length constrain on the index, not on the column

we actually don't need a limit on the column itself for MySQL to work.
we need to set a length on the index.

db/migrate/20170222111732_create_gpg_keys.rb

@@ -7,12 +7,12 @@ class CreateGpgKeys < ActiveRecord::Migration
       t.references :user, index: true, foreign_key: { on_delete: :cascade }
-      t.binary :primary_keyid, limit: Gitlab::Database.mysql? ? 20 : nil
-      t.binary :fingerprint, limit: Gitlab::Database.mysql? ? 20 : nil
+      t.binary :primary_keyid
+      t.binary :fingerprint
       t.text :key
-      t.index :primary_keyid
+      t.index :primary_keyid, length: Gitlab::Database.mysql? ? 20 : nil
     end
   end
 end

db/migrate/20170613154149_create_gpg_signatures.rb

@@ -10,14 +10,14 @@ class CreateGpgSignatures < ActiveRecord::Migration
       t.boolean :valid_signature
-      t.binary :commit_sha, limit: Gitlab::Database.mysql? ? 20 : nil
-      t.binary :gpg_key_primary_keyid, limit: Gitlab::Database.mysql? ? 20 : nil
+      t.binary :commit_sha
+      t.binary :gpg_key_primary_keyid
       t.text :gpg_key_user_name
       t.text :gpg_key_user_email
-      t.index :commit_sha
-      t.index :gpg_key_primary_keyid
+      t.index :commit_sha, length: Gitlab::Database.mysql? ? 20 : nil
+      t.index :gpg_key_primary_keyid, length: Gitlab::Database.mysql? ? 20 : nil
     end
   end
 end

db/migrate/limits_to_mysql.rb

 # rubocop:disable all
+require Rails.root.join('lib/gitlab/database/migration_helpers.rb')
+
 class LimitsToMysql < ActiveRecord::Migration
+  include Gitlab::Database::MigrationHelpers
+
   def up
     return unless ActiveRecord::Base.configurations[Rails.env]['adapter'] =~ /^mysql/
@@ -8,9 +12,14 @@ class LimitsToMysql < ActiveRecord::Migration
     change_column :snippets, :content, :text, limit: 2147483647
     change_column :notes, :st_diff, :text, limit: 2147483647
     change_column :events, :data, :text, limit: 2147483647
-    change_column :gpg_keys, :primary_keyid, :binary, limit: 20
-    change_column :gpg_keys, :fingerprint, :binary, limit: 20
-    change_column :gpg_signatures, :commit_sha, :binary, limit: 20
-    change_column :gpg_signatures, :gpg_key_primary_keyid, :binary, limit: 20
+
+    [
+      [:gpg_keys, :primary_keyid],
+      [:gpg_signatures, :commit_sha],
+      [:gpg_signatures, :gpg_key_primary_keyid]
+    ].each do |table_name, column_name|
+      remove_index table_name, column_name if index_exists?(table_name, column_name)
+      add_concurrent_index table_name, column_name, length: 20
+    end
   end
 end