class ActiveRecord::ConnectionAdapters::ConnectionPool

def reap

or a thread dies unexpectedly.
a programmer forgets to checkin a connection at the end of a thread
Recover lost connections for the pool. A lost connection can occur if 
def reap
  stale_connections = synchronize do
    return if self.discarded?
    @connections.select do |conn|
      conn.in_use? && !conn.owner.alive?
    end.each do |conn|
      conn.steal!
    end
  end
  stale_connections.each do |conn|
    if conn.active?
      conn.reset!
      checkin conn
    else
      remove conn
    end
  end
end