core: convert Slice= into a proper dependency (and add a back dependency)

The slice a unit is assigned to is currently a UnitRef reference. Let's
turn it into a proper dependency, to simplify and clean up code a bit.
Now that new dep types are cheaper, deps should generally be preferable
over everything else, if the concept applies.

This brings one major benefit: we often have to iterate through all unit
a slice contains. So far we iterated through all Before= dependencies of
the slice unit to achieve that, filtering out unrelated units, and
taking benefit of the fact that slice units are implicitly ordered
Before= the units they contain. By making Slice= a proper dependency,
and having an accompanying SliceOf= dependency type, this is much
simpler and nicer as we can directly enumerate the units a slice
contains.

The forward dependency is actually called InSlice internally, since we
already used the UNIT_SLICE name as UnitType field. However, since we
don't intend to expose the dependency to users as dep anyway (we already
have the regular Slice D-Bus property for this) this shouldn't matter.
The SliceOf= implicit dependency type (the erverse of Slice=/InSlice=)
is exported over the bus, to make things a bit nicer to debug and
discoverable.

1 files changed, 4 insertions, 19 deletions

diff --git a/src/core/cgroup.c b/src/core/cgroup.c
index 97820eb264..5b00faa6f6 100644
--- a/src/core/cgroup.c
+++ b/src/core/cgroup.c
@@ -1702,12 +1702,8 @@ CGroupMask unit_get_members_mask(Unit *u) {
         if (u->type == UNIT_SLICE) {
                 Unit *member;
 
-                UNIT_FOREACH_DEPENDENCY(member, u, UNIT_ATOM_BEFORE) {
-                        if (UNIT_DEREF(member->slice) != u)
-                                continue;
-
+                UNIT_FOREACH_DEPENDENCY(member, u, UNIT_ATOM_SLICE_OF)
                         u->cgroup_members_mask |= unit_get_subtree_mask(member); /* note that this calls ourselves again, for the children */
-                }
         }
 
         u->cgroup_members_mask_valid = true;
@@ -2362,13 +2358,10 @@ static int unit_realize_cgroup_now_disable(Unit *u, ManagerState state) {
         if (u->type != UNIT_SLICE)
                 return 0;
 
-        UNIT_FOREACH_DEPENDENCY(m, u, UNIT_ATOM_BEFORE) {
+        UNIT_FOREACH_DEPENDENCY(m, u, UNIT_ATOM_SLICE_OF) {
                 CGroupMask target_mask, enable_mask, new_target_mask, new_enable_mask;
                 int r;
 
-                if (UNIT_DEREF(m->slice) != u)
-                        continue;
-
                 /* The cgroup for this unit might not actually be fully realised yet, in which case it isn't
                  * holding any controllers open anyway. */
                 if (!m->cgroup_realized)
@@ -2530,11 +2523,7 @@ void unit_add_family_to_cgroup_realize_queue(Unit *u) {
                 /* Children of u likely changed when we're called */
                 u->cgroup_members_mask_valid = false;
 
-                UNIT_FOREACH_DEPENDENCY(m, u, UNIT_ATOM_BEFORE) {
-
-                        /* Skip units that have a dependency on the slice but aren't actually in it. */
-                        if (UNIT_DEREF(m->slice) != u)
-                                continue;
+                UNIT_FOREACH_DEPENDENCY(m, u, UNIT_ATOM_SLICE_OF) {
 
                         /* No point in doing cgroup application for units without active processes. */
                         if (UNIT_IS_INACTIVE_OR_FAILED(unit_active_state(m)))
@@ -3799,12 +3788,8 @@ void unit_invalidate_cgroup_bpf(Unit *u) {
         if (u->type == UNIT_SLICE) {
                 Unit *member;
 
-                UNIT_FOREACH_DEPENDENCY(member, u, UNIT_ATOM_BEFORE) {
-                        if (UNIT_DEREF(member->slice) != u)
-                                continue;
-
+                UNIT_FOREACH_DEPENDENCY(member, u, UNIT_ATOM_SLICE_OF)
                         unit_invalidate_cgroup_bpf(member);
-                }
         }
 }