Prevent cross joins for all while allowing existing failures

This uses a filename based approach to allowlist spec files

Add documentation about cross-database joins detection

.cross-join-allowlist.yml

+- ./ee/spec/lib/analytics/devops_adoption/snapshot_calculator_spec.rb

doc/development/database/multiple_databases.md

@@ -109,6 +109,10 @@ already many such examples that need to be fixed in
 The following are some real examples that have resulted from this and these
 patterns may apply to future cases.
 
+[Introduced](https://gitlab.com/gitlab-org/gitlab/-/merge_requests/68620) in GitLab 14.3, any
+queries detected that join across databases raises an error (except
+for pre-existing queries).
+
 #### Remove the code
 
 The simplest solution we've seen several times now has been an existing scope

spec/support/database/prevent_cross_joins.rb

@@ -33,7 +33,8 @@ module Database
       tables = PgQuery.parse(sql).tables
       schemas = Database::GitlabSchema.table_schemas(tables)
 
-      if schemas.many?
+      if schemas.include?(:gitlab_ci) && schemas.include?(:gitlab_main)
+        Thread.current[:has_cross_join_exception] = true
         raise CrossJoinAcrossUnsupportedTablesError,
           "Unsupported cross-join across '#{tables.join(", ")}' modifying '#{schemas.to_a.join(", ")}' discovered " \
           "when executing query '#{sql}'"
@@ -66,11 +67,18 @@ end
 Gitlab::Database.singleton_class.prepend(
   Database::PreventCrossJoins::GitlabDatabaseMixin)
 
+ALLOW_LIST = Set.new(YAML.load_file(Rails.root.join('.cross-join-allowlist.yml'))).freeze
+
 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|
-    with_cross_joins_prevented { example.run }
+  config.around do |example|
+    Thread.current[:has_cross_join_exception] = false
+
+    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