core: rework the "no_gc" unit flag to become a more generic "perpetual" flag

So far "no_gc" was set on -.slice and init.scope, to units that are always
running, cannot be stopped and never exist in an "inactive" state. Since these
units are the only users of this flag, let's remodel it and rename it
"perpetual" and let's derive more funcitonality off it. Specifically, refuse
enqueing stop jobs for these units, and report that they are "unstoppable" in
the CanStop bus property.

1 files changed, 24 insertions, 3 deletions

diff --git a/src/core/unit.c b/src/core/unit.c
index cabb1050a8..8d74b44f73 100644
--- a/src/core/unit.c
+++ b/src/core/unit.c
@@ -323,7 +323,7 @@ bool unit_check_gc(Unit *u) {
         if (state != UNIT_INACTIVE)
                 return true;
 
-        if (u->no_gc)
+        if (u->perpetual)
                 return true;
 
         if (u->refs)
@@ -924,6 +924,7 @@ void unit_dump(Unit *u, FILE *f, const char *prefix) {
                 "%s\tGC Check Good: %s\n"
                 "%s\tNeed Daemon Reload: %s\n"
                 "%s\tTransient: %s\n"
+                "%s\tPerpetual: %s\n"
                 "%s\tSlice: %s\n"
                 "%s\tCGroup: %s\n"
                 "%s\tCGroup realized: %s\n"
@@ -942,6 +943,7 @@ void unit_dump(Unit *u, FILE *f, const char *prefix) {
                 prefix, yes_no(unit_check_gc(u)),
                 prefix, yes_no(unit_need_daemon_reload(u)),
                 prefix, yes_no(u->transient),
+                prefix, yes_no(u->perpetual),
                 prefix, strna(unit_slice_name(u)),
                 prefix, strna(u->cgroup_path),
                 prefix, yes_no(u->cgroup_realized),
@@ -1616,6 +1618,18 @@ int unit_stop(Unit *u) {
         return UNIT_VTABLE(u)->stop(u);
 }
 
+bool unit_can_stop(Unit *u) {
+        assert(u);
+
+        if (!unit_supported(u))
+                return false;
+
+        if (u->perpetual)
+                return false;
+
+        return !!UNIT_VTABLE(u)->stop;
+}
+
 /* Errors:
  *         -EBADR:    This unit type does not support reloading.
  *         -ENOEXEC:  Unit is not started.
@@ -2150,13 +2164,20 @@ bool unit_job_is_applicable(Unit *u, JobType j) {
 
         case JOB_VERIFY_ACTIVE:
         case JOB_START:
-        case JOB_STOP:
         case JOB_NOP:
+                /* Note that we don't check unit_can_start() here. That's because .device units and suchlike are not
+                 * startable by us but may appear due to external events, and it thus makes sense to permit enqueing
+                 * jobs for it. */
                 return true;
 
+        case JOB_STOP:
+                /* Similar as above. However, perpetual units can never be stopped (neither explicitly nor due to
+                 * external events), hence it makes no sense to permit enqueing such a request either. */
+                return !u->perpetual;
+
         case JOB_RESTART:
         case JOB_TRY_RESTART:
-                return unit_can_start(u);
+                return unit_can_stop(u) && unit_can_start(u);
 
         case JOB_RELOAD:
         case JOB_TRY_RELOAD: