Introduce StatusThreadingConcern (#3490)

* Add a StatusFilter class to identify visibility of statuses by accounts

* Extract StatusThreadingConcern from Status

* Clarify purpose of checking for nil account

app/models/concerns/status_threading_concern.rb

@@ -0,0 +1,89 @@
+# frozen_string_literal: true
+
+module StatusThreadingConcern
+  extend ActiveSupport::Concern
+
+  def ancestors(account = nil)
+    find_statuses_from_tree_path(ancestor_ids, account)
+  end
+
+  def descendants(account = nil)
+    find_statuses_from_tree_path(descendant_ids, account)
+  end
+
+  private
+
+  def ancestor_ids
+    Rails.cache.fetch("ancestors:#{id}") do
+      ancestors_without_self.pluck(:id)
+    end
+  end
+
+  def ancestors_without_self
+    ancestor_statuses - [self]
+  end
+
+  def ancestor_statuses
+    Status.find_by_sql([<<-SQL.squish, id: id])
+      WITH RECURSIVE search_tree(id, in_reply_to_id, path)
+      AS (
+        SELECT id, in_reply_to_id, ARRAY[id]
+        FROM statuses
+        WHERE id = :id
+        UNION ALL
+        SELECT statuses.id, statuses.in_reply_to_id, path || statuses.id
+        FROM search_tree
+        JOIN statuses ON statuses.id = search_tree.in_reply_to_id
+        WHERE NOT statuses.id = ANY(path)
+      )
+      SELECT id
+      FROM search_tree
+      ORDER BY path DESC
+    SQL
+  end
+
+  def descendant_ids
+    descendants_without_self.pluck(:id)
+  end
+
+  def descendants_without_self
+    descendant_statuses - [self]
+  end
+
+  def descendant_statuses
+    Status.find_by_sql([<<-SQL.squish, id: id])
+      WITH RECURSIVE search_tree(id, path)
+      AS (
+        SELECT id, ARRAY[id]
+        FROM statuses
+        WHERE id = :id
+        UNION ALL
+        SELECT statuses.id, path || statuses.id
+        FROM search_tree
+        JOIN statuses ON statuses.in_reply_to_id = search_tree.id
+        WHERE NOT statuses.id = ANY(path)
+      )
+      SELECT id
+      FROM search_tree
+      ORDER BY path
+    SQL
+  end
+
+  def find_statuses_from_tree_path(ids, account)
+    statuses = statuses_with_accounts(ids).to_a
+
+    # FIXME: n+1 bonanza
+    statuses.reject! { |status| filter_from_context?(status, account) }
+
+    # Order ancestors/descendants by tree path
+    statuses.sort_by! { |status| ids.index(status.id) }
+  end
+
+  def statuses_with_accounts(ids)
+    Status.where(id: ids).includes(:account)
+  end
+
+  def filter_from_context?(status, account)
+    StatusFilter.new(status, account).filtered?
+  end
+end