Allow prevent_cross_joins to be used directly in tests

This method can be passed a block and we can validate that no SQL query
in that code block attempts to join across ci/main databases. This is
useful in the intermediate time while we attempt to fix many small
issues until we enable this as the default in all RSpec tests.

doc/development/database/multiple_databases.md

@@ -243,3 +243,38 @@ A quick checklist for fixing a specific join query would be:
    adding a new column
 1. Can we remove the join by adding a new table in the correct database that
    replicates the minimum data needed to do the join
+
+#### How to validate you have correctly removed a cross-join
+
+We have introduced a method you can use in RSpec tests to validate all SQL
+queries within a code block to ensure that none of them are joining across the
+2 databases. This is a useful tool to confirm you have correctly fixed an
+existing cross-join.
+
+At some point in the future we will have fixed all cross-joins and this tool
+will run by default in all tests but for now it needs to be explicitly enabled
+for your test.
+
+You can use this method like so:
+
+```ruby
+it 'does not join across databases' do
+  with_cross_joins_prevented do
+    ::Ci::Build.joins(:project).to_a
+  end
+end
+```
+
+This will raise an exception if the query joins across the 2 databases. The
+above example is fixed by removing the join like so:
+
+```ruby
+it 'does not join across databases' do
+  with_cross_joins_prevented do
+    ::Ci::Build.preload(:project).to_a
+  end
+end
+```
+
+You can see a real example of using this method for fixing a cross-join in
+<https://gitlab.com/gitlab-org/gitlab/-/merge_requests/67655>.

spec/support/database/prevent_cross_joins.rb

@@ -56,6 +56,20 @@ module Database
       end
     end
 
+    module SpecHelpers
+      def with_cross_joins_prevented
+        subscriber = ActiveSupport::Notifications.subscribe('sql.active_record') do |event|
+          ::Database::PreventCrossJoins.validate_cross_joins!(event.payload[:sql])
+        end
+
+        Thread.current[:allow_cross_joins_across_databases] = false
+
+        yield
+      ensure
+        ActiveSupport::Notifications.unsubscribe(subscriber) if subscriber
+      end
+    end
+
     module GitlabDatabaseMixin
       def allow_cross_joins_across_databases(url:)
         Thread.current[:allow_cross_joins_across_databases] = true
@@ -69,16 +83,10 @@ Gitlab::Database.singleton_class.prepend(
   Database::PreventCrossJoins::GitlabDatabaseMixin)
 
 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|
-    subscriber = ActiveSupport::Notifications.subscribe('sql.active_record') do |event|
-      ::Database::PreventCrossJoins.validate_cross_joins!(event.payload[:sql])
-    end
-
-    Thread.current[:allow_cross_joins_across_databases] = false
-
-    example.run
-  ensure
-    ActiveSupport::Notifications.unsubscribe(subscriber) if subscriber
+    with_cross_joins_prevented { example.run }
   end
 end