Add optimistic lock to avoid race conditions when handling votes (mastodon#10196)

* Add optimistic lock to avoid race conditions when handling votes

* Force-reload polls when getting `ActiveRecord::StaleObjectError`

Author: ClearlyClaire

app/models/poll.rb

@@ -15,6 +15,7 @@
 #  last_fetched_at :datetime
 #  created_at      :datetime         not null
 #  updated_at      :datetime         not null
+#  lock_version    :integer          default(0), not null
 #
 
 class Poll < ApplicationRecord

app/models/poll_vote.rb

@@ -32,5 +32,8 @@ def object_type
   def increment_counter_cache
     poll.cached_tallies[choice] = (poll.cached_tallies[choice] || 0) + 1
     poll.save
+  rescue ActiveRecord::StaleObjectError
+    poll.reload
+    retry
   end
 end

db/migrate/20190306145741_add_lock_version_to_polls.rb

@@ -0,0 +1,6 @@
+class AddLockVersionToPolls < ActiveRecord::Migration[5.2]
+  def change
+    add_column :polls, :lock_version, :integer, null: false, default: 0
+  end
+end
+

db/schema.rb

@@ -10,7 +10,7 @@
 #
 # It's strongly recommended that you check this file into your version control system.
 
-ActiveRecord::Schema.define(version: 2019_03_04_152020) do
+ActiveRecord::Schema.define(version: 2019_03_06_145741) do
 
   # These are extensions that must be enabled in order to support this database
   enable_extension "plpgsql"
@@ -464,6 +464,7 @@
     t.datetime "last_fetched_at"
     t.datetime "created_at", null: false
     t.datetime "updated_at", null: false
+    t.integer "lock_version", default: 0, null: false
     t.index ["account_id"], name: "index_polls_on_account_id"
     t.index ["status_id"], name: "index_polls_on_status_id"
   end