touch module outside content_tag save txn to prevent deadlock

fixes CNVS-7757

test plan:
  - open two Rails consoles
  - in each, create a bunch of items in the same
    module at the same time, e.g.

    mod = ContextModule.last
    100.times do |x|
      mod.add_item type: 'context_module_sub_header',
                   title: "item #{x}"
    end
  - each console should finish adding its items

 or, when deployed in a production-like environment
 (with multiple app servers, or at least a multithreaded server)
  - generate a curl command line to add an item to a module
    (works with a subheader, so you don't need corresponding
    assets)

    curl -H "Authorization: Bearer {token} \
      https://<canvas>/api/v1/courses/XXX/modules/YYY/items \
      -F module_item[title]=newthing -F module_item[type]=SubHeader &

    (the trailing &, after a space, means run in the background;
     that is, don't wait for the preceding request to finish
     before returning to the command line)
  - run the preceding command several times in rapid succession
    (up, enter, up, enter, etc.)
    (I was able to reproduce in less than 10 requests)
  - look at the json blocks returned, and make sure none of them
    are an internal server error (if they are, there will be an
    error report number you can look up and see if the deadlock
    happened)

Change-Id: I63834cfd98393e5207625db27d18451103aa2944
Reviewed-on: https://gerrit.instructure.com/23783
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Rob Orton <rob@instructure.com>
QA-Review: August Thornton <august@instructure.com>
Product-Review: Jeremy Stanley <jeremy@instructure.com>

app/controllers/context_modules_controller.rb

@@ -325,8 +325,7 @@ class ContextModulesController < ApplicationController
   def add_item
     @module = @context.context_modules.not_deleted.find(params[:context_module_id])
     if authorized_action(@module, @current_user, :update)
-      @tag = @module.add_item(params[:item]) #@item)
-      @module.touch
+      @tag = @module.add_item(params[:item])
       render :json => @tag.to_json
     end
   end
@@ -336,7 +335,6 @@ class ContextModulesController < ApplicationController
     if authorized_action(@tag.context_module, @current_user, :update)
       @module = @tag.context_module
       @tag.destroy
-      @module.touch
       render :json => @tag.to_json
     end
   end

app/models/content_tag.rb

@@ -42,7 +42,7 @@ class ContentTag < ActiveRecord::Base
   validates_length_of :comments, :maximum => maximum_text_length, :allow_nil => true, :allow_blank => true
   before_save :default_values
   after_save :update_could_be_locked
-  after_save :touch_context_module
+  after_save :touch_context_module_after_transaction
   after_save :touch_context_if_learning_outcome
   include CustomValidations
   validates_as_url :url
@@ -71,8 +71,16 @@ class ContentTag < ActiveRecord::Base
 
   attr_accessor :skip_touch
   def touch_context_module
-    ContentTag.touch_context_modules([self.context_module_id]) unless skip_touch.present?
+    return true if skip_touch.present?
+    ContentTag.touch_context_modules([self.context_module_id])
   end
+
+  def touch_context_module_after_transaction
+    connection.after_transaction_commit {
+      touch_context_module
+    }
+  end
+  private :touch_context_module_after_transaction
   
   def self.touch_context_modules(ids=[])
     ContextModule.where(:id => ids).update_all(:updated_at => Time.now.utc) unless ids.empty?

spec/integration/context_module_spec.rb

@@ -34,6 +34,8 @@ describe ContextModule do
       response.body.should match(/My Sub Header Title/)
 
       content_tag.update_attributes(:title => "My New Title")
+      run_transaction_commit_callbacks
+
       get "/courses/#{@course.id}/modules"
       response.body.should match(/My New Title/)
     end

spec/models/content_tag_spec.rb

@@ -259,6 +259,17 @@ describe ContentTag do
     test_url_validation(ContentTag.create!(:content => quiz, :context => @course))
   end
 
+  it "should touch the module after committing the save" do
+    course
+    mod = @course.context_modules.create!
+    yesterday = 1.day.ago
+    ContextModule.where(:id => mod).update_all(:updated_at => yesterday)
+    tag = mod.add_item :type => 'context_module_sub_header', :title => 'blah'
+    mod.reload.updated_at.to_i.should == yesterday.to_i
+    run_transaction_commit_callbacks
+    mod.reload.updated_at.should > 5.seconds.ago
+  end
+
   it "should allow skipping touches on save" do
     course
     @assignment = @course.assignments.create!(:title => "some assignment")
@@ -268,6 +279,7 @@ describe ContentTag do
       :title => 'some assignment (renamed)',
       :id => @assignment.id
     })
+    run_transaction_commit_callbacks
     @tag.update_asset_name!
     @tag.reload
 
@@ -276,7 +288,9 @@ describe ContentTag do
 
     @tag.skip_touch = true
     @tag.save
+    run_transaction_commit_callbacks
 
     @module.reload.updated_at.to_i.should == yesterday.to_i
   end
+
 end