pid1: include the source unit in UnitRef

No functional change.

The source unit manages the reference. It allocates the UnitRef structure and
registers it in the target unit, and then the reference must be destroyed
before the source unit is destroyed. Thus, is should be OK to include the
pointer to the source unit, it should be live as long as the reference exists.

v2:
- rename refs to refs_by_target

src/core/load-fragment.c

@@ -1860,7 +1860,7 @@ int config_parse_socket_service(
                 return -ENOEXEC;
         }
 
-        unit_ref_set(&s->service, x);
+        unit_ref_set(&s->service, UNIT(s), x);
 
         return 0;
 }

src/core/service.c

@@ -2684,7 +2684,7 @@ static int service_deserialize_item(Unit *u, const char *key, const char *value,
                 if (r < 0)
                         log_unit_debug_errno(u, r, "Failed to load accept-socket unit: %s", value);
                 else {
-                        unit_ref_set(&s->accept_socket, socket);
+                        unit_ref_set(&s->accept_socket, u, socket);
                         SOCKET(socket)->n_connections++;
                 }
 
@@ -3770,7 +3770,7 @@ int service_set_socket_fd(Service *s, int fd, Socket *sock, bool selinux_context
         s->socket_fd = fd;
         s->socket_fd_selinux_context_net = selinux_context_net;
 
-        unit_ref_set(&s->accept_socket, UNIT(sock));
+        unit_ref_set(&s->accept_socket, UNIT(s), UNIT(sock));
         return 0;
 }
 

src/core/slice.c

@@ -76,7 +76,7 @@ static int slice_add_parent_slice(Slice *s) {
         if (r < 0)
                 return r;
 
-        unit_ref_set(&u->slice, parent);
+        unit_ref_set(&u->slice, u, parent);
         return 0;
 }
 

src/core/socket.c

@@ -250,7 +250,7 @@ int socket_instantiate_service(Socket *s) {
         if (r < 0)
                 return r;
 
-        unit_ref_set(&s->service, u);
+        unit_ref_set(&s->service, UNIT(s), u);
 
         return unit_add_two_dependencies(UNIT(s), UNIT_BEFORE, UNIT_TRIGGERS, u, false, UNIT_DEPENDENCY_IMPLICIT);
 }
@@ -377,7 +377,7 @@ static int socket_add_extras(Socket *s) {
                         if (r < 0)
                                 return r;
 
-                        unit_ref_set(&s->service, x);
+                        unit_ref_set(&s->service, u, x);
                 }
 
                 r = unit_add_two_dependencies(u, UNIT_BEFORE, UNIT_TRIGGERS, UNIT_DEREF(s->service), true, UNIT_DEPENDENCY_IMPLICIT);

src/core/unit.c

@@ -362,7 +362,7 @@ bool unit_may_gc(Unit *u) {
         if (u->perpetual)
                 return false;
 
-        if (u->refs)
+        if (u->refs_by_target)
                 return false;
 
         if (sd_bus_track_count(u->bus_track) > 0)
@@ -662,9 +662,8 @@ void unit_free(Unit *u) {
         free(u->reboot_arg);
 
         unit_ref_unset(&u->slice);
-
-        while (u->refs)
-                unit_ref_unset(u->refs);
+        while (u->refs_by_target)
+                unit_ref_unset(u->refs_by_target);
 
         safe_close(u->ip_accounting_ingress_map_fd);
         safe_close(u->ip_accounting_egress_map_fd);
@@ -896,8 +895,8 @@ int unit_merge(Unit *u, Unit *other) {
                 return r;
 
         /* Redirect all references */
-        while (other->refs)
-                unit_ref_set(other->refs, u);
+        while (other->refs_by_target)
+                unit_ref_set(other->refs_by_target, other->refs_by_target->source, u);
 
         /* Merge dependencies */
         for (d = 0; d < _UNIT_DEPENDENCY_MAX; d++)
@@ -2976,8 +2975,7 @@ int unit_set_slice(Unit *u, Unit *slice) {
         if (UNIT_ISSET(u->slice) && u->cgroup_realized)
                 return -EBUSY;
 
-        unit_ref_unset(&u->slice);
-        unit_ref_set(&u->slice, slice);
+        unit_ref_set(&u->slice, u, slice);
         return 1;
 }
 
@@ -3986,30 +3984,32 @@ int unit_get_unit_file_preset(Unit *u) {
         return u->unit_file_preset;
 }
 
-Unit* unit_ref_set(UnitRef *ref, Unit *u) {
+Unit* unit_ref_set(UnitRef *ref, Unit *source, Unit *target) {
         assert(ref);
-        assert(u);
+        assert(source);
+        assert(target);
 
-        if (ref->unit)
+        if (ref->target)
                 unit_ref_unset(ref);
 
-        ref->unit = u;
-        LIST_PREPEND(refs, u->refs, ref);
-        return u;
+        ref->source = source;
+        ref->target = target;
+        LIST_PREPEND(refs_by_target, target->refs_by_target, ref);
+        return target;
 }
 
 void unit_ref_unset(UnitRef *ref) {
         assert(ref);
 
-        if (!ref->unit)
+        if (!ref->target)
                 return;
 
         /* We are about to drop a reference to the unit, make sure the garbage collection has a look at it as it might
          * be unreferenced now. */
-        unit_add_to_gc_queue(ref->unit);
+        unit_add_to_gc_queue(ref->target);
 
-        LIST_REMOVE(refs, ref->unit->refs, ref);
-        ref->unit = NULL;
+        LIST_REMOVE(refs_by_target, ref->target->refs_by_target, ref);
+        ref->source = ref->target = NULL;
 }
 
 static int user_from_unit_name(Unit *u, char **ret) {

src/core/unit.h

@@ -123,8 +123,8 @@ struct UnitRef {
          * that we can merge two units if necessary and correct all
          * references to them */
 
-        Unit* unit;
-        LIST_FIELDS(UnitRef, refs);
+        Unit *source, *target;
+        LIST_FIELDS(UnitRef, refs_by_target);
 };
 
 typedef enum UnitCGroupBPFState {
@@ -187,7 +187,7 @@ struct Unit {
         char *job_timeout_reboot_arg;
 
         /* References to this */
-        LIST_HEAD(UnitRef, refs);
+        LIST_HEAD(UnitRef, refs_by_target);
 
         /* Conditions to check */
         LIST_HEAD(Condition, conditions);
@@ -724,11 +724,11 @@ void unit_trigger_notify(Unit *u);
 UnitFileState unit_get_unit_file_state(Unit *u);
 int unit_get_unit_file_preset(Unit *u);
 
-Unit* unit_ref_set(UnitRef *ref, Unit *u);
+Unit* unit_ref_set(UnitRef *ref, Unit *source, Unit *target);
 void unit_ref_unset(UnitRef *ref);
 
-#define UNIT_DEREF(ref) ((ref).unit)
-#define UNIT_ISSET(ref) (!!(ref).unit)
+#define UNIT_DEREF(ref) ((ref).target)
+#define UNIT_ISSET(ref) (!!(ref).target)
 
 int unit_patch_contexts(Unit *u);