destroy all signatures and add with default value

To avoid having to implement legacy code handling for the obsolete
`verified_signature` attribute and to avoid any race conditions during
the zero-downtime-deployment we do the following:

1. Destroy all records
2. Migration: Use add_column_with_default to add the new attribute and
   update the verification_status values on records created between 1.
   and 2.
3. Deploy the new code
4. Post migration: Destroy all records

Like this we make sure that at no point there is a record with a `nil`
value for the new `verification_status` attribute.

db/migrate/20170817123339_add_verification_status_to_gpg_signatures.rb

 class AddVerificationStatusToGpgSignatures < ActiveRecord::Migration
   DOWNTIME = false
 
-  def change
-    add_column :gpg_signatures, :verification_status, :smallint
+  include Gitlab::Database::MigrationHelpers
+  disable_ddl_transaction!
+
+  class GpgSignature < ActiveRecord::Base
+    self.table_name = 'gpg_signatures'
+
+    include EachBatch
+  end
+
+  def up
+    # First we remove all signatures because we need to re-verify them all
+    # again anyway (because of the updated verification logic).
+    #
+    # This makes adding the column with default values faster
+    GpgSignature.each_batch do |relation|
+      relation.delete_all
+    end
+
+    add_column_with_default(:gpg_signatures, :verification_status, :smallint, default: 0)
+  end
+
+  def down
+    remove_column(:gpg_signatures, :verification_status)
   end
 end

db/schema.rb

@@ -614,7 +614,7 @@ ActiveRecord::Schema.define(version: 20170830125940) do
     t.binary "gpg_key_primary_keyid"
     t.text "gpg_key_user_name"
     t.text "gpg_key_user_email"
-    t.integer "verification_status", limit: 2
+    t.integer "verification_status", limit: 2, default: 0, null: false
   end
 
   add_index "gpg_signatures", ["commit_sha"], name: "index_gpg_signatures_on_commit_sha", unique: true, using: :btree