Fix handling for `GitlabSchemasValidateConnection` for early start

It appears that each connection needs to be adopted. However,
in it's initial phase (before being adopted) the Rails
might execute SQL queries to configure the connection settings
(collation, timeouts, etc.). This will happen before
the connection is actually assigned to the connection pool,
as if such connection will be failed to be initialized
it will be orphaned.

lib/gitlab/database.rb

@@ -203,8 +203,13 @@ def self.db_config_names
     # This does not look at literal connection names, but rather compares
     # models that are holders for a given db_config_name
     def self.gitlab_schemas_for_connection(connection)
-      db_name = self.db_config_name(connection)
-      primary_model = self.database_base_models.fetch(db_name.to_sym)
+      db_config = self.db_config_for_connection(connection)
+
+      # connection might not be yet adopted (returning NullPool, and no connection_klass)
+      # in such cases it is fine to ignore such connections
+      return unless db_config
+
+      primary_model = self.database_base_models.fetch(db_config.name.to_sym)
       self.schemas_to_base_models.select do |_, child_models|
         child_models.any? do |child_model|

spec/lib/gitlab/database_spec.rb

@@ -222,10 +222,6 @@
   end
   describe '.gitlab_schemas_for_connection' do
-    it 'does raise exception for invalid connection' do
-      expect { described_class.gitlab_schemas_for_connection(:invalid) }.to raise_error /key not found: "unknown"/
-    end
-
     it 'does return a valid schema depending on a base model used', :request_store do
       # FF due to lib/gitlab/database/load_balancing/configuration.rb:92
       stub_feature_flags(force_no_sharing_primary_model: true)
@@ -282,6 +278,15 @@
         end
       end
     end
+
+    it 'does return empty for non-adopted connections' do
+      new_connection = ActiveRecord::Base.postgresql_connection(
+        ActiveRecord::Base.connection_db_config.configuration_hash)
+
+      expect(described_class.gitlab_schemas_for_connection(new_connection)).to be_nil
+    ensure
+      new_connection&.disconnect!
+    end
   end
   describe '#true_value' do