Make Geo::PruneEventLogWorker delete rows more gently

Instead of attempting to delete all rows in a single SQL query, do it
in batches.

Closes gitlab-org/gitlab-ee#6140.

ee/app/models/geo/event_log.rb

 module Geo
   class EventLog < ActiveRecord::Base
     include Geo::Model
+    include ::EachBatch
 
     belongs_to :repository_created_event,
       class_name: 'Geo::RepositoryCreatedEvent',

ee/app/workers/geo/prune_event_log_worker.rb

@@ -16,8 +16,8 @@ module Geo
 
       try_obtain_lease do
         if Gitlab::Geo.secondary_nodes.empty?
-          log_info('No secondary nodes, delete all Geo Event Log entries')
-          Geo::EventLog.delete_all
+          log_info('No secondary nodes, truncate the Geo Event Log table')
+          ActiveRecord::Base.connection.truncate(Geo::EventLog.table_name)
           break
         end
 
@@ -31,7 +31,8 @@ module Geo
         end
 
         log_info('Delete Geo Event Log entries up to id', geo_event_log_id: cursor_last_event_ids.min)
-        Geo::EventLog.where('id < ?', cursor_last_event_ids.min).delete_all
+        Geo::EventLog.where('id <= ?', cursor_last_event_ids.min)
+                     .each_batch { |batch| batch.delete_all }
       end
     end
   end

ee/changelogs/unreleased/tc-geo-gently-log-prune.yml

+---
+title: Make Geo::PruneEventLogWorker delete rows more gently
+merge_request: 5835
+author:
+type: performance

ee/spec/workers/geo/prune_event_log_worker_spec.rb

@@ -45,7 +45,8 @@ describe Geo::PruneEventLogWorker, :geo do
         it 'deletes everything from the Geo event log' do
           create_list(:geo_event_log, 2)
 
-          expect(worker).to receive(:log_info).with('No secondary nodes, delete all Geo Event Log entries')
+          expect(worker).to receive(:log_info).with('No secondary nodes, truncate the Geo Event Log table')
+          expect(ActiveRecord::Base.connection).to receive(:truncate).with('geo_event_log').and_call_original
 
           expect { worker.perform }.to change { Geo::EventLog.count }.by(-2)
         end
@@ -79,13 +80,13 @@ describe Geo::PruneEventLogWorker, :geo do
         end
 
         it 'takes the integer-minimum value of all cursor_last_event_ids' do
-          events = create_list(:geo_event_log, 12)
+          events = create_list(:geo_event_log, 5)
 
           create(:geo_node_status, :healthy, cursor_last_event_id: events[3].id, geo_node_id: secondary.id)
           create(:geo_node_status, :healthy, cursor_last_event_id: events.last.id, geo_node_id: secondary2.id)
           expect(worker).to receive(:log_info).with('Delete Geo Event Log entries up to id', geo_event_log_id: events[3].id)
 
-          expect { worker.perform }.to change { Geo::EventLog.count }.by(-3)
+          expect { worker.perform }.to change { Geo::EventLog.count }.by(-4)
         end
       end
     end