FIX: Stop race condition when topic notification jobs are scheduled during a database transaction

This was not picked up by tests because scheduled jobs are run immediately
and in the current thread (and therefore the current database transaction).

This particular case sometimes occurs inside multiple nested transactions,
so simply moving the offending line outside of the transaction is not enough.

Implemented TransactionHelper, which allows us to use `TransactionHelper.after_commit`
to define code to be run after the current transaction has been committed.

app/helpers/transaction_helper.rb

@@ -0,0 +1,38 @@
+##
+# Allows running arbitrary code after the current transaction has been committed.
+# Works even with nexted transactions. Useful for scheduling sidekiq jobs.
+# Slightly simplified version of https://dev.to/evilmartians/rails-aftercommit-everywhere--4j9g
+# Usage:
+#    Topic.transaction do
+#        puts "Some work before scheduling"
+#        TransactionHelper.after_commit do
+#            puts "Running after commit"
+#        end
+#        puts "Some work after scheduling"
+#    end
+#
+# Produces:
+#     > Some work before scheduling
+#     > Some work after scheduling
+#     > Running after commit
+
+module TransactionHelper
+  class AfterCommitWrapper
+    def initialize
+      @callback = Proc.new
+    end
+
+    def committed!(*)
+      @callback.call
+    end
+
+    def before_committed!(*); end
+    def rolledback!(*); end
+  end
+
+  def self.after_commit(&blk)
+    ActiveRecord::Base.connection.add_transaction_record(
+        AfterCommitWrapper.new(&blk)
+    )
+  end
+end

app/models/topic.rb

@@ -686,7 +686,9 @@ class Topic < ActiveRecord::Base
 
         if post = self.ordered_posts.first
           notified_user_ids = [post.user_id, post.last_editor_id].uniq
-          Jobs.enqueue(:notify_category_change, post_id: post.id, notified_user_ids: notified_user_ids)
+          TransactionHelper.after_commit do
+            Jobs.enqueue(:notify_category_change, post_id: post.id, notified_user_ids: notified_user_ids)
+          end
         end
       end
 

spec/helpers/transaction_helper_spec.rb

@@ -0,0 +1,32 @@
+require 'rails_helper'
+
+describe TransactionHelper do
+
+  it "runs callbacks after outermost transaction is committed" do
+    outputString = "1"
+
+      # Main transaction
+      ActiveRecord::Base.transaction do
+        outputString += "2"
+
+          # Nested transaction
+          ActiveRecord::Base.transaction do
+            outputString += "3"
+
+              TransactionHelper.after_commit do
+                outputString += "6"
+              end
+              outputString += "4"
+          end
+
+          TransactionHelper.after_commit do
+            outputString += "7"
+          end
+
+          outputString += "5"
+      end
+
+      expect(outputString).to eq("1234567")
+  end
+
+end