chuckya/lib/mastodon/migration_helpers.rb

# frozen_string_literal: true

# This file is copied almost entirely from GitLab, which has done a large
# amount of work to ensure that migrations can happen with minimal downtime.
# Many thanks to those engineers.

# Changes have been made to remove dependencies on other GitLab files and to
# shorten temporary column names.

# Documentation on using these functions (and why one might do so):
# https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/development/what_requires_downtime.md

# The file itself:
# https://gitlab.com/gitlab-org/gitlab-ce/blob/master/lib/gitlab/database/migration_helpers.rb

# It is licensed as follows:

# Copyright (c) 2011-2017 GitLab B.V.

# Permission is hereby granted, free of charge, to any person obtaining a copy
# of this software and associated documentation files (the "Software"), to deal
# in the Software without restriction, including without limitation the rights
# to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
# copies of the Software, and to permit persons to whom the Software is
# furnished to do so, subject to the following conditions:

# The above copyright notice and this permission notice shall be included in
# all copies or substantial portions of the Software.

# THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
# IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
# FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
# AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
# LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
# OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
# THE SOFTWARE.

# This is bad form, but there are enough differences that it's impractical to do
# otherwise:

module Mastodon
  module MigrationHelpers
    class CorruptionError < StandardError
      attr_reader :index_name

      def initialize(index_name)
        @index_name = index_name

        super "The index `#{index_name}` seems to be corrupted, it contains duplicate rows. " \
          'For information on how to fix this, see our documentation: ' \
          'https://docs.joinmastodon.org/admin/troubleshooting/index-corruption/'
      end

      def cause
        nil
      end

      def backtrace
        []
      end
    end

    # Model that can be used for querying permissions of a SQL user.
    class Grant < ActiveRecord::Base
      self.table_name = 'information_schema.role_table_grants'

      def self.scope_to_current_user
        where('grantee = user')
      end

      # Returns true if the current user can create and execute triggers on the
      # given table.
      def self.create_and_execute_trigger?(table)
        priv = where(privilege_type: 'TRIGGER', table_name: table)

        priv.scope_to_current_user.any?
      end
    end

    BACKGROUND_MIGRATION_BATCH_SIZE = 1000 # Number of rows to process per job
    BACKGROUND_MIGRATION_JOB_BUFFER_SIZE = 1000 # Number of jobs to bulk queue at a time

    # Gets an estimated number of rows for a table
    def estimate_rows_in_table(table_name)
      exec_query('SELECT reltuples FROM pg_class WHERE relname = ' +
        "'#{table_name}'").to_a.first['reltuples']
    end

    # Adds `created_at` and `updated_at` columns with timezone information.
    #
    # This method is an improved version of Rails' built-in method `add_timestamps`.
    #
    # Available options are:
    # default - The default value for the column.
    # null - When set to `true` the column will allow NULL values.
    #        The default is to not allow NULL values.
    def add_timestamps_with_timezone(table_name, **options)
      options[:null] = false if options[:null].nil?

      [:created_at, :updated_at].each do |column_name|
        if options[:default] && transaction_open?
          raise '`add_timestamps_with_timezone` with default value cannot be run inside a transaction. ' \
            'You can disable transactions by calling `disable_ddl_transaction!` ' \
            'in the body of your migration class'
        end

        # If default value is presented, use `add_column_with_default` method instead.
        if options[:default]
          add_column_with_default(
            table_name,
            column_name,
            :datetime_with_timezone,
            default: options[:default],
            allow_null: options[:null]
          )
        else
          add_column(table_name, column_name, :datetime_with_timezone, **options)
        end
      end
    end

    # 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(table_name, column_name, **options)
      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

      options = options.merge({ algorithm: :concurrently })
      disable_statement_timeout

      add_index(table_name, column_name, **options)
    end

    # Removes an existed index, concurrently when supported
    #
    # On PostgreSQL this method removes an index concurrently.
    #
    # Example:
    #
    #     remove_concurrent_index :users, :some_column
    #
    # See Rails' `remove_index` for more info on the available arguments.
    def remove_concurrent_index(table_name, column_name, **options)
      if transaction_open?
        raise 'remove_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 supports_drop_index_concurrently?
        options = options.merge({ algorithm: :concurrently })
        disable_statement_timeout
      end

      remove_index(table_name, **options.merge({ column: column_name }))
    end

    # Removes an existing index, concurrently when supported
    #
    # On PostgreSQL this method removes an index concurrently.
    #
    # Example:
    #
    #     remove_concurrent_index :users, "index_X_by_Y"
    #
    # See Rails' `remove_index` for more info on the available arguments.
    def remove_concurrent_index_by_name(table_name, index_name, **options)
      if transaction_open?
        raise 'remove_concurrent_index_by_name 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 supports_drop_index_concurrently?
        options = options.merge({ algorithm: :concurrently })
        disable_statement_timeout
      end

      remove_index(table_name, **options.merge({ name: index_name }))
    end

    # Only available on Postgresql >= 9.2
    def supports_drop_index_concurrently?
      version = select_one("SELECT current_setting('server_version_num') AS v")['v'].to_i

      version >= 90_200
    end

    # Only available on Postgresql >= 11
    def supports_add_column_with_default?
      version = select_one("SELECT current_setting('server_version_num') AS v")['v'].to_i

      version >= 110_000
    end

    # Adds a foreign key with only minimal locking on the tables involved.
    #
    # This method only requires minimal locking when using PostgreSQL. When
    # using MySQL this method will use Rails' default `add_foreign_key`.
    #
    # source - The source table containing the foreign key.
    # target - The target table the key points to.
    # column - The name of the column to create the foreign key on.
    # on_delete - The action to perform when associated data is removed,
    #             defaults to "CASCADE".
    def add_concurrent_foreign_key(source, target, column:, on_delete: :cascade, target_col: 'id')
      # Transactions would result in ALTER TABLE locks being held for the
      # duration of the transaction, defeating the purpose of this method.
      if transaction_open?
        raise 'add_concurrent_foreign_key can not be run inside a transaction'
      end

      # While MySQL does allow disabling of foreign keys it has no equivalent
      # of PostgreSQL's "VALIDATE CONSTRAINT". As a result we'll just fall
      # back to the normal foreign key procedure.
      on_delete = 'SET NULL' if on_delete == :nullify

      disable_statement_timeout

      key_name = concurrent_foreign_key_name(source, column, target_col)

      # Using NOT VALID allows us to create a key without immediately
      # validating it. This means we keep the ALTER TABLE lock only for a
      # short period of time. The key _is_ enforced for any newly created
      # data.
      execute <<-EOF.strip_heredoc
      ALTER TABLE #{source}
      ADD CONSTRAINT #{key_name}
      FOREIGN KEY (#{column})
      REFERENCES #{target} (#{target_col})
      #{on_delete ? "ON DELETE #{on_delete.upcase}" : ''}
      NOT VALID;
      EOF

      # Validate the existing constraint. This can potentially take a very
      # long time to complete, but fortunately does not lock the source table
      # while running.
      execute("ALTER TABLE #{source} VALIDATE CONSTRAINT #{key_name};")
    end

    # Returns the name for a concurrent foreign key.
    #
    # PostgreSQL constraint names have a limit of 63 bytes. The logic used
    # here is based on Rails' foreign_key_name() method, which unfortunately
    # is private so we can't rely on it directly.
    def concurrent_foreign_key_name(table, column, target_col)
      "fk_#{Digest::SHA256.hexdigest("#{table}_#{column}_#{target_col}_fk").first(10)}"
    end

    # Long-running migrations may take more than the timeout allowed by
    # the database. Disable the session's statement timeout to ensure
    # migrations don't get killed prematurely. (PostgreSQL only)
    def disable_statement_timeout
      execute('SET statement_timeout TO 0')
    end

    # Updates the value of a column in batches.
    #
    # This method updates the table in batches of 5% of the total row count.
    # This method will continue updating rows until no rows remain.
    #
    # When given a block this method will yield two values to the block:
    #
    # 1. An instance of `Arel::Table` for the table that is being updated.
    # 2. The query to run as an Arel object.
    #
    # By supplying a block one can add extra conditions to the queries being
    # executed. Note that the same block is used for _all_ queries.
    #
    # Example:
    #
    #     update_column_in_batches(:projects, :foo, 10) do |table, query|
    #       query.where(table[:some_column].eq('hello'))
    #     end
    #
    # This would result in this method updating only rows where
    # `projects.some_column` equals "hello".
    #
    # table - The name of the table.
    # column - The name of the column to update.
    # value - The value for the column.
    #
    # Rubocop's Metrics/AbcSize metric is disabled for this method as Rubocop
    # determines this method to be too complex while there's no way to make it
    # less "complex" without introducing extra methods (which actually will
    # make things _more_ complex).
    def update_column_in_batches(table_name, column, value)
      if transaction_open?
        raise 'update_column_in_batches can not be run inside a transaction, ' \
          'you can disable transactions by calling disable_ddl_transaction! ' \
          'in the body of your migration class'
      end

      table = Arel::Table.new(table_name)

      total = estimate_rows_in_table(table_name).to_i
      if total < 1
        count_arel = table.project(Arel.star.count.as('count'))
        count_arel = yield table, count_arel if block_given?

        total = exec_query(count_arel.to_sql).to_ary.first['count'].to_i

        return if total == 0
      end

      # Update in batches of 5% until we run out of any rows to update.
      batch_size = ((total / 100.0) * 5.0).ceil
      max_size = 1000

      # The upper limit is 1000 to ensure we don't lock too many rows. For
      # example, for "merge_requests" even 1% of the table is around 35 000
      # rows for GitLab.com.
      batch_size = max_size if batch_size > max_size

      start_arel = table.project(table[:id]).order(table[:id].asc).take(1)
      start_arel = yield table, start_arel if block_given?
      first_row = exec_query(start_arel.to_sql).to_ary.first
      # In case there are no rows but we didn't catch it in the estimated size:
      return unless first_row
      start_id = first_row['id'].to_i

      say "Migrating #{table_name}.#{column} (~#{total.to_i} rows)"

      started_time = Time.zone.now
      last_time = Time.zone.now
      migrated = 0
      loop do
        stop_row = nil

        suppress_messages do
          stop_arel = table.project(table[:id])
            .where(table[:id].gteq(start_id))
            .order(table[:id].asc)
            .take(1)
            .skip(batch_size)

          stop_arel = yield table, stop_arel if block_given?
          stop_row = exec_query(stop_arel.to_sql).to_ary.first

          update_arel = Arel::UpdateManager.new
            .table(table)
            .set([[table[column], value]])
            .where(table[:id].gteq(start_id))

          if stop_row
            stop_id = stop_row['id'].to_i
            start_id = stop_id
            update_arel = update_arel.where(table[:id].lt(stop_id))
          end

          update_arel = yield table, update_arel if block_given?

          execute(update_arel.to_sql)
        end

        migrated += batch_size
        if Time.zone.now - last_time > 1
          status = "Migrated #{migrated} rows"

          percentage = 100.0 * migrated / total
          status += " (~#{sprintf('%.2f', percentage)}%, "

          remaining_time = (100.0 - percentage) * (Time.zone.now - started_time) / percentage

          status += "#{(remaining_time / 60).to_i}:"
          status += sprintf('%02d', remaining_time.to_i % 60)
          status += ' remaining, '

          # Tell users not to interrupt if we're almost done.
          if remaining_time > 10
            status += 'safe to interrupt'
          else
            status += 'DO NOT interrupt'
          end

          status += ')'

          say status, true
          last_time = Time.zone.now
        end

        # There are no more rows left to update.
        break unless stop_row
      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. Change the default value of the column to the specified value.
    # 3. Update all existing rows in batches.
    # 4. Set a `NOT NULL` constraint on the column if desired (the default).
    #
    # 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.
    # limit - Sets a column limit. For example, for :integer, the default is
    #         4-bytes. Set `limit: 8` to allow 8-byte integers.
    # allow_null - When set to `true` the column will allow NULL values, the
    #              default is to not allow NULL values.
    #
    # This method can also take a block which is passed directly to the
    # `update_column_in_batches` method.
    def add_column_with_default(table, column, type, default:, limit: nil, allow_null: false, &block)
      if supports_add_column_with_default?
        add_column(table, column, type, default: default, limit: limit, null: allow_null)
        return
      end

      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

      disable_statement_timeout

      transaction do
        if limit
          add_column(table, column, type, default: nil, limit: limit)
        else
          add_column(table, column, type, default: nil)
        end

        # 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
        update_column_in_batches(table, column, default, &block)

        change_column_null(table, column, false) unless allow_null
      # We want to rescue _all_ exceptions here, even those that don't inherit
      # from StandardError.
      rescue Exception => error # rubocop: disable all
        remove_column(table, column)

        raise error
      end
    end

    # Renames a column without requiring downtime.
    #
    # Concurrent renames work by using database triggers to ensure both the
    # old and new column are in sync. However, this method will _not_ remove
    # the triggers or the old column automatically; this needs to be done
    # manually in a post-deployment migration. This can be done using the
    # method `cleanup_concurrent_column_rename`.
    #
    # table - The name of the database table containing the column.
    # old - The old column name.
    # new - The new column name.
    # type - The type of the new column. If no type is given the old column's
    #        type is used.
    def rename_column_concurrently(table, old, new, type: nil)
      if transaction_open?
        raise 'rename_column_concurrently can not be run inside a transaction'
      end

      check_trigger_permissions!(table)
      trigger_name = rename_trigger_name(table, old, new)

      # If we were in the middle of update_column_in_batches, we should remove
      # the old column and start over, as we have no idea where we were.
      if column_for(table, new)
        remove_rename_triggers_for_postgresql(table, trigger_name)

        remove_column(table, new)
      end

      old_col = column_for(table, old)
      new_type = type || old_col.type

      col_opts = {
        precision: old_col.precision,
        scale: old_col.scale,
      }

      # We may be trying to reset the limit on an integer column type, so let
      # Rails handle that.
      unless [:bigint, :integer].include?(new_type)
        col_opts[:limit] = old_col.limit
      end

      add_column(table, new, new_type, **col_opts)

      # We set the default value _after_ adding the column so we don't end up
      # updating any existing data with the default value. This isn't
      # necessary since we copy over old values further down.
      change_column_default(table, new, old_col.default) if old_col.default

      quoted_table = quote_table_name(table)
      quoted_old = quote_column_name(old)
      quoted_new = quote_column_name(new)

      install_rename_triggers_for_postgresql(trigger_name, quoted_table,
                                             quoted_old, quoted_new)

      update_column_in_batches(table, new, Arel::Table.new(table)[old])

      change_column_null(table, new, false) unless old_col.null

      copy_indexes(table, old, new)
      copy_foreign_keys(table, old, new)
    end

    # Changes the type of a column concurrently.
    #
    # table - The table containing the column.
    # column - The name of the column to change.
    # new_type - The new column type.
    def change_column_type_concurrently(table, column, new_type)
      temp_column = rename_column_name(column)

      rename_column_concurrently(table, column, temp_column, type: new_type)

      # Primary keys don't necessarily have an associated index.
      if ActiveRecord::Base.get_primary_key(table) == column.to_s
        old_pk_index_name = "index_#{table}_on_#{column}"
        new_pk_index_name = "index_#{table}_on_#{column}_cm"

        unless indexes_for(table, column).find{|i| i.name == old_pk_index_name}
          add_concurrent_index(table, [temp_column],
            unique: true,
            name: new_pk_index_name
          )
        end
      end
    end

    # Performs cleanup of a concurrent type change.
    #
    # table - The table containing the column.
    # column - The name of the column to change.
    # new_type - The new column type.
    def cleanup_concurrent_column_type_change(table, column)
      temp_column = rename_column_name(column)

      # Wait for the indices to be built
      indexes_for(table, column).each do |index|
        expected_name = index.name + '_cm'

        puts "Waiting for index #{expected_name}"
        sleep 1 until indexes_for(table, temp_column).find {|i| i.name == expected_name }
      end

      was_primary = (ActiveRecord::Base.get_primary_key(table) == column.to_s)
      old_default_fn = column_for(table, column).default_function

      old_fks = []
      if was_primary
        # Get any foreign keys pointing at this column we need to recreate, and
        # remove the old ones.
        # Based on code from:
        # http://errorbank.blogspot.com/2011/03/list-all-foreign-keys-references-for.html
        old_fks_res = execute <<-EOF.strip_heredoc
          select m.relname as src_table,
            (select a.attname
              from pg_attribute a
              where a.attrelid = m.oid
                and a.attnum = o.conkey[1]
                and a.attisdropped = false) as src_col,
            o.conname as name,
            o.confdeltype as on_delete
          from pg_constraint o
          left join pg_class f on f.oid = o.confrelid
          left join pg_class c on c.oid = o.conrelid
          left join pg_class m on m.oid = o.conrelid
          where o.contype = 'f'
            and o.conrelid in (
              select oid from pg_class c where c.relkind = 'r')
            and f.relname = '#{table}';
          EOF
        old_fks = old_fks_res.to_a
        old_fks.each do |old_fk|
          add_concurrent_foreign_key(
            old_fk['src_table'],
            table,
            column: old_fk['src_col'],
            target_col: temp_column,
            on_delete: extract_foreign_key_action(old_fk['on_delete'])
          )

          remove_foreign_key(old_fk['src_table'], name: old_fk['name'])
        end
      end

      # If there was a sequence owned by the old column, make it owned by the
      # new column, as it will otherwise be deleted when we get rid of the
      # old column.
      if (seq_match = /^nextval\('([^']*)'(::text|::regclass)?\)/.match(old_default_fn))
        seq_name = seq_match[1]
        execute("ALTER SEQUENCE #{seq_name} OWNED BY #{table}.#{temp_column}")
      end

      transaction do
        # This has to be performed in a transaction as otherwise we might have
        # inconsistent data.

        cleanup_concurrent_column_rename(table, column, temp_column)
        rename_column(table, temp_column, column)

        # If there was an old default function, we didn't copy it. Do that now
        # in the transaction, so we don't miss anything.
        change_column_default(table, column, -> { old_default_fn }) if old_default_fn
      end

      # Rename any indices back to what they should be.
      indexes_for(table, column).each do |index|
        next unless index.name.end_with?('_cm')

        real_index_name = index.name.sub(/_cm$/, '')
        rename_index(table, index.name, real_index_name)
      end

      # Rename any foreign keys back to names based on the real column.
      foreign_keys_for(table, column).each do |fk|
        old_fk_name = concurrent_foreign_key_name(fk.from_table, temp_column, 'id')
        new_fk_name = concurrent_foreign_key_name(fk.from_table, column, 'id')
        execute("ALTER TABLE #{fk.from_table} RENAME CONSTRAINT " +
          "#{old_fk_name} TO #{new_fk_name}")
      end

      # Rename any foreign keys from other tables to names based on the real
      # column.
      old_fks.each do |old_fk|
        old_fk_name = concurrent_foreign_key_name(old_fk['src_table'],
          old_fk['src_col'], temp_column)
        new_fk_name = concurrent_foreign_key_name(old_fk['src_table'],
          old_fk['src_col'], column)
        execute("ALTER TABLE #{old_fk['src_table']} RENAME CONSTRAINT " +
          "#{old_fk_name} TO #{new_fk_name}")
      end

      # If the old column was a primary key, mark the new one as a primary key.
      if was_primary
        execute("ALTER TABLE #{table} ADD PRIMARY KEY USING INDEX " +
          "index_#{table}_on_#{column}")
      end
    end

    # Cleans up a concurrent column name.
    #
    # This method takes care of removing previously installed triggers as well
    # as removing the old column.
    #
    # table - The name of the database table.
    # old - The name of the old column.
    # new - The name of the new column.
    def cleanup_concurrent_column_rename(table, old, new)
      trigger_name = rename_trigger_name(table, old, new)

      check_trigger_permissions!(table)

      remove_rename_triggers_for_postgresql(table, trigger_name)

      remove_column(table, old)
    end

    # Performs a concurrent column rename when using PostgreSQL.
    def install_rename_triggers_for_postgresql(trigger, table, old, new)
      execute <<-EOF.strip_heredoc
      CREATE OR REPLACE FUNCTION #{trigger}()
      RETURNS trigger AS
      $BODY$
      BEGIN
        NEW.#{new} := NEW.#{old};
        RETURN NEW;
      END;
      $BODY$
      LANGUAGE 'plpgsql'
      VOLATILE
      EOF

      execute <<-EOF.strip_heredoc
      CREATE TRIGGER #{trigger}
      BEFORE INSERT OR UPDATE
      ON #{table}
      FOR EACH ROW
      EXECUTE PROCEDURE #{trigger}()
      EOF
    end

    # Installs the triggers necessary to perform a concurrent column rename on
    # MySQL.
    def install_rename_triggers_for_mysql(trigger, table, old, new)
      execute <<-EOF.strip_heredoc
      CREATE TRIGGER #{trigger}_insert
      BEFORE INSERT
      ON #{table}
      FOR EACH ROW
      SET NEW.#{new} = NEW.#{old}
      EOF

      execute <<-EOF.strip_heredoc
      CREATE TRIGGER #{trigger}_update
      BEFORE UPDATE
      ON #{table}
      FOR EACH ROW
      SET NEW.#{new} = NEW.#{old}
      EOF
    end

    # Removes the triggers used for renaming a PostgreSQL column concurrently.
    def remove_rename_triggers_for_postgresql(table, trigger)
      execute("DROP TRIGGER IF EXISTS #{trigger} ON #{table}")
      execute("DROP FUNCTION IF EXISTS #{trigger}()")
    end

    # Removes the triggers used for renaming a MySQL column concurrently.
    def remove_rename_triggers_for_mysql(trigger)
      execute("DROP TRIGGER IF EXISTS #{trigger}_insert")
      execute("DROP TRIGGER IF EXISTS #{trigger}_update")
    end

    # Returns the (base) name to use for triggers when renaming columns.
    def rename_trigger_name(table, old, new)
      'trigger_' + Digest::SHA256.hexdigest("#{table}_#{old}_#{new}").first(12)
    end

    # Returns the name to use for temporary rename columns.
    def rename_column_name(base)
      base.to_s + '_cm'
    end

    # Returns an Array containing the indexes for the given column
    def indexes_for(table, column)
      column = column.to_s

      indexes(table).select { |index| index.columns.include?(column) }
    end

    # Returns an Array containing the foreign keys for the given column.
    def foreign_keys_for(table, column)
      column = column.to_s

      foreign_keys(table).select { |fk| fk.column == column }
    end

    # Copies all indexes for the old column to a new column.
    #
    # table - The table containing the columns and indexes.
    # old - The old column.
    # new - The new column.
    def copy_indexes(table, old, new)
      old = old.to_s
      new = new.to_s

      indexes_for(table, old).each do |index|
        new_columns = index.columns.map do |column|
          column == old ? new : column
        end

        # This is necessary as we can't properly rename indexes such as
        # "ci_taggings_idx".
        name = index.name + '_cm'

        # If the order contained the old column, map it to the new one.
        order = index.orders
        if order.key?(old)
          order[new] = order.delete(old)
        end

        options = {
          unique: index.unique,
          name: name,
          length: index.lengths,
          order: order
        }

        # These options are not supported by MySQL, so we only add them if
        # they were previously set.
        options[:using] = index.using if index.using
        options[:where] = index.where if index.where

        add_concurrent_index(table, new_columns, **options)
      end
    end

    # Copies all foreign keys for the old column to the new column.
    #
    # table - The table containing the columns and indexes.
    # old - The old column.
    # new - The new column.
    def copy_foreign_keys(table, old, new)
      foreign_keys_for(table, old).each do |fk|
        add_concurrent_foreign_key(fk.from_table,
                                   fk.to_table,
                                   column: new,
                                   on_delete: fk.on_delete)
      end
    end

    # Returns the column for the given table and column name.
    def column_for(table, name)
      name = name.to_s

      columns(table).find { |column| column.name == name }
    end

    # Update the configuration of an index by creating a new one and then
    # removing the old one
    def update_index(table_name, index_name, columns, **index_options)
      if index_name_exists?(table_name, "#{index_name}_new") && index_name_exists?(table_name, index_name)
        remove_index table_name, name: "#{index_name}_new"
      elsif index_name_exists?(table_name, "#{index_name}_new")
        # Very unlikely case where the script has been interrupted during/after removal but before renaming
        rename_index table_name, "#{index_name}_new", index_name
      end

      begin
        add_index table_name, columns, **index_options.merge(name: "#{index_name}_new", algorithm: :concurrently)
      rescue ActiveRecord::RecordNotUnique
        remove_index table_name, name: "#{index_name}_new"
        raise CorruptionError.new(index_name)
      end

      remove_index table_name, name: index_name if index_name_exists?(table_name, index_name)
      rename_index table_name, "#{index_name}_new", index_name
    end

    # This will replace the first occurrence of a string in a column with
    # the replacement
    # On postgresql we can use `regexp_replace` for that.
    # On mysql we find the location of the pattern, and overwrite it
    # with the replacement
    def replace_sql(column, pattern, replacement)
      quoted_pattern = Arel::Nodes::Quoted.new(pattern.to_s)
      quoted_replacement = Arel::Nodes::Quoted.new(replacement.to_s)

      replace = Arel::Nodes::NamedFunction
        .new("regexp_replace", [column, quoted_pattern, quoted_replacement])
      Arel::Nodes::SqlLiteral.new(replace.to_sql)
    end

    def remove_foreign_key_without_error(*args)
      remove_foreign_key(*args)
    rescue ArgumentError
    end

    def sidekiq_queue_migrate(queue_from, to:)
      while sidekiq_queue_length(queue_from) > 0
        Sidekiq.redis do |conn|
          conn.rpoplpush "queue:#{queue_from}", "queue:#{to}"
        end
      end
    end

    def sidekiq_queue_length(queue_name)
      Sidekiq.redis do |conn|
        conn.llen("queue:#{queue_name}")
      end
    end

    def check_trigger_permissions!(table)
      unless Grant.create_and_execute_trigger?(table)
        dbname = ActiveRecord::Base.configurations[Rails.env]['database']
        user = ActiveRecord::Base.configurations[Rails.env]['username'] || ENV['USER']

        raise <<-EOF
Your database user is not allowed to create, drop, or execute triggers on the
table #{table}.

If you are using PostgreSQL you can solve this by logging in to the Mastodon
database (#{dbname}) using a super user and running:

    ALTER USER #{user} WITH SUPERUSER

The query will grant the user super user permissions, ensuring you don't run
into similar problems in the future (e.g. when new tables are created).
        EOF
      end
    end

    # Bulk queues background migration jobs for an entire table, batched by ID range.
    # "Bulk" meaning many jobs will be pushed at a time for efficiency.
    # If you need a delay interval per job, then use `queue_background_migration_jobs_by_range_at_intervals`.
    #
    # model_class - The table being iterated over
    # job_class_name - The background migration job class as a string
    # batch_size - The maximum number of rows per job
    #
    # Example:
    #
    #     class Route < ActiveRecord::Base
    #       include EachBatch
    #       self.table_name = 'routes'
    #     end
    #
    #     bulk_queue_background_migration_jobs_by_range(Route, 'ProcessRoutes')
    #
    # Where the model_class includes EachBatch, and the background migration exists:
    #
    #     class Gitlab::BackgroundMigration::ProcessRoutes
    #       def perform(start_id, end_id)
    #         # do something
    #       end
    #     end
    def bulk_queue_background_migration_jobs_by_range(model_class, job_class_name, batch_size: BACKGROUND_MIGRATION_BATCH_SIZE)
      raise "#{model_class} does not have an ID to use for batch ranges" unless model_class.column_names.include?('id')

      jobs = []

      model_class.each_batch(of: batch_size) do |relation|
        start_id, end_id = relation.pluck('MIN(id), MAX(id)').first

        if jobs.length >= BACKGROUND_MIGRATION_JOB_BUFFER_SIZE
          # Note: This code path generally only helps with many millions of rows
          # We push multiple jobs at a time to reduce the time spent in
          # Sidekiq/Redis operations. We're using this buffer based approach so we
          # don't need to run additional queries for every range.
          BackgroundMigrationWorker.perform_bulk(jobs)
          jobs.clear
        end

        jobs << [job_class_name, [start_id, end_id]]
      end

      BackgroundMigrationWorker.perform_bulk(jobs) unless jobs.empty?
    end

    # Queues background migration jobs for an entire table, batched by ID range.
    # Each job is scheduled with a `delay_interval` in between.
    # If you use a small interval, then some jobs may run at the same time.
    #
    # model_class - The table being iterated over
    # job_class_name - The background migration job class as a string
    # delay_interval - The duration between each job's scheduled time (must respond to `to_f`)
    # batch_size - The maximum number of rows per job
    #
    # Example:
    #
    #     class Route < ActiveRecord::Base
    #       include EachBatch
    #       self.table_name = 'routes'
    #     end
    #
    #     queue_background_migration_jobs_by_range_at_intervals(Route, 'ProcessRoutes', 1.minute)
    #
    # Where the model_class includes EachBatch, and the background migration exists:
    #
    #     class Gitlab::BackgroundMigration::ProcessRoutes
    #       def perform(start_id, end_id)
    #         # do something
    #       end
    #     end
    def queue_background_migration_jobs_by_range_at_intervals(model_class, job_class_name, delay_interval, batch_size: BACKGROUND_MIGRATION_BATCH_SIZE)
      raise "#{model_class} does not have an ID to use for batch ranges" unless model_class.column_names.include?('id')

      model_class.each_batch(of: batch_size) do |relation, index|
        start_id, end_id = relation.pluck('MIN(id), MAX(id)').first

        # `BackgroundMigrationWorker.bulk_perform_in` schedules all jobs for
        # the same time, which is not helpful in most cases where we wish to
        # spread the work over time.
        BackgroundMigrationWorker.perform_in(delay_interval * index, job_class_name, [start_id, end_id])
      end
    end

    private

    # https://github.com/rails/rails/blob/v5.2.0/activerecord/lib/active_record/connection_adapters/postgresql/schema_statements.rb#L678-L684
    def extract_foreign_key_action(specifier)
      case specifier
      when 'c'; :cascade
      when 'n'; :nullify
      when 'r'; :restrict
      end
    end
  end
end