Fix error 500s creating new projects due to empty weights

GitLab 13.1.0 added the
`application_settings.repository_storages_weighted` column, but it was
possible that the value was not seeded properly due to a stale schema
cache due to the use of ActiveRecord inside the migrations. Admins that
upgraded might see 500 errors when creating projects.

To ensure that the data is seeded properly, we surgically add a
migration that seeds the data only if it is not present. The schema
version was intentionally used to avoid backporting conflicts and to
ensure any background migrations will have the correct data when they
run. The changes were largely taken from
https://gitlab.com/gitlab-org/gitlab/-/merge_requests/35814.

Closes https://gitlab.com/gitlab-org/gitlab/-/issues/225203

changelogs/unreleased/sh-reseed-repository-storages.yml

+---
+title: Fix error 500s creating new projects due to empty weights
+merge_request: 35829
+author:
+type: fixed

db/migrate/20200509203901_reseed_repository_storages_weighted.rb

+# frozen_string_literal: true
+
+class ReseedRepositoryStoragesWeighted < ActiveRecord::Migration[6.0]
+  DOWNTIME = false
+
+  class ApplicationSetting < ActiveRecord::Base
+    serialize :repository_storages
+    self.table_name = 'application_settings'
+  end
+
+  def up
+    reseed_repository_storages_weighted
+  end
+
+  private
+
+  def reseed_repository_storages_weighted
+    # We need to flush the cache to ensure the newly-added column is loaded
+    ApplicationSetting.reset_column_information
+
+    # There should only be one row here due to
+    # 20200420162730_remove_additional_application_settings_rows.rb
+    ApplicationSetting.all.each do |settings|
+      # Admins may have already tweaked these values, so don't do anything
+      # if there is data already.
+      next if settings.repository_storages_weighted.present?
+
+      storages = Gitlab.config.repositories.storages.keys.collect do |storage|
+        weight = settings.repository_storages.include?(storage) ? 100 : 0
+        [storage.to_sym, weight]
+      end
+
+      settings.repository_storages_weighted = Hash[storages]
+      settings.save!
+    end
+  end
+end

db/structure.sql

@@ -23396,6 +23396,7 @@ COPY "schema_migrations" (version) FROM STDIN;
 20200508091106
 20200508140959
 20200508203901
+20200509203901
 20200511080113
 20200511083541
 20200511092246

spec/migrations/reseed_repository_storages_weighted_spec.rb

+# frozen_string_literal: true
+
+require 'spec_helper'
+require Rails.root.join('db', 'migrate', '20200509203901_reseed_repository_storages_weighted.rb')
+
+RSpec.describe ReseedRepositoryStoragesWeighted do
+  let(:storages) { { "foo" => {}, "baz" => {} } }
+  let(:application_settings) do
+    table(:application_settings).tap do |klass|
+      klass.class_eval do
+        serialize :repository_storages
+      end
+    end
+  end
+
+  before do
+    allow(Gitlab.config.repositories).to receive(:storages).and_return(storages)
+  end
+
+  let(:repository_storages) { ["foo"] }
+  let!(:application_setting) { application_settings.create!(repository_storages: repository_storages) }
+
+  context 'with empty repository_storages_weighted column' do
+    it 'populates repository_storages_weighted properly' do
+      migrate!
+
+      expect(application_settings.find(application_setting.id).repository_storages_weighted).to eq({ "foo" => 100, "baz" => 0 })
+    end
+  end
+
+  context 'with already-populated repository_storages_weighted column' do
+    let(:existing_weights) { { "foo" => 100, "baz" => 50 } }
+
+    it 'does not change repository_storages_weighted properly' do
+      application_setting.repository_storages_weighted = existing_weights
+      application_setting.save!
+
+      migrate!
+
+      expect(application_settings.find(application_setting.id).repository_storages_weighted).to eq(existing_weights)
+    end
+  end
+end