Fix RUBY-2347 GSSAPI service name is not defaulted when any other auth mechanism properties are set (#2042)

Co-authored-by: Oleg Pudeyev <oleg@bsdpower.com>

Author: p-mongo

lib/mongo/client.rb

@@ -466,7 +466,18 @@ def initialize(addresses_or_uri, options = nil)
       # construction
       sdam_proc = options.delete(:sdam_proc)
 
-      options = default_options(options).merge(options)
+      # For gssapi service_name, the default option is given in a hash
+      # (one level down from the top level).
+      merged_options = default_options(options)
+      options.each do |k, v|
+        default_v = merged_options[k]
+        if Hash === default_v
+          v = default_v.merge(v)
+        end
+        merged_options[k] = v
+      end
+      options = merged_options
+
       @options = validate_new_options!(options)
 =begin WriteConcern object support
       if @options[:write_concern].is_a?(WriteConcern::Base)

spec/integration/client_authentication_options_spec.rb

@@ -256,6 +256,43 @@
         expect(client.options[:auth_mech_properties]).to eq({ 'service_name' => 'mongodb' })
       end
     end
+
+    context 'when properties are given but not service name' do
+      context 'with URI options' do
+        let(:credentials) { "#{user}:#{pwd}@" }
+
+        context 'with default auth mech properties' do
+          let(:options) { '?authMechanism=GSSAPI&authMechanismProperties=service_realm:foo' }
+
+          it 'sets service name to mongodb' do
+            expect(client.options[:auth_mech_properties]).to eq(
+              'service_name' => 'mongodb',
+              'service_realm' => 'foo',
+            )
+          end
+        end
+      end
+
+      context 'with client options' do
+        let(:client_opts) do
+          {
+            auth_mech: :gssapi,
+            user: user,
+            password: pwd,
+            auth_mech_properties: {
+              service_realm: 'foo',
+            }.freeze,
+          }.freeze
+        end
+
+        it 'sets default auth mech properties' do
+          expect(client.options[:auth_mech_properties]).to eq(
+            'service_name' => 'mongodb',
+            'service_realm' => 'foo',
+          )
+        end
+      end
+    end
   end
 
   context 'with PLAIN auth mechanism' do