RUBY-1791 Raise if transactions not supported (mongodb#2822)

Author: comandeo-mongo

Diff for: lib/mongo/error.rb

@@ -217,6 +217,7 @@ def write_concern_error_labels
 require 'mongo/error/server_api_conflict'
 require 'mongo/error/server_api_not_supported'
 require 'mongo/error/server_not_usable'
+require 'mongo/error/transactions_not_supported'
 require 'mongo/error/unknown_payload_type'
 require 'mongo/error/unmet_dependency'
 require 'mongo/error/unsupported_option'

Diff for: lib/mongo/error/transactions_not_supported.rb

@@ -0,0 +1,34 @@
+# frozen_string_literal: true
+
+# Copyright (C) 2019-2020 MongoDB Inc.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#   http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+module Mongo
+  class Error
+    # Transactions are not supported by the cluster. There might be the
+    # following reasons:
+    #  - topology is standalone
+    #  - topology is replica set and server version is < 4.0
+    #  - topology is sharded and server version is < 4.2
+    #
+    # @param [ String ] reason The reason why transactions are no supported.
+    #
+    # @since 2.7.0
+    class TransactionsNotSupported < Error
+      def initialize(reason)
+        super("Transactions are not supported for the cluster: #{reason}")
+      end
+    end
+  end
+end

Diff for: lib/mongo/server/description/features.rb

@@ -48,6 +48,7 @@ class Features
           # provided by the client during findAndModify operations, requiring the
           # driver to raise client-side errors when those options are provided.
           find_and_modify_option_validation: 8,
+          sharded_transactions: 8,
           transactions: 7,
           scram_sha_256: 7,
           array_filters: 6,

Diff for: lib/mongo/session.rb

@@ -555,6 +555,8 @@ def with_transaction(options=nil)
     #
     # @since 2.6.0
     def start_transaction(options = nil)
+      check_transactions_supported!
+
       if options
         Lint.validate_read_concern_option(options[:read_concern])
 
@@ -1202,5 +1204,18 @@ def check_matching_cluster!(client)
         raise Mongo::Error::InvalidSession.new(MISMATCHED_CLUSTER_ERROR_MSG)
       end
     end
+
+    def check_transactions_supported!
+      raise Mongo::Error::TransactionsNotSupported, "standalone topology" if cluster.single?
+
+      cluster.next_primary.with_connection do |conn|
+        if cluster.replica_set? && !conn.features.transactions_enabled?
+          raise Mongo::Error::TransactionsNotSupported, "server version is < 4.0"
+        end
+        if cluster.sharded? && !conn.features.sharded_transactions_enabled?
+          raise Mongo::Error::TransactionsNotSupported, "sharded transactions require server version >= 4.2"
+        end
+      end
+    end
   end
 end

Diff for: spec/mongo/session_transaction_spec.rb

@@ -26,6 +26,17 @@ class SessionTransactionSpecError < StandardError; end
     collection.delete_many
   end
 
+  describe 'start_transaction' do
+    context 'when topology is sharded and server is < 4.2' do
+      max_server_fcv '4.1'
+      require_topology :sharded
+
+      it 'raises an error' do
+        expect { session.start_transaction }.to raise_error(Mongo::Error::TransactionsNotSupported, /sharded transactions require server version/)
+      end
+    end
+  end
+
   describe '#abort_transaction' do
     require_topology :replica_set
 
@@ -75,6 +86,8 @@ class SessionTransactionSpecError < StandardError; end
   end
 
   describe '#with_transaction' do
+    require_topology :replica_set
+
     context 'callback successful' do
       it 'commits' do
         session.with_transaction do
@@ -123,6 +136,7 @@ class SessionTransactionSpecError < StandardError; end
         expect(Mongo::Utils).to receive(:monotonic_time).ordered.and_return(start + 1)
         expect(Mongo::Utils).to receive(:monotonic_time).ordered.and_return(start + 2)
         expect(Mongo::Utils).to receive(:monotonic_time).ordered.and_return(start + 200)
+        allow(session).to receive('check_transactions_supported!').and_return true
 
         expect do
           session.with_transaction do
@@ -156,6 +170,7 @@ class SessionTransactionSpecError < StandardError; end
             expect(Mongo::Utils).to receive(:monotonic_time).ordered.and_return(start + i)
           end
           expect(Mongo::Utils).to receive(:monotonic_time).ordered.and_return(start + 200)
+          allow(session).to receive('check_transactions_supported!').and_return true
 
           exc = Mongo::Error::OperationFailure.new('timeout test')
           exc.add_label(label)