Refactor and improve Geo::LogCursor and tests

It also make it more efficient

ee/lib/gitlab/geo/log_cursor/event_logs.rb

@@ -10,7 +10,7 @@ module Gitlab
         # fetches up to BATCH_SIZE next events and keep track of batches
         # rubocop: disable CodeReuse/ActiveRecord
         def fetch_in_batches(batch_size: BATCH_SIZE)
-          last_id = last_processed_id
+          last_id = last_processed_id || last_event_log_id
 
           ::Geo::EventLog.where('id > ?', last_id).find_in_batches(batch_size: batch_size) do |batch|
             yield(batch, last_id)
@@ -31,18 +31,19 @@ module Gitlab
           event_state.update!(event_id: event_id)
         end
 
-        # @return [Integer] id of last replicated event
+        # @return [Integer] id of last replicated event or nil if it does not exist
         def last_processed_id
-          last = ::Geo::EventLogState.last_processed&.id
-          return last if last
-
-          if ::Geo::EventLog.any?
-            event_id = ::Geo::EventLog.last.id
-            save_processed(event_id)
-            event_id
-          else
-            -1
+          ::Geo::EventLogState.last_processed&.id
         end
+
+        # @return [Integer] id of the event we need to start processing from
+        def last_event_log_id
+          last_id = ::Geo::EventLog.last&.id
+
+          return 0 unless last_id
+
+          save_processed(last_id)
+          last_id
         end
       end
     end

ee/spec/lib/gitlab/geo/log_cursor/event_logs_spec.rb

@@ -15,14 +15,16 @@ describe Gitlab::Geo::LogCursor::EventLogs, :postgresql, :clean_gitlab_redis_sha
       let!(:event_log_2) { create(:geo_event_log) }
 
       context 'when there is no event_log_state' do
-        it 'does not yields a group of events' do
+        it 'does not yield a group of events' do
           expect { |b| subject.fetch_in_batches(&b) }.not_to yield_with_args([event_log_1, event_log_2])
         end
       end
 
       context 'when there is already an event_log_state' do
+        let(:last_event_id) { event_log_1.id - 1 }
+
         before do
-          create(:geo_event_log_state, event_id: event_log_1.id - 1)
+          create(:geo_event_log_state, event_id: last_event_id)
         end
 
         it 'saves last event as last processed after yielding' do
@@ -30,6 +32,10 @@ describe Gitlab::Geo::LogCursor::EventLogs, :postgresql, :clean_gitlab_redis_sha
 
           expect(Geo::EventLogState.last.event_id).to eq(event_log_2.id)
         end
+
+        it 'yields a group of events' do
+          expect { |b| subject.fetch_in_batches(&b) }.to yield_with_args([event_log_1, event_log_2], last_event_id)
+        end
       end
     end
   end