Fix edge cases while iterating over model and registry table

There are some edge cases to take into acocunt:

1. When there are unused registries, but there no
   replicable records next_range! returns nil;
2. When the unused registry foreign key ids are greater
   than the last replicable record id;
3. When the unused registry foreign key ids are lower
   than the first replicable record id;

ee/lib/gitlab/geo/registry_batcher.rb

@@ -2,8 +2,9 @@
 
 module Gitlab
   module Geo
-    # Returns an ID range within a table so it can be iterated over. Repeats from
-    # the beginning after it reaches the end.
+    # Returns an ID range to allow iteration over a registry table and its
+    # source replicable table. Repeats from the beginning after it reaches
+    # the end.
     #
     # Used by Geo in particular to iterate over a replicable and its registry
     # table.
@@ -93,11 +94,18 @@ module Gitlab
 
       # @private
       #
-      # This method is used to remove orphaned registries where the foreign key
-      # IDs are greather than last replicable ID. The difference here is that
-      # we need to check against the foreign key IDS not the registry ID and
-      # for the existence of more rows to check we need to check against the
-      # the first ID of the batch.
+      # Get the last ID of the of the batch (not the table) for the registry
+      # and check if there are more rows in the table.
+      #
+      # This query differs from the replicable query by:
+      #
+      # - We check against the foreign key IDs not the registry IDs;
+      # - In the where clause of the more_rows part, we use greater
+      #   than or equal. This allows the batcher to switch to the
+      #   registry table while getting the last ID of the batch
+      #   when the previous batch included the end of the replicable
+      #   table but there are orphaned registries where the foreign key
+      #   ids are higher than the last replicable id;
       #
       # @param [Integer] batch_first_id the first ID of the batch
       # @return [Integer, Boolean] A tuple with the the last ID of the batch (not the table),
@@ -108,12 +116,12 @@ module Gitlab
           EXISTS (
             SELECT #{model_foreign_key}
             FROM #{registry_class.table_name}
-            WHERE #{model_foreign_key} > #{batch_first_id}
+            WHERE #{model_foreign_key} >= MAX(batch.#{model_foreign_key})
           ) AS more_rows
           FROM (
             SELECT #{model_foreign_key}
             FROM #{registry_class.table_name}
-            WHERE #{model_foreign_key} > #{batch_first_id}
+            WHERE #{model_foreign_key} >= #{batch_first_id}
             ORDER BY #{model_foreign_key}
             LIMIT #{batch_size}) AS batch;
         SQL

ee/spec/lib/gitlab/geo/registry_batcher_spec.rb

@@ -41,13 +41,29 @@ describe Gitlab::Geo::RegistryBatcher, :geo, :use_clean_rails_memory_store_cachi
       end
 
       context 'when it was called before' do
+        context 'when the previous batch included the end of the table' do
           before do
-          described_class.new(registry_class, key: key, batch_size: batch_size).next_range!
+            described_class.new(registry_class, key: key, batch_size: registry_class.count).next_range!
           end
 
           it { is_expected.to be_nil }
+        end
+
+        context 'when the previous batch did not include the end of the table' do
+          before do
+            described_class.new(registry_class, key: key, batch_size: registry_class.count - 1).next_range!
+          end
+
+          it 'starts after the previous batch' do
+            expect(subject).to eq(registries.last.public_send(model_foreign_key)..registries.last.public_send(model_foreign_key))
+          end
+        end
 
         context 'if cache is cleared' do
+          before do
+            described_class.new(registry_class, key: key, batch_size: batch_size).next_range!
+          end
+
           it 'starts from the beginning' do
             Rails.cache.clear