Add tombstones for remote statuses (#9830)

* Add Tombstone model to remember object deletion

* Do not recreate a status if it has been deleted

* Record Tombstone for remote deleted items

Also, only record deleted items from same-host actors

* Clear an user's tombstones when their key change

app/lib/activitypub/activity/create.rb

@@ -6,6 +6,7 @@ class ActivityPub::Activity::Create < ActivityPub::Activity
 
   def perform
     return if unsupported_object_type? || invalid_origin?(@object['id'])
+    return if Tombstone.exists?(uri: @object['id'])
 
     RedisLock.acquire(lock_options) do |lock|
       if lock.acquired?

app/lib/activitypub/activity/delete.rb

@@ -21,8 +21,9 @@ class ActivityPub::Activity::Delete < ActivityPub::Activity
   def delete_note
     return if object_uri.nil?
 
-    RedisLock.acquire(lock_options) do |_lock|
-      delete_later!(object_uri)
+    unless invalid_origin?(object_uri)
+      RedisLock.acquire(lock_options) { |_lock| delete_later!(object_uri) }
+      Tombstone.find_or_create_by(uri: object_uri, account: @account)
     end
 
     @status   = Status.find_by(uri: object_uri, account: @account)
@@ -74,4 +75,13 @@ class ActivityPub::Activity::Delete < ActivityPub::Activity
   def lock_options
     { redis: Redis.current, key: "create:#{object_uri}" }
   end
+
+  def invalid_origin?(url)
+    return true if unsupported_uri_scheme?(url)
+
+    needle   = Addressable::URI.parse(url).host
+    haystack = Addressable::URI.parse(@account.uri).host
+
+    !haystack.casecmp(needle).zero?
+  end
 end

app/models/tombstone.rb

@@ -0,0 +1,15 @@
+# frozen_string_literal: true
+
+# == Schema Information
+#
+# Table name: tombstones
+#
+#  id         :bigint(8)        not null, primary key
+#  account_id :bigint(8)
+#  uri        :string           not null
+#  created_at :datetime         not null
+#  updated_at :datetime         not null
+#
+
+class Tombstone < ApplicationRecord
+end

app/services/activitypub/process_account_service.rb

@@ -33,6 +33,8 @@ class ActivityPub::ProcessAccountService < BaseService
 
     after_protocol_change! if protocol_changed?
     after_key_change! if key_changed? && !@options[:signed_with_known_key]
+    clear_tombstones! if key_changed?
+
     unless @options[:only_key]
       check_featured_collection! if @account.featured_collection_url.present?
       check_links! unless @account.fields.empty?
@@ -209,6 +211,10 @@ class ActivityPub::ProcessAccountService < BaseService
     !@old_public_key.nil? && @old_public_key != @account.public_key
   end
 
+  def clear_tombstones!
+    Tombstone.delete_all(account_id: @account.id)
+  end
+
   def protocol_changed?
     !@old_protocol.nil? && @old_protocol != @account.protocol
   end