Fix ArgumentError when a server is marked unknown (#2949)

Author: drcapulet

lib/mongo/server.rb

@@ -616,7 +616,9 @@ def unknown!(options = {})
         return
       end
 
-      if options[:generation] && options[:generation] < pool&.generation
+      # NOTE: You cannot use safe navigation here because if pool is nil you end
+      # up trying to evaluate Integer < nil which is invalid.
+      if options[:generation] && pool && options[:generation] < pool.generation
         return
       end
 

spec/mongo/server/connection_spec.rb

@@ -260,6 +260,19 @@ class ConnectionSpecTestException < Exception; end
         it_behaves_like 'marks server unknown'
         it_behaves_like 'logs a warning'
         it_behaves_like 'adds server diagnostics'
+
+        context 'when the error includes a generation' do
+          let(:exception) do
+            Mongo::Error::SocketError.new.tap do |exc|
+              allow(exc).to receive(:generation).and_return(1234)
+              allow(exc).to receive(:service_id).and_return('fake')
+            end
+          end
+
+          it 'does not fail marking the server unknown' do
+            expect(error).to eq(exception)
+          end
+        end
       end
 
       context 'when #authenticate! raises an exception' do