core: simplify cg_[all_]unified()

cg_[all_]unified() test whether a specific controller or all controllers are on
the unified hierarchy.  While what's being asked is a simple binary question,
the callers must assume that the functions may fail any time, which
unnecessarily complicates their usages.  This complication is unnecessary.
Internally, the test result is cached anyway and there are only a few places
where the test actually needs to be performed.

This patch simplifies cg_[all_]unified().

* cg_[all_]unified() are updated to return bool.  If the result can't be
  decided, assertion failure is triggered.  Error handlings from their callers
  are dropped.

* cg_unified_flush() is updated to calculate the new result synchrnously and
  return whether it succeeded or not.  Places which need to flush the test
  result are updated to test for failure.  This ensures that all the following
  cg_[all_]unified() tests succeed.

* Places which expected possible cg_[all_]unified() failures are updated to
  call and test cg_unified_flush() before calling cg_[all_]unified().  This
  includes functions used while setting up mounts during boot and
  manager_setup_cgroup().

src/nspawn/nspawn-cgroup.c

@@ -78,13 +78,9 @@ 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 unified, r;
+        int r;
 
-        unified = cg_unified(SYSTEMD_CGROUP_CONTROLLER);
-        if (unified < 0)
-                return log_error_errno(unified, "Failed to determine whether the unified hierarchy is used: %m");
-
-        if ((unified > 0) == (unified_requested >= CGROUP_UNIFIED_SYSTEMD))
+        if (cg_unified(SYSTEMD_CGROUP_CONTROLLER) == (unified_requested >= CGROUP_UNIFIED_SYSTEMD))
                 return 0;
 
         /* When the host uses the legacy cgroup setup, but the
@@ -100,7 +96,7 @@ 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 (unified)
+        if (cg_unified(SYSTEMD_CGROUP_CONTROLLER))
                 r = mount_verbose(LOG_ERR, "cgroup", tree, "cgroup",
                                   MS_NOSUID|MS_NOEXEC|MS_NODEV, "none,name=systemd,xattr");
         else
@@ -142,7 +138,7 @@ finish:
 int create_subcgroup(pid_t pid, CGroupUnified unified_requested) {
         _cleanup_free_ char *cgroup = NULL;
         const char *child;
-        int unified, r;
+        int r;
         CGroupMask supported;
 
         /* In the unified hierarchy inner nodes may only contain
@@ -154,10 +150,7 @@ int create_subcgroup(pid_t pid, CGroupUnified unified_requested) {
         if (unified_requested == CGROUP_UNIFIED_NONE)
                 return 0;
 
-        unified = cg_unified(SYSTEMD_CGROUP_CONTROLLER);
-        if (unified < 0)
-                return log_error_errno(unified, "Failed to determine whether the unified hierarchy is used: %m");
-        if (unified == 0)
+        if (!cg_unified(SYSTEMD_CGROUP_CONTROLLER))
                 return 0;
 
         r = cg_mask_supported(&supported);

src/nspawn/nspawn-mount.c

@@ -994,7 +994,7 @@ static int mount_legacy_cgns_supported(
                         return r;
         }
 
-        if (cg_all_unified() > 0)
+        if (cg_all_unified())
                 goto skip_controllers;
 
         controllers = set_new(&string_hash_ops);
@@ -1091,7 +1091,7 @@ static int mount_legacy_cgns_unsupported(
                         return r;
         }
 
-        if (cg_all_unified() > 0)
+        if (cg_all_unified())
                 goto skip_controllers;
 
         controllers = set_new(&string_hash_ops);

src/nspawn/nspawn.c

@@ -316,7 +316,7 @@ static int custom_mount_check_all(void) {
 
 static int detect_unified_cgroup_hierarchy(const char *directory) {
         const char *e;
-        int r, all_unified, systemd_unified;
+        int r;
 
         /* Allow the user to control whether the unified hierarchy is used */
         e = getenv("UNIFIED_CGROUP_HIERARCHY");
@@ -332,15 +332,8 @@ static int detect_unified_cgroup_hierarchy(const char *directory) {
                 return 0;
         }
 
-        all_unified = cg_all_unified();
-        systemd_unified = cg_unified(SYSTEMD_CGROUP_CONTROLLER);
-
-        if (all_unified < 0 || systemd_unified < 0)
-                return log_error_errno(all_unified < 0 ? all_unified : systemd_unified,
-                                       "Failed to determine whether the unified cgroups hierarchy is used: %m");
-
         /* Otherwise inherit the default from the host system */
-        if (all_unified > 0) {
+        if (cg_all_unified()) {
                 /* 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);
@@ -350,7 +343,7 @@ 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 (systemd_unified > 0) {
+        } else if (cg_unified(SYSTEMD_CGROUP_CONTROLLER)) {
                 /* Mixed cgroup hierarchy support was added in 232 */
                 r = systemd_installation_has_version(directory, 232);
                 if (r < 0)
@@ -3533,7 +3526,10 @@ int main(int argc, char *argv[]) {
 
         log_parse_environment();
         log_open();
-        cg_unified_flush();
+
+        r = cg_unified_flush();
+        if (r < 0)
+                return log_error_errno(r, "Failed to determine whether the unified cgroups hierarchy is used: %m");
 
         /* Make sure rename_process() in the stub init process can work */
         saved_argv = argv;