Fix an exception in Geo scheduler workers

app/workers/geo/base_scheduler_worker.rb

@@ -124,6 +124,8 @@ module Geo
     def schedule_jobs
       capacity = max_capacity
       num_to_schedule = [capacity - scheduled_job_ids.size, pending_resources.size].min
+      num_to_schedule = 0 if num_to_schedule < 0
+
       to_schedule = pending_resources.shift(num_to_schedule)
 
       scheduled = to_schedule.map do |args|

changelogs/unreleased-ee/4309-fix-negative-array-size.yml

+---
+title: Fix an exception in Geo scheduler workers
+merge_request: 3740
+author:
+type: fixed

spec/workers/geo/repository_shard_sync_worker_spec.rb

@@ -175,6 +175,15 @@ describe Geo::RepositoryShardSyncWorker, :geo, :delete, :clean_gitlab_redis_cach
         Sidekiq::Testing.inline! { subject.perform(shard_name) }
       end
     end
+
+    context 'number of scheduled jobs exceeds capacity' do
+      it 'schedules 0 jobs' do
+        is_expected.to receive(:scheduled_job_ids).and_return(1..1000).at_least(:once)
+        is_expected.not_to receive(:schedule_job)
+
+        Sidekiq::Testing.inline! { subject.perform(shard_name) }
+      end
+    end
   end
 
   describe 'when PostgreSQL FDW is available', :geo do