Module: Gitlab::Database::MigrationHelpers::RequireDisableDdlTransactionForMultipleLocks

Extended by:
ActiveSupport::Concern
Included in:
Gitlab::Database::Migration::V2_3
Defined in:
lib/gitlab/database/migration_helpers/require_disable_ddl_transaction_for_multiple_locks.rb

Overview

This cop detects multiple table locks across different statements in a single migration. This scenario has caused incidents in the past due to deadlocks (for example app.incident.io/gitlab/incidents/202).

Constant Summary collapse

LOCK_ACQUIRING_COMMANDS = 
%w[ALTER CREATE DROP TRUNCATE LOCK UPDATE DELETE INSERT].freeze
LOCK_TYPES =

Reference: www.postgresql.org/docs/current/explicit-locking.html

{
  high_severity: [
    'AccessExclusiveLock',  # Conflicts with all lock modes
    'ExclusiveLock'         # Conflicts with all except ROW SHARE
  ],

  low_severity: [
    'RowShareLock',         # Conflicts with EXCLUSIVE
    'AccessShareLock'       # Conflicts with ACCESS EXCLUSIVE only
  ]
}.freeze

Instance Method Summary collapse

Instance Method Details

#exec_migration(connection, direction) ⇒ Object 



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# File 'lib/gitlab/database/migration_helpers/require_disable_ddl_transaction_for_multiple_locks.rb', line 39

def exec_migration(connection, direction)
  return super if should_skip_check?

  # In-memory tracking structures
  statement_tracking = []
  tables_locked_up_till_now = Set.new

  begin
    # Subscribe to SQL execution to track each statement
    subscription_id = ActiveSupport::Notifications.subscribe('sql.active_record') do |*args|
      event = ActiveSupport::Notifications::Event.new(*args)
      sql = event.payload[:sql].strip

      next if should_skip_sql_statement?(sql)

      newly_locked_tables = []
      if likely_to_acquire_locks?(sql)
        newly_locked_tables = check_current_locks(connection).excluding(tables_locked_up_till_now.to_a)
      end

      newly_locked_tables.each { |table| tables_locked_up_till_now.add(table) }

      # Record new statement
      current_statement = {
        number: 1 + statement_tracking.size,
        sql: sql,
        locked_tables: newly_locked_tables.uniq
      }
      statement_tracking << current_statement
    end

    # Run the migration
    super.tap do
      # After the migration completes, analyze the collected lock data
      verify_single_table_per_statement(statement_tracking)
    end
  ensure
    # Cleanup
    ActiveSupport::Notifications.unsubscribe(subscription_id) if subscription_id
  end
end