Cache column_exists? for application settings

This is most a backport of
https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/4987/diffs but includes
other columns that use column_exists? in a way that may cause unnecessary
schema loads.

app/models/application_setting.rb

@@ -347,15 +347,15 @@ class ApplicationSetting < ActiveRecord::Base
   end
   def home_page_url_column_exists?
-    ActiveRecord::Base.connection.column_exists?(:application_settings, :home_page_url)
+    ::Gitlab::Database.cached_column_exists?(:application_settings, :home_page_url)
   end
   def help_page_support_url_column_exists?
-    ActiveRecord::Base.connection.column_exists?(:application_settings, :help_page_support_url)
+    ::Gitlab::Database.cached_column_exists?(:application_settings, :help_page_support_url)
   end
   def sidekiq_throttling_column_exists?
-    ActiveRecord::Base.connection.column_exists?(:application_settings, :sidekiq_throttling_enabled)
+    ::Gitlab::Database.cached_column_exists?(:application_settings, :sidekiq_throttling_enabled)
   end
   def domain_whitelist_raw

changelogs/unreleased/sh-cache-column-exists.yml

+---
+title: Cache column_exists? for application settings
+merge_request:
+author:
+type: performance

lib/gitlab/database.rb

@@ -183,6 +183,10 @@ module Gitlab
       ActiveRecord::Base.connection
     end
+    def self.cached_column_exists?(table_name, column_name)
+      connection.schema_cache.columns_hash(table_name).has_key?(column_name.to_s)
+    end
+
     private_class_method :connection
     def self.database_version

spec/lib/gitlab/database_spec.rb

@@ -287,6 +287,17 @@ describe Gitlab::Database do
     end
   end
+  describe '.cached_column_exists?' do
+    it 'only retrieves data once' do
+      expect(ActiveRecord::Base.connection).to receive(:columns).once.and_call_original
+
+      2.times do
+        expect(described_class.cached_column_exists?(:projects, :id)).to be_truthy
+        expect(described_class.cached_column_exists?(:projects, :bogus_column)).to be_falsey
+      end
+    end
+  end
+
   describe '#true_value' do
     it 'returns correct value for PostgreSQL' do
       expect(described_class).to receive(:postgresql?).and_return(true)