RUBY-1588 Do not update round trip time on failed ismaster calls

lib/mongo/server/description.rb

@@ -189,10 +189,7 @@ class Description
       #   call took to complete.
       #
       # @since 2.0.0
-      def initialize(address, config = {}, average_round_trip_time = 0)
-        if average_round_trip_time.nil?
-          raise ArgumentError, 'Average round trip time cannot be nil'
-        end
+      def initialize(address, config = {}, average_round_trip_time = nil)
         @address = address
         @config = config
         unless unknown?

lib/mongo/server/round_trip_time_averager.rb

@@ -36,9 +36,15 @@ def measure
           rv = yield
         rescue Exception => exc
         end
-        @last_round_trip_time = Time.now - start
+        last_round_trip_time = Time.now - start
 
-        update_average_round_trip_time
+        # If ismaster fails, we need to return the last round trip time
+        # because it is used in the heartbeat failed SDAM event,
+        # but we must not update the round trip time recorded in the server.
+        unless exc
+          @last_round_trip_time = last_round_trip_time
+          update_average_round_trip_time
+        end
 
         [rv, exc, last_round_trip_time, average_round_trip_time]
       end

spec/mongo/server/description_spec.rb

@@ -327,8 +327,8 @@
       described_class.new(address, { 'secondary' => false }, 4.5)
     end
 
-    it 'defaults to 0' do
-      expect(described_class.new(address).average_round_trip_time).to eq(0)
+    it 'defaults to nil' do
+      expect(described_class.new(address).average_round_trip_time).to be nil
     end
 
     it 'can be set via the constructor' do

spec/mongo/server/round_trip_time_averager_spec.rb

@@ -21,4 +21,23 @@
       end
     end
   end
+
+  describe '#measure' do
+    context 'block does not raise' do
+      it 'updates average rtt' do
+        expect(averager).to receive(:update_average_round_trip_time)
+        averager.measure do
+        end
+      end
+    end
+
+    context 'block raises' do
+      it 'does not update average rtt' do
+        expect(averager).not_to receive(:update_average_round_trip_time)
+        averager.measure do
+          raise "Problem"
+        end
+      end
+    end
+  end
 end