RUBY-2806 Rate limit push monitor handshake sends (#2357)

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

Author: p-mongo

lib/mongo/server/push_monitor.rb

@@ -123,6 +123,9 @@ def do_work
           log_prefix: options[:log_prefix],
           bg_error_backtrace: options[:bg_error_backtrace],
         )
+
+        # Avoid tight looping in push monitor - see RUBY-2806.
+        sleep(0.5)
       end
 
       def check

spec/mongo/server/push_monitor_spec.rb

@@ -76,6 +76,25 @@
           push_monitor.do_work
         end.should_not raise_error
       end
+
+      it 'throttles checks' do
+        push_monitor
+
+        start = Mongo::Utils.monotonic_time
+
+        expect(Socket).to receive(:getaddrinfo).and_raise(SocketError.new('Test exception'))
+        lambda do
+          push_monitor.do_work
+        end.should_not raise_error
+
+        expect(Socket).to receive(:getaddrinfo).and_raise(SocketError.new('Test exception'))
+        lambda do
+          push_monitor.do_work
+        end.should_not raise_error
+
+        elapsed = Mongo::Utils.monotonic_time - start
+        elapsed.should > 0.5
+      end
     end
   end