Update forks visibility_level after parent project visibility_level change

app/models/project.rb

@@ -731,7 +731,11 @@ class Project < ActiveRecord::Base
   end
 
   def forks_count
-    ForkedProjectLink.where(forked_from_project_id: self.id).count
+    forks.count
+  end
+
+  def forks
+    ForkedProjectLink.where(forked_from_project_id: self.id)
   end
 
   def find_label(name)

app/services/projects/update_service.rb

@@ -3,7 +3,8 @@ module Projects
     def execute
       # check that user is allowed to set specified visibility_level
       new_visibility = params[:visibility_level]
-      if new_visibility && new_visibility.to_i != project.visibility_level
+      if new_visibility
+        if new_visibility.to_i != project.visibility_level
           unless can?(current_user, :change_visibility_level, project) &&
             Gitlab::VisibilityLevel.allowed_for?(current_user, new_visibility)
             deny_visibility_level(project, new_visibility)
@@ -11,13 +12,8 @@ module Projects
           end
         end
 
-      unless project.visibility_level_allowed?(new_visibility)
-        level_name = Gitlab::VisibilityLevel.level_name(new_visibility)
-        project.errors.add(
-          :visibility_level,
-          "#{level_name} could not be set as visibility level of this project - parent project settings are more restrictive"
-        )
-        return false
+        return false unless visibility_level_allowed?(new_visibility)
+        update_forks_visibility_level(new_visibility)
       end
 
       new_branch = params[:default_branch]
@@ -32,5 +28,31 @@ module Projects
         end
       end
     end
+
+    private
+
+    def visibility_level_allowed?(level)
+      return true if project.visibility_level_allowed?(level)
+
+      level_name = Gitlab::VisibilityLevel.level_name(level)
+      project.errors.add(
+        :visibility_level,
+        "#{level_name} could not be set as visibility level of this project - parent project settings are more restrictive"
+      )
+
+      false
+    end
+
+    def update_forks_visibility_level(new_level)
+      project.forks.each do |forked_link|
+        forked_project = forked_link.forked_to_project
+        fork_level = forked_project.visibility_level
+
+        if fork_level > new_level.to_i
+          forked_project.visibility_level = new_level.to_i
+          forked_project.save!
+        end
+      end
+    end
   end
 end