core: make SYSTEMD_CGROUP_CONTROLLER a special string

SYSTEMD_CGROUP_CONTROLLER is currently defined as "name=elogind" which cgroup
utility functions interpret as a named cgroup hierarchy with the specified
named.  With the planned cgroup hybrid mode changes, SYSTEMD_CGROUP_CONTROLLER
would map to different hierarchy names.

This patch makes SYSTEMD_CGROUP_CONTROLLER a special string "_elogind" which is
substituted to "name=elogind" by the cgroup utility functions.  This allows the
callers to address the elogind hierarchy without actually specifying the
hierarchy name allowing the cgroup utility functions to map it to whatever is
appropriate.

Note that SYSTEMD_CGROUP_CONTROLLER was already special on full unified cgroup
hierarchy even before this patch.

2 files changed, 17 insertions, 5 deletions

diff --git a/src/basic/cgroup-util.c b/src/basic/cgroup-util.c
index bcb022896..272fb1973 100644
--- a/src/basic/cgroup-util.c
+++ b/src/basic/cgroup-util.c
@@ -548,6 +548,9 @@ static const char *controller_to_dirname(const char *controller) {
          * just cuts off the name= prefixed used for named
          * hierarchies, if it is specified. */
 
+        if (streq(controller, SYSTEMD_CGROUP_CONTROLLER))
+                controller = SYSTEMD_CGROUP_CONTROLLER_LEGACY;
+
         e = startswith(controller, "name=");
         if (e)
                 return e;
@@ -920,7 +923,7 @@ int cg_get_xattr(const char *controller, const char *path, const char *name, voi
 int cg_pid_get_path(const char *controller, pid_t pid, char **path) {
         _cleanup_fclose_ FILE *f = NULL;
         char line[LINE_MAX];
-        const char *fs;
+        const char *fs, *controller_str;
         size_t cs = 0;
         bool unified;
 
@@ -934,8 +937,14 @@ int cg_pid_get_path(const char *controller, pid_t pid, char **path) {
                 controller = SYSTEMD_CGROUP_CONTROLLER;
 
         unified = cg_unified(controller);
-        if (!unified)
-                cs = strlen(controller);
+        if (!unified) {
+                if (streq(controller, SYSTEMD_CGROUP_CONTROLLER))
+                        controller_str = SYSTEMD_CGROUP_CONTROLLER_LEGACY;
+                else
+                        controller_str = controller;
+
+                cs = strlen(controller_str);
+        }
 
         fs = procfs_file_alloca(pid, "cgroup");
         log_debug_elogind("Searching for PID %u in \"%s\" (controller \"%s\")",
@@ -974,7 +983,7 @@ int cg_pid_get_path(const char *controller, pid_t pid, char **path) {
 
                         *e = 0;
                         FOREACH_WORD_SEPARATOR(word, k, l, ",", state) {
-                                if (k == cs && memcmp(word, controller, cs) == 0) {
+                                if (k == cs && memcmp(word, controller_str, cs) == 0) {
                                         found = true;
                                         break;
                                 }
@@ -1860,6 +1869,9 @@ bool cg_controller_is_valid(const char *p) {
         if (!p)
                 return false;
 
+        if (streq(p, SYSTEMD_CGROUP_CONTROLLER))
+                return true;
+
         s = startswith(p, "name=");
         if (s)
                 p = s;
diff --git a/src/core/cgroup.c b/src/core/cgroup.c
index e306b8bce..8f6e5e072 100644
--- a/src/core/cgroup.c
+++ b/src/core/cgroup.c
@@ -1806,7 +1806,7 @@ int manager_setup_cgroup(Manager *m) {
         else if (cg_unified(SYSTEMD_CGROUP_CONTROLLER))
                 log_debug("Unified cgroup hierarchy is located at %s. Controllers are on legacy hierarchies.", path);
         else
-                log_debug("Using cgroup controller " SYSTEMD_CGROUP_CONTROLLER ". File system hierarchy is at %s.", path);
+                log_debug("Using cgroup controller " SYSTEMD_CGROUP_CONTROLLER_LEGACY ". File system hierarchy is at %s.", path);
 
         if (!m->test_run) {
                 const char *scope_path;