module ActiveSupport::LoggerThreadSafeLevel

def add(severity, message = nil, progname = nil, &block) # :nodoc:

:nodoc:
FIXME: Remove when the minimum Ruby version supports overriding Logger#level.
Redefined to check severity against #level, and thus the thread-local level, rather than +@level+. 
def add(severity, message = nil, progname = nil, &block) # :nodoc:
  severity ||= UNKNOWN
  progname ||= @progname
  return true if @logdev.nil? || severity < level
  if message.nil?
    if block_given?
      message  = yield
    else
      message  = progname
      progname = @progname
    end
  end
  @logdev.write \
    format_message(format_severity(severity), Time.now, progname, message)
end

def level 

def level
  local_level || super
end

def local_level 

def local_level
  IsolatedExecutionState[:logger_thread_safe_level]
end

def local_level=(level) 

def local_level=(level)
  case level
  when Integer
  when Symbol
    level = Logger::Severity.const_get(level.to_s.upcase)
  when nil
  else
    raise ArgumentError, "Invalid log level: #{level.inspect}"
  end
  IsolatedExecutionState[:logger_thread_safe_level] = level
end

def log_at(level)

Change the thread-local level for the duration of the given block. 
def log_at(level)
  old_local_level, self.local_level = local_level, level
  yield
ensure
  self.local_level = old_local_level
end