Allow inbox owner to view implicitly targeted ActivityPub payload (#9093)

Fix #9091

app/controllers/activitypub/inboxes_controller.rb

@@ -36,6 +36,6 @@ class ActivityPub::InboxesController < Api::BaseController
   end
 
   def process_payload
-    ActivityPub::ProcessingWorker.perform_async(signed_request_account.id, body.force_encoding('UTF-8'))
+    ActivityPub::ProcessingWorker.perform_async(signed_request_account.id, body.force_encoding('UTF-8'), @account&.id)
   end
 end

app/lib/activitypub/activity/create.rb

@@ -81,11 +81,22 @@ class ActivityPub::Activity::Create < ActivityPub::Activity
       @mentions << Mention.new(account: account, silent: true)
 
       # If there is at least one silent mention, then the status can be considered
-      # as a limited-audience status, and not strictly a direct message
+      # as a limited-audience status, and not strictly a direct message, but only
+      # if we considered a direct message in the first place
       next unless @params[:visibility] == :direct
 
       @params[:visibility] = :limited
     end
+
+    # If the payload was delivered to a specific inbox, the inbox owner must have
+    # access to it, unless they already have access to it anyway
+    return if @options[:delivered_to_account_id].nil? || @mentions.any? { mention.account_id == @options[:delivered_to_account_id] }
+
+    @mentions << Mention.new(account_id: @options[:delivered_to_account_id], silent: true)
+
+    return unless @param[:visibility] == :direct
+
+    @params[:visibility] = :limited
   end
 
   def attach_tags(status)

app/workers/activitypub/processing_worker.rb

@@ -5,7 +5,7 @@ class ActivityPub::ProcessingWorker
 
   sidekiq_options backtrace: true
 
-  def perform(account_id, body)
+  def perform(account_id, body, delivered_to_account_id = nil)
-    ActivityPub::ProcessCollectionService.new.call(body, Account.find(account_id), override_timestamps: true)
+    ActivityPub::ProcessCollectionService.new.call(body, Account.find(account_id), override_timestamps: true, delivered_to_account_id: delivered_to_account_id)
   end
 end