cgroup: change cg_unified() to possibly return errors again

We use our cgroup APIs in various contexts, including from our libraries
sd-login, sd-bus. As we don#t control those environments we can't rely
that the unified cgroup setup logic succeeds, and hence really shouldn't
assert on it.

This more or less reverts 415fc41cea.

src/nspawn/nspawn-cgroup.c

@@ -78,9 +78,12 @@ int sync_cgroup(pid_t pid, CGroupUnified unified_requested, uid_t arg_uid_shift)
         char tree[] = "/tmp/unifiedXXXXXX", pid_string[DECIMAL_STR_MAX(pid) + 1];
         bool undo_mount = false;
         const char *fn;
-        int r;
+        int r, unified_controller;
 
-        if (cg_unified(SYSTEMD_CGROUP_CONTROLLER) == (unified_requested >= CGROUP_UNIFIED_SYSTEMD))
+        unified_controller = cg_unified(SYSTEMD_CGROUP_CONTROLLER);
+        if (unified_controller < 0)
+                return log_error_errno(unified_controller, "Failed to determine whether the systemd hierarchy is unified: %m");
+        if ((unified_controller > 0) == (unified_requested >= CGROUP_UNIFIED_SYSTEMD))
                 return 0;
 
         /* When the host uses the legacy cgroup setup, but the
@@ -96,7 +99,7 @@ int sync_cgroup(pid_t pid, CGroupUnified unified_requested, uid_t arg_uid_shift)
         if (!mkdtemp(tree))
                 return log_error_errno(errno, "Failed to generate temporary mount point for unified hierarchy: %m");
 
-        if (cg_unified(SYSTEMD_CGROUP_CONTROLLER))
+        if (unified_controller > 0)
                 r = mount_verbose(LOG_ERR, "cgroup", tree, "cgroup",
                                   MS_NOSUID|MS_NOEXEC|MS_NODEV, "none,name=systemd,xattr");
         else
@@ -150,7 +153,10 @@ int create_subcgroup(pid_t pid, CGroupUnified unified_requested) {
         if (unified_requested == CGROUP_UNIFIED_NONE)
                 return 0;
 
-        if (!cg_unified(SYSTEMD_CGROUP_CONTROLLER))
+        r = cg_unified(SYSTEMD_CGROUP_CONTROLLER);
+        if (r < 0)
+                return log_error_errno(r, "Failed to determine whether the systemd controller is unified: %m");
+        if (r == 0)
                 return 0;
 
         r = cg_mask_supported(&supported);

src/nspawn/nspawn-mount.c

@@ -991,7 +991,10 @@ static int mount_legacy_cgns_supported(
                         return r;
         }
 
-        if (cg_all_unified())
+        r = cg_all_unified();
+        if (r < 0)
+                return r;
+        if (r > 0)
                 goto skip_controllers;
 
         controllers = set_new(&string_hash_ops);
@@ -1094,7 +1097,10 @@ static int mount_legacy_cgns_unsupported(
                         return r;
         }
 
-        if (cg_all_unified())
+        r = cg_all_unified();
+        if (r < 0)
+                return r;
+        if (r > 0)
                 goto skip_controllers;
 
         controllers = set_new(&string_hash_ops);

src/nspawn/nspawn.c

@@ -333,7 +333,10 @@ static int detect_unified_cgroup_hierarchy(const char *directory) {
         }
 
         /* Otherwise inherit the default from the host system */
-        if (cg_all_unified()) {
+        r = cg_all_unified();
+        if (r < 0)
+                return log_error_errno(r, "Failed to determine whether we are in all unified mode.");
+        if (r > 0) {
                 /* Unified cgroup hierarchy support was added in 230. Unfortunately the detection
                  * routine only detects 231, so we'll have a false negative here for 230. */
                 r = systemd_installation_has_version(directory, 230);
@@ -343,7 +346,7 @@ static int detect_unified_cgroup_hierarchy(const char *directory) {
                         arg_unified_cgroup_hierarchy = CGROUP_UNIFIED_ALL;
                 else
                         arg_unified_cgroup_hierarchy = CGROUP_UNIFIED_NONE;
-        } else if (cg_unified(SYSTEMD_CGROUP_CONTROLLER)) {
+        } else if (cg_unified(SYSTEMD_CGROUP_CONTROLLER) > 0) {
                 /* Mixed cgroup hierarchy support was added in 233 */
                 r = systemd_installation_has_version(directory, 233);
                 if (r < 0)