Fix RUBY-2427 Server description on 4.4+ servers flips between having compression on & off (#2535)

Co-authored-by: Oleg Pudeyev <code@olegp.name>

Author: p-mongo

lib/mongo/server/monitor/connection.rb

@@ -227,15 +227,21 @@ def handshake!
         # @api private
         def check_document
           server_api = @app_metadata.server_api || options[:server_api]
-          if hello_ok? || server_api
-            doc = HELLO_DOC
+          doc = if hello_ok? || server_api
+            _doc = HELLO_DOC
             if server_api
-              doc = doc.merge(Utils.transform_server_api(server_api))
+              _doc = _doc.merge(Utils.transform_server_api(server_api))
             end
-            doc
+            _doc
           else
             LEGACY_HELLO_DOC
           end
+          # compressors must be set to maintain correct compression status
+          # in the server description. See RUBY-2427
+          if compressors = options[:compressors]
+            doc = doc.merge(compression: compressors)
+          end
+          doc
         end
 
         private

spec/integration/sdam_events_spec.rb

@@ -135,4 +135,44 @@
       end
     end
   end
+
+  describe 'server description changed' do
+    require_topology :single
+
+    let(:sdam_proc) do
+      Proc.new do |client|
+        client.subscribe(Mongo::Monitoring::SERVER_DESCRIPTION_CHANGED, subscriber)
+      end
+    end
+
+    let(:client) do
+      new_local_client(SpecConfig.instance.addresses,
+        # Heartbeat interval is bound by 500 ms
+        SpecConfig.instance.test_options.merge(client_options).merge(
+          heartbeat_frequency: 0.5,
+          sdam_proc: sdam_proc,
+        ),
+      )
+    end
+
+    let(:client_options) do
+      {}
+    end
+
+    it 'is not published when there are no changes in server state' do
+      client
+      sleep 6
+      client.close
+
+      events = subscriber.select_succeeded_events(Mongo::Monitoring::Event::ServerDescriptionChanged)
+
+      # In 6 seconds we should have about 10 or 12 heartbeats.
+      # We expect 1 or 2 description changes:
+      # The first one from unknown to known,
+      # The second one because server changes the fields it returns based on
+      # driver server check payload (e.g. ismaster/isWritablePrimary).
+      events.length.should >= 1
+      events.length.should <= 2
+    end
+  end
 end