	}
 
 	has_cgns = cgns_supported();