Remove caching of Connection#config

Caching was added in
https://gitlab.com/gitlab-org/gitlab/-/merge_requests/65262, which lead
to an incident and support for using uncached configs in
https://gitlab.com/gitlab-org/gitlab/-/merge_requests/66988.

In this commit we remove caching of the config settings, and add a test
and comment to explain why we don't want the data to be cached. The
performance impact is likely not worth potentially breaking code that
expects uncached settings.

This fixes https://gitlab.com/gitlab-org/gitlab/-/issues/337054.

lib/gitlab/database/connection.rb

@@ -34,12 +34,17 @@ module Gitlab
         Gitlab::Runtime.max_threads + headroom
       end
 
-      def uncached_config
-        scope.connection_db_config.configuration_hash.with_indifferent_access
-      end
-
       def config
-        @config ||= uncached_config
+        # The result of this method must not be cached, as other methods may use
+        # it after making configuration changes and expect those changes to be
+        # present. For example, `disable_prepared_statements` expects the
+        # configuration settings to always be up to date.
+        #
+        # See the following for more information:
+        #
+        # - https://gitlab.com/gitlab-org/release/retrospectives/-/issues/39
+        # - https://gitlab.com/gitlab-com/gl-infra/production/-/issues/5238
+        scope.connection_db_config.configuration_hash.with_indifferent_access
       end
 
       def pool_size
@@ -72,7 +77,7 @@ module Gitlab
 
       # Disables prepared statements for the current database connection.
       def disable_prepared_statements
-        scope.establish_connection(uncached_config.merge(prepared_statements: false))
+        scope.establish_connection(config.merge(prepared_statements: false))
       end
 
       def read_only?

spec/lib/gitlab/database/connection_spec.rb

@@ -29,12 +29,19 @@ RSpec.describe Gitlab::Database::Connection do
     it 'returns a default pool size' do
       expect(connection.config).to include(pool: connection.default_pool_size)
     end
+
+    it 'does not cache its results' do
+      a = connection.config
+      b = connection.config
+
+      expect(a).not_to equal(b)
+    end
   end
 
   describe '#pool_size' do
     context 'when no explicit size is configured' do
       it 'returns the default pool size' do
-        expect(connection.config).to receive(:[]).with(:pool).and_return(nil)
+        expect(connection).to receive(:config).and_return({ pool: nil })
 
         expect(connection.pool_size).to eq(connection.default_pool_size)
       end
@@ -42,7 +49,7 @@ RSpec.describe Gitlab::Database::Connection do
 
     context 'when an explicit pool size is set' do
       it 'returns the pool size' do
-        expect(connection.config).to receive(:[]).with(:pool).and_return(4)
+        expect(connection).to receive(:config).and_return({ pool: 4 })
 
         expect(connection.pool_size).to eq(4)
       end