core: introduce Unit.dependency_generation counter and restart loop when dependency is updated in the loop

When starting unit A, a dependent unit B may be loaded if it is not
loaded yet, and the dependencies in unit A may be updated.
As Hashmap does not allow a new entry to be added in a loop, we need to
restart loop in such case.

Fixes a bug introduced by cda667722c.
Fixes #36031.

src/core/unit.c

@@ -631,12 +631,14 @@ static void unit_clear_dependencies(Unit *u) {
                                 hashmap_remove(other_deps, u);
 
                         unit_add_to_gc_queue(other);
+                        other->dependency_generation++;
                 }
 
                 hashmap_free(deps);
         }
 
         u->dependencies = hashmap_free(u->dependencies);
+        u->dependency_generation++;
 }
 
 static void unit_remove_transient(Unit *u) {
@@ -1094,6 +1096,9 @@ static void unit_merge_dependencies(Unit *u, Unit *other) {
         }
 
         other->dependencies = hashmap_free(other->dependencies);
+
+        u->dependency_generation++;
+        other->dependency_generation++;
 }
 
 int unit_merge(Unit *u, Unit *other) {
@@ -3088,6 +3093,7 @@ static int unit_add_dependency_impl(
                         return r;
 
                 flags = NOTIFY_DEPENDENCY_UPDATE_FROM;
+                u->dependency_generation++;
         }
 
         if (other_info.data != other_info_old.data) {
@@ -3104,6 +3110,7 @@ static int unit_add_dependency_impl(
                 }
 
                 flags |= NOTIFY_DEPENDENCY_UPDATE_TO;
+                other->dependency_generation++;
         }
 
         return flags;
@@ -5618,6 +5625,9 @@ void unit_remove_dependencies(Unit *u, UnitDependencyMask mask) {
                                 /* The unit 'other' may not be wanted by the unit 'u'. */
                                 unit_submit_to_stop_when_unneeded_queue(other);
 
+                                u->dependency_generation++;
+                                other->dependency_generation++;
+
                                 done = false;
                                 break;
                         }