Hide dry run partition pruning behind feature flag

Adds the partition_pruning_dry_run feature flag.

Even though the partition manager does not actually detach any
partitions yet, putting the logic for determining the set of partitions
to detach behind a flag makes rolling out this code safer, because the
logic changed touches the logic for creating new partitions.

Feature flag issue: https://gitlab.com/gitlab-org/gitlab/-/issues/335315

config/feature_flags/development/partition_pruning_dry_run.yml

+---
+name: partition_pruning_dry_run
+introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/65093
+rollout_issue_url: 
+milestone: '14.1'
+type: development
+group: group::database
+default_enabled: false

lib/gitlab/database/partitioning/monthly_strategy.rb

@@ -86,7 +86,7 @@ module Gitlab
         end
 
         def pruning_old_partitions?
-          retain_for.present?
+          Feature.enabled?(:partition_pruning_dry_run) && retain_for.present?
         end
 
         def oldest_active_date

lib/gitlab/database/partitioning/partition_manager.rb

@@ -46,6 +46,8 @@ module Gitlab
         end
 
         def detach_partitions
+          return unless Feature.enabled?(:partition_pruning_dry_run)
+
           models.each do |model|
             next if extra_partitions(model).empty?
 

spec/lib/gitlab/database/partitioning/monthly_strategy_spec.rb

@@ -237,6 +237,16 @@ RSpec.describe Gitlab::Database::Partitioning::MonthlyStrategy do
 
           expect(subject).to contain_exactly(min_value_to_may)
         end
+
+        context 'when the feature flag is toggled off' do
+          before do
+            stub_feature_flags(partition_pruning_dry_run: false)
+          end
+
+          it 'is empty' do
+            expect(subject).to eq([])
+          end
+        end
       end
 
       context 'with a time retention policy of 2 months' do
@@ -248,6 +258,16 @@ RSpec.describe Gitlab::Database::Partitioning::MonthlyStrategy do
                                Gitlab::Database::Partitioning::TimePartition.new(model.table_name, '2020-05-01', '2020-06-01', partition_name: 'partitioned_test_202005')
           )
         end
+
+        context 'when the feature flag is toggled off' do
+          before do
+            stub_feature_flags(partition_pruning_dry_run: false)
+          end
+
+          it 'is empty' do
+            expect(subject).to eq([])
+          end
+        end
       end
     end
   end

spec/lib/gitlab/database/partitioning/partition_manager_spec.rb

@@ -118,25 +118,41 @@ RSpec.describe Gitlab::Database::Partitioning::PartitionManager do
       ]
     end
 
-    it 'detaches each extra partition' do
-      extra_partitions.each { |p| expect(manager).to receive(:detach_one_partition).with(p) }
-
-      subject
-    end
+    context 'with the partition_pruning_dry_run feature flag enabled' do
+      before do
+        stub_feature_flags(partition_pruning_dry_run: true)
+      end
+      it 'detaches each extra partition' do
+        extra_partitions.each { |p| expect(manager).to receive(:detach_one_partition).with(p) }
 
-    context 'error handling' do
-      let(:models) do
-        [
-          double(partitioning_strategy: error_strategy, table_name: table),
-          model
-        ]
+        subject
       end
 
-      let(:error_strategy) { double }
+      context 'error handling' do
+        let(:models) do
+          [
+            double(partitioning_strategy: error_strategy, table_name: table),
+            model
+          ]
+        end
 
-      it 'still drops partitions for the other model' do
-        expect(error_strategy).to receive(:extra_partitions).and_raise('injected error!')
-        extra_partitions.each { |p| expect(manager).to receive(:detach_one_partition).with(p) }
+        let(:error_strategy) { double }
+
+        it 'still drops partitions for the other model' do
+          expect(error_strategy).to receive(:extra_partitions).and_raise('injected error!')
+          extra_partitions.each { |p| expect(manager).to receive(:detach_one_partition).with(p) }
+
+          subject
+        end
+      end
+    end
+
+    context 'with the partition_pruning_dry_run feature flag disabled' do
+      before do
+        stub_feature_flags(partition_pruning_dry_run: false)
+      end
+      it 'returns immediately' do
+        expect(manager).not_to receive(:detach)
 
         subject
       end