Update epic cache counts on project/group deletion

What does this MR do and why?

When issue's count or weight is changed for an epic, we update its cached values. These cached values are sum of both direct epic's issues and also sub-epic's issues.

Because we want to include subepics weights/counts too, when cached value is changed we propagate this change to all epic ancestors. This is achieved by scheduling cache update for epic's parent in epic's after_commit callback.

Related to #356683 (closed)

DB changes:

• queries to get impacted epics when destroying group or project - !90615 (comment 1000841520) and !90615 (comment 1000999580)
• queries to get epic's cached sum/counts - !90615 (comment 1004585335) and !90615 (comment 1004589231)

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ee/app/models/ee/epic.rb

@@ -179,6 +179,11 @@ def etag_caching_enabled?
 
       after_commit do
         ::Epics::UpdateIssueCachedMetadataWorker.perform_async(parent_id) if propagate_issue_metadata_change?
+
+        parent_change = previous_changes['parent_id'] || []
+        if parent_change[0] && parent_change[0] != parent_change[1]
+          ::Epics::UpdateIssueCachedMetadataWorker.perform_async(parent_change[0])
+        end
       end
 
       def epic_tree_root?
@@ -342,8 +347,6 @@ def search(query)
       end
 
       def ids_for_base_and_decendants(epic_ids)
-        return self.class.none unless parent_id
-
         ::Gitlab::ObjectHierarchy.new(self.id_in(epic_ids)).base_and_descendants.pluck(:id)
       end
 
@@ -559,6 +562,8 @@ def level_depth_exceeded?(parent_epic)
     private :level_depth_exceeded?
 
     def base_and_ancestors
+      return self.class.none unless parent_id
+
       hierarchy.base_and_ancestors(hierarchy_order: :asc)
     end
 
@@ -630,12 +635,14 @@ def total_issue_weight_and_count
       }
     end
 
+    # TODO: private
+
     def subepics_weight_and_count
       sum = children.select(
-        'SUM(COALESCE(total_opened_issue_weight, 0)) AS opened_issue_weight',
-        'SUM(COALESCE(total_closed_issue_weight, 0)) AS closed_issue_weight',
-        'SUM(COALESCE(total_opened_issue_count, 0)) AS opened_issue_count',
-        'SUM(COALESCE(total_closed_issue_count, 0)) AS closed_issue_count'
+        'SUM(total_opened_issue_weight) AS opened_issue_weight',
+        'SUM(total_closed_issue_weight) AS closed_issue_weight',
+        'SUM(total_opened_issue_count) AS opened_issue_count',
+        'SUM(total_closed_issue_count) AS closed_issue_count'
       ).reorder(nil).first
 
       {
@@ -669,11 +676,13 @@ def issues_weight_and_count
 
     def propagate_issue_metadata_change?
       return false unless parent_id
+      return true if destroyed?
 
-      cached_attrs = %w(total_opened_issue_weight total_closed_issue_weight
-                        total_opened_issue_count total_closed_issue_count)
+      attrs = %w(total_opened_issue_weight total_closed_issue_weight
+                 total_opened_issue_count total_closed_issue_count
+                 parent_id)
 
-      (previous_changes.keys & cached_attrs).any?
+      (previous_changes.keys & attrs).any?
     end
   end
 end