Added helper methods for database migrations

These helpers can be used to perform migrations without taking down the
entire application.

For example, the method "add_column_with_default" can be used to add a
new column with a default value without locking the entire table.

lib/gitlab/database/migration_helpers.rb

+module Gitlab
+  module Database
+    module MigrationHelpers
+      # Creates a new index, concurrently when supported
+      #
+      # On PostgreSQL this method creates an index concurrently, on MySQL this
+      # creates a regular index.
+      #
+      # Example:
+      #
+      #     add_concurrent_index :users, :some_column
+      #
+      # See Rails' `add_index` for more info on the available arguments.
+      def add_concurrent_index(*args)
+        if transaction_open?
+          raise 'add_concurrent_index can not be run inside a transaction, ' \
+            'you can disable transactions by calling disable_ddl_transaction! ' \
+            'in the body of your migration class'
+        end
+
+        if Database.postgresql?
+          args << { algorithm: :concurrently }
+        end
+
+        add_index(*args)
+      end
+
+      # Updates the value of a column in batches.
+      #
+      # This method updates the table in batches of 5% of the total row count.
+      # Any data inserted while running this method (or after it has finished
+      # running) is _not_ updated automatically.
+      #
+      # This method _only_ updates rows where the column's value is set to NULL.
+      #
+      # table - The name of the table.
+      # column - The name of the column to update.
+      # value - The value for the column.
+      def update_column_in_batches(table, column, value)
+        quoted_table = quote_table_name(table)
+        quoted_column = quote_column_name(column)
+        quoted_value = quote(value)
+        processed = 0
+
+        total = exec_query("SELECT COUNT(*) AS count FROM #{quoted_table}").
+          to_hash.
+          first['count'].
+          to_i
+
+        # Update in batches of 5% with an upper limit of 5000 rows.
+        batch_size = ((total / 100.0) * 5.0).ceil
+
+        while processed < total
+          start_row = exec_query(%Q{
+            SELECT id
+            FROM #{quoted_table}
+            ORDER BY id ASC
+            LIMIT 1 OFFSET #{processed}
+          }).to_hash.first
+
+          stop_row = exec_query(%Q{
+            SELECT id
+            FROM #{quoted_table}
+            ORDER BY id ASC
+            LIMIT 1 OFFSET #{processed + batch_size}
+          }.strip_heredoc).to_hash.first
+
+          query = %Q{
+            UPDATE #{quoted_table}
+            SET #{quoted_column} = #{quoted_value}
+            WHERE id >= #{start_row['id']}
+          }
+
+          if stop_row
+            query += " AND id < #{stop_row['id']}"
+          end
+
+          execute(query.strip_heredoc)
+
+          processed += batch_size
+        end
+      end
+
+      # Adds a column with a default value without locking an entire table.
+      #
+      # This method runs the following steps:
+      #
+      # 1. Add the column with a default value of NULL.
+      # 2. Update all existing rows in batches.
+      # 3. Change the default value of the column to the specified value.
+      # 4. Update any remaining rows.
+      #
+      # These steps ensure a column can be added to a large and commonly used
+      # table without locking the entire table for the duration of the table
+      # modification.
+      #
+      # table - The name of the table to update.
+      # column - The name of the column to add.
+      # type - The column type (e.g. `:integer`).
+      # default - The default value for the column.
+      # allow_null - When set to `true` the column will allow NULL values, the
+      #              default is to not allow NULL values.
+      def add_column_with_default(table, column, type, default:, allow_null: false)
+        if transaction_open?
+          raise 'add_column_with_default can not be run inside a transaction, ' \
+            'you can disable transactions by calling disable_ddl_transaction! ' \
+            'in the body of your migration class'
+        end
+
+        transaction do
+          add_column(table, column, type, default: nil)
+
+          # Changing the default before the update ensures any newly inserted
+          # rows already use the proper default value.
+          change_column_default(table, column, default)
+        end
+
+        begin
+          transaction do
+            update_column_in_batches(table, column, default)
+          end
+        rescue Exception => error
+          remove_column(table, column)
+
+          raise error
+        end
+
+        change_column_null(table, column, false) unless allow_null
+      end
+    end
+  end
+end

spec/lib/gitlab/database/migration_helpers_spec.rb

+require 'spec_helper'
+
+describe Gitlab::Database::MigrationHelpers, lib: true do
+  let(:model) do
+    Class.new do
+      include Gitlab::Database::MigrationHelpers
+
+      def method_missing(name, *args, &block)
+        ActiveRecord::Base.connection.send(name, *args, &block)
+      end
+    end.new
+  end
+
+  describe '#add_concurrent_index' do
+    context 'outside a transaction' do
+      before do
+        expect(model).to receive(:transaction_open?).and_return(false)
+      end
+
+      context 'using PostgreSQL' do
+        it 'creates the index concurrently' do
+          expect(Gitlab::Database).to receive(:postgresql?).and_return(true)
+
+          expect(model).to receive(:add_index).
+            with(:users, :foo, algorithm: :concurrently)
+
+          model.add_concurrent_index(:users, :foo)
+        end
+      end
+
+      context 'using MySQL' do
+        it 'creates a regular index' do
+          expect(Gitlab::Database).to receive(:postgresql?).and_return(false)
+
+          expect(model).to receive(:add_index).
+            with(:users, :foo)
+
+          model.add_concurrent_index(:users, :foo)
+        end
+      end
+    end
+
+    context 'inside a transaction' do
+      it 'raises RuntimeError' do
+        expect(model).to receive(:transaction_open?).and_return(true)
+
+        expect { model.add_concurrent_index(:users, :foo) }.
+          to raise_error(RuntimeError)
+      end
+    end
+  end
+
+  describe '#update_column_in_batches' do
+    before do
+      create_list(:empty_project, 5)
+    end
+
+    it 'updates all the rows in a table' do
+      model.update_column_in_batches(:projects, :import_error, 'foo')
+
+      expect(Project.where(import_error: 'foo').count).to eq(5)
+    end
+  end
+
+  describe '#add_column_with_default' do
+    context 'outside of a transaction' do
+      before do
+        expect(model).to receive(:transaction_open?).and_return(false)
+
+        expect(model).to receive(:transaction).twice.and_yield
+
+        expect(model).to receive(:add_column).
+          with(:projects, :foo, :integer, default: nil)
+
+        expect(model).to receive(:change_column_default).
+          with(:projects, :foo, 10)
+      end
+
+      it 'adds the column while allowing NULL values' do
+        expect(model).to receive(:update_column_in_batches).
+          with(:projects, :foo, 10)
+
+        expect(model).not_to receive(:change_column_null)
+
+        model.add_column_with_default(:projects, :foo, :integer,
+                                      default: 10,
+                                      allow_null: true)
+      end
+
+      it 'adds the column while not allowing NULL values' do
+        expect(model).to receive(:update_column_in_batches).
+          with(:projects, :foo, 10)
+
+        expect(model).to receive(:change_column_null).
+          with(:projects, :foo, false)
+
+        model.add_column_with_default(:projects, :foo, :integer, default: 10)
+      end
+
+      it 'removes the added column whenever updating the rows fails' do
+        expect(model).to receive(:update_column_in_batches).
+          with(:projects, :foo, 10).
+          and_raise(RuntimeError)
+
+        expect(model).to receive(:remove_column).
+          with(:projects, :foo)
+
+        expect {
+          model.add_column_with_default(:projects, :foo, :integer, default: 10)
+        }.to raise_error(RuntimeError)
+      end
+    end
+
+    context 'inside a transaction' do
+      it 'raises RuntimeError' do
+        expect(model).to receive(:transaction_open?).and_return(true)
+
+        expect {
+          model.add_column_with_default(:projects, :foo, :integer, default: 10)
+        }.to raise_error(RuntimeError)
+      end
+    end
+  end
+end