Additional fix to handle NULL lock_version

If the UI sends a string value for lock_version (e.g. "0"), then the
previous monkey patch did not properly handle that properly. This
commit casts the value to an integer to determine whether to look for
NULL lock_versions.

For merge requests, GitLab sends a POST request to
`namespace/project/merge_requests/:iid` with the
`merge_request[lock_version]` parameter with a string `0`. The string
value comes from the form field, which explains why
https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/28145 wasn't
sufficient.

config/initializers/config_initializers_active_record_locking.rb

 # frozen_string_literal: true
+
+# ensure ActiveRecord's version has been required already
+require 'active_record/locking/optimistic'
+
 # rubocop:disable Lint/RescueException
 module ActiveRecord
   module Locking
@@ -16,7 +20,7 @@ module ActiveRecord
           self[locking_column] += 1
 
           # Patched because when `lock_version` is read as `0`, it may actually be `NULL` in the DB.
-          possible_previous_lock_value = previous_lock_value == 0 ? [nil, 0] : previous_lock_value
+          possible_previous_lock_value = previous_lock_value.to_i == 0 ? [nil, 0] : previous_lock_value
 
           affected_rows = self.class.unscoped._update_record(
             arel_attributes_with_values(attribute_names),

spec/models/issue_spec.rb

@@ -56,16 +56,27 @@ describe Issue do
   end
 
   describe 'locking' do
+    using RSpec::Parameterized::TableSyntax
+
+    where(:lock_version) do
+      [
+        [0],
+        ["0"]
+      ]
+    end
+
+    with_them do
       it 'works when an issue has a NULL lock_version' do
         issue = create(:issue)
 
         described_class.where(id: issue.id).update_all('lock_version = NULL')
 
-      issue.update!(lock_version: 0, title: 'locking test')
+        issue.update!(lock_version: lock_version, title: 'locking test')
 
         expect(issue.reload.title).to eq('locking test')
       end
     end
+  end
 
   describe '#order_by_position_and_priority' do
     let(:project) { create :project }

spec/models/merge_request_spec.rb

@@ -32,16 +32,27 @@ describe MergeRequest do
   end
 
   describe 'locking' do
+    using RSpec::Parameterized::TableSyntax
+
+    where(:lock_version) do
+      [
+        [0],
+        ["0"]
+      ]
+    end
+
+    with_them do
       it 'works when a merge request has a NULL lock_version' do
         merge_request = create(:merge_request)
 
         described_class.where(id: merge_request.id).update_all('lock_version = NULL')
 
-      merge_request.update!(lock_version: 0, title: 'locking test')
+        merge_request.update!(lock_version: lock_version, title: 'locking test')
 
         expect(merge_request.reload.title).to eq('locking test')
       end
     end
+  end
 
   describe '#squash_in_progress?' do
     let(:repo_path) do