Add post migration for checksum calculation

app/workers/all_queues.yml

@@ -104,3 +104,5 @@
 - update_user_activity
 - upload_checksum
 - web_hook
+
+- object_storage:update_artifact_checksum_worker
\ No newline at end of file

app/workers/update_artifact_checksum_worker.rb

+class UpdateArtifactChecksumWorker
+  include ApplicationWorker
+  include ObjectStorageQueue
+
+  def perform(job_artifact_id)
+    Ci::JobArtifact.find_by(id: job_artifact_id).try do |job_artifact|
+      job_artifact.set_checksum
+      job_artifact.save!
+    end
+  end
+end

db/post_migrate/20180228121020_update_checksum_for_ci_job_artifacts.rb

+class UpdateChecksumForCiJobArtifacts < ActiveRecord::Migration
+  include Gitlab::Database::MigrationHelpers
+
+  DOWNTIME = false
+  BATCH_SIZE = 2500
+
+  class JobArtifact < ActiveRecord::Base
+    include EachBatch
+    self.table_name = 'ci_job_artifacts'
+  end
+
+  def up
+    UpdateChecksumForCiJobArtifacts::JobArtifact
+      .where('checksum IS NULL')
+      .each_batch(of: BATCH_SIZE) do |relation|
+      ids = relation.pluck(:id).map { |id| [id] }
+
+      UpdateArtifactChecksumWorker.bulk_perform_async(ids)
+    end
+  end
+
+  def down
+    # no-op
+  end
+end