Merge branch '42239-fix-mysql-source-installs' into 'master'

Resolve "mysql-only installation from source broken: Gem::LoadError: pg is not part of the bundle"

Closes #42239

See merge request gitlab-org/gitlab-ce!16810

config/initializers/ar5_pg_10_support.rb

 raise "Vendored ActiveRecord 5 code! Delete #{__FILE__}!" if ActiveRecord::VERSION::MAJOR >= 5
 
-require 'active_record/connection_adapters/postgresql_adapter'
-require 'active_record/connection_adapters/postgresql/schema_statements'
-
-#
-# Monkey-patch the refused Rails 4.2 patch at https://github.com/rails/rails/pull/31330
-#
-# Updates sequence logic to support PostgreSQL 10.
-#
-# rubocop:disable all
-module ActiveRecord
+if Gitlab::Database.postgresql?
+  require 'active_record/connection_adapters/postgresql_adapter'
+  require 'active_record/connection_adapters/postgresql/schema_statements'
+
+  #
+  # Monkey-patch the refused Rails 4.2 patch at https://github.com/rails/rails/pull/31330
+  #
+  # Updates sequence logic to support PostgreSQL 10.
+  #
+  # rubocop:disable all
+  module ActiveRecord
     module ConnectionAdapters
 
       # We need #postgresql_version to be public as in ActiveRecord 5 for seed_fu
@@ -53,5 +54,6 @@ module ActiveRecord
         end
       end
     end
+  end
+  # rubocop:enable all
 end
-# rubocop:enable all

config/initializers/peek.rb

@@ -7,10 +7,12 @@ if Gitlab::Database.mysql?
   require 'peek-mysql2'
   PEEK_DB_CLIENT = ::Mysql2::Client
   PEEK_DB_VIEW = Peek::Views::Mysql2
-else
+elsif Gitlab::Database.postgresql?
   require 'peek-pg'
   PEEK_DB_CLIENT = ::PG::Connection
   PEEK_DB_VIEW = Peek::Views::PG
+else
+  raise "Unsupported database adapter for peek!"
 end
 
 Peek.into PEEK_DB_VIEW