Retry migration removing stages in case of duplicates

db/post_migrate/20180119121225_remove_redundant_pipeline_stages.rb

@@ -3,12 +3,15 @@ class RemoveRedundantPipelineStages < ActiveRecord::Migration
 
   DOWNTIME = false
 
-  def up
-    remove_concurrent_index :ci_stages, [:pipeline_id, :name]
+  disable_ddl_transaction!
 
+  def up(attempts: 100)
+    remove_outdated_index!
     remove_redundant_pipeline_stages!
-
-    add_concurrent_index :ci_stages, [:pipeline_id, :name], unique: true
+    add_unique_index!
+  rescue ActiveRecord::RecordNotUnique
+    retry if (attempts -= 1) > 0
+    raise
   end
 
   def down
@@ -18,6 +21,16 @@ class RemoveRedundantPipelineStages < ActiveRecord::Migration
 
   private
 
+  def remove_outdated_index!
+    return unless index_exists?(:ci_stages, [:pipeline_id, :name])
+
+    remove_concurrent_index :ci_stages, [:pipeline_id, :name]
+  end
+
+  def add_unique_index!
+    add_concurrent_index :ci_stages, [:pipeline_id, :name], unique: true
+  end
+
   def remove_redundant_pipeline_stages!
     redundant_stages_ids = <<~SQL
       SELECT id FROM ci_stages WHERE (pipeline_id, name) IN (

spec/migrations/remove_redundant_pipeline_stages_spec.rb

@@ -36,4 +36,13 @@ describe RemoveRedundantPipelineStages, :migration do
     expect(builds.all.count).to eq 6
     expect(builds.all.pluck(:stage_id).compact).to eq [102]
   end
+
+  it 'retries when duplicated stages are being created during migration' do
+    allow(subject).to receive(:remove_outdated_index!)
+    expect(subject).to receive(:remove_redundant_pipeline_stages!).exactly(3).times
+    allow(subject).to receive(:add_unique_index!)
+      .and_raise(ActiveRecord::RecordNotUnique.new('Duplicated stages present!'))
+
+    expect { subject.up(attempts: 3) }.to raise_error ActiveRecord::RecordNotUnique
+  end
 end