work around the new exception class not being present in older driver…

… versions

gemfiles/driver_stable.gemfile

@@ -1,7 +1,7 @@
 # rubocop:todo all
 source "https://rubygems.org"
 
-gem 'mongo', git: "https://github.com/mongodb/mongo-ruby-driver", branch: '2.18-stable'
+gem 'mongo', git: "https://github.com/mongodb/mongo-ruby-driver", branch: '2.19-stable'
 
 gem 'actionpack'
 

lib/mongoid/clients/sessions.rb

@@ -90,7 +90,7 @@ def transaction(options = {}, session_options: {})
               session.start_transaction(options)
               yield
               commit_transaction(session)
-            rescue Mongo::Error::TransactionsNotSupported
+            rescue *transactions_not_supported_exceptions
               raise Mongoid::Errors::TransactionsNotSupported
             rescue Mongoid::Errors::Rollback
               abort_transaction(session)
@@ -152,6 +152,22 @@ def after_rollback(*args, &block)
 
         private
 
+        # Driver version 2.20 introduced a new exception for reporting that
+        # transactions are not supported. Prior to that, the condition was
+        # discovered by the rescue clause falling through to a different
+        # exception.
+        #
+        # This method ensures that Mongoid continues to work with older driver
+        # versions, by only returning the new exception.
+        #
+        # Once support is removed for all versions prior to 2.20.0, we can
+        # replace this method.
+        def transactions_not_supported_exceptions
+          return nil unless defined? Mongo::Error::TransactionsNotSupported
+
+          Mongo::Error::TransactionsNotSupported
+        end
+
         # @return [ Mongo::Session ] Session for the current client.
         def _session
           Threaded.get_session(client: persistence_context.client)