Cleanup legacy artifact background migration

Add migration and spec

commit 3cc12e1268a6865f524d8fab1804f018312fdf5a
Author: Shinya Maeda <shinya@gitlab.com>
Date:   Tue Jan 8 19:34:31 2019 +0900

    Add changelog to this change

commit 5006fc96e38db514956a35f53ae8ee536548a2e9
Author: Shinya Maeda <shinya@gitlab.com>
Date:   Tue Jan 8 19:32:56 2019 +0900

    Remove partial index from ci_builds artifact_file

    ok

    Update schema

commit 3c956bdc02b195bc67d0327bf3748a631ea5466d
Author: Shinya Maeda <shinya@gitlab.com>
Date:   Mon Jan 7 21:41:27 2019 +0900

    Add frozen_string_literal: true

commit 8c827cd616890160e6e8908843403a6f20c03236
Author: Shinya Maeda <shinya@gitlab.com>
Date:   Mon Jan 7 21:40:06 2019 +0900

    Set batch size 100

commit aeee559777d3bdeadfd2b9bb61d460f2dc1fa8a6
Author: Shinya Maeda <shinya@gitlab.com>
Date:   Fri Jan 4 18:33:05 2019 +0900

    Cleanup legacy artifact background migration

3 files changed, 52 insertions, 1 deletions

diff --git a/db/migrate/20190104182041_cleanup_legacy_artifact_migration.rb b/db/migrate/20190104182041_cleanup_legacy_artifact_migration.rb
new file mode 100644
index 00000000000..11659846a06
--- /dev/null
+++ b/db/migrate/20190104182041_cleanup_legacy_artifact_migration.rb
@@ -0,0 +1,34 @@
+# frozen_string_literal: true
+
+class CleanupLegacyArtifactMigration < ActiveRecord::Migration[5.0]
+  include Gitlab::Database::MigrationHelpers
+
+  DOWNTIME = false
+
+  disable_ddl_transaction!
+
+  class Build < ActiveRecord::Base
+    include EachBatch
+
+    self.table_name = 'ci_builds'
+    self.inheritance_column = :_type_disabled
+
+    scope :with_legacy_artifacts, -> { where("artifacts_file <> ''") }
+  end
+
+  def up
+    Gitlab::BackgroundMigration.steal('MigrateLegacyArtifacts')
+
+    CleanupLegacyArtifactMigration::Build
+      .with_legacy_artifacts
+      .each_batch(of: 100) do |batch|
+      range = batch.pluck('MIN(id)', 'MAX(id)').first
+
+      Gitlab::BackgroundMigration::MigrateLegacyArtifacts.new.perform(*range)
+    end
+  end
+
+  def down
+    # no-op
+  end
+end
diff --git a/db/migrate/20190108192941_remove_partial_index_from_ci_builds_artifacts_file.rb b/db/migrate/20190108192941_remove_partial_index_from_ci_builds_artifacts_file.rb
new file mode 100644
index 00000000000..073faf721ae
--- /dev/null
+++ b/db/migrate/20190108192941_remove_partial_index_from_ci_builds_artifacts_file.rb
@@ -0,0 +1,18 @@
+# frozen_string_literal: true
+
+class RemovePartialIndexFromCiBuildsArtifactsFile < ActiveRecord::Migration[5.0]
+  include Gitlab::Database::MigrationHelpers
+
+  DOWNTIME = false
+  INDEX_NAME = 'partial_index_ci_builds_on_id_with_legacy_artifacts'.freeze
+
+  disable_ddl_transaction!
+
+  def up
+    remove_concurrent_index_by_name(:ci_builds, INDEX_NAME)
+  end
+
+  def down
+    add_concurrent_index(:ci_builds, :id, where: "artifacts_file <> ''", name: INDEX_NAME)
+  end
+end
diff --git a/db/schema.rb b/db/schema.rb
index c4902116a3a..c6fef9b5d11 100644
--- a/db/schema.rb
+++ b/db/schema.rb
@@ -354,7 +354,6 @@ ActiveRecord::Schema.define(version: 20190115054216) do
     t.index ["commit_id", "status", "type"], name: "index_ci_builds_on_commit_id_and_status_and_type", using: :btree
     t.index ["commit_id", "type", "name", "ref"], name: "index_ci_builds_on_commit_id_and_type_and_name_and_ref", using: :btree
     t.index ["commit_id", "type", "ref"], name: "index_ci_builds_on_commit_id_and_type_and_ref", using: :btree
-    t.index ["id"], name: "partial_index_ci_builds_on_id_with_legacy_artifacts", where: "(artifacts_file <> ''::text)", using: :btree
     t.index ["project_id", "id"], name: "index_ci_builds_on_project_id_and_id", using: :btree
     t.index ["project_id", "status"], name: "index_ci_builds_project_id_and_status_for_live_jobs_partial2", where: "(((type)::text = 'Ci::Build'::text) AND ((status)::text = ANY (ARRAY[('running'::character varying)::text, ('pending'::character varying)::text, ('created'::character varying)::text])))", using: :btree
     t.index ["protected"], name: "index_ci_builds_on_protected", using: :btree