Reintroduce cross-database query check

But we disable GC around the sql parser to workaround segfault. The
segfault looks like it's improperly accessing memory that has been
garbage collected.

spec/support/database/prevent_cross_joins.rb

@@ -30,7 +30,15 @@ module Database
 
       # PgQuery might fail in some cases due to limited nesting:
       # https://github.com/pganalyze/pg_query/issues/209
-      tables = PgQuery.parse(sql).tables
+      #
+      # Also, we disable GC while parsing because of https://github.com/pganalyze/pg_query/issues/226
+      begin
+        GC.disable
+        tables = PgQuery.parse(sql).tables
+      ensure
+        GC.enable
+      end
+
       schemas = Database::GitlabSchema.table_schemas(tables)
 
       if schemas.include?(:gitlab_ci) && schemas.include?(:gitlab_main)
@@ -72,10 +80,13 @@ ALLOW_LIST = Set.new(YAML.load_file(Rails.root.join('.cross-join-allowlist.yml')
 RSpec.configure do |config|
   config.include(::Database::PreventCrossJoins::SpecHelpers)
 
-  # TODO: remove `:prevent_cross_joins` to enable the check by default
-  config.around(:each, :prevent_cross_joins) do |example|
+  config.around do |example|
     Thread.current[:has_cross_join_exception] = false
 
-    with_cross_joins_prevented { example.run }
+    if ALLOW_LIST.include?(example.file_path)
+      example.run
+    else
+      with_cross_joins_prevented { example.run }
+    end
   end
 end

spec/support_specs/database/prevent_cross_joins_spec.rb

@@ -3,7 +3,7 @@
 require 'spec_helper'
 
 RSpec.describe Database::PreventCrossJoins do
-  context 'when running in :prevent_cross_joins scope', :prevent_cross_joins do
+  context 'when running in a default scope' do
     context 'when only non-CI tables are used' do
       it 'does not raise exception' do
         expect { main_only_query }.not_to raise_error
@@ -32,14 +32,6 @@ RSpec.describe Database::PreventCrossJoins do
     end
   end
 
-  context 'when running in a default scope' do
-    context 'when CI and non-CI tables are used' do
-      it 'does not raise exception' do
-        expect { main_and_ci_query }.not_to raise_error
-      end
-    end
-  end
-
   private
 
   def main_only_query