    # File lib/rack/session/memcache.rb, line 78
          def with_lock(env)
            @mutex.lock if env['rack.multithread']
            yield
          rescue MemCache::MemCacheError, Errno::ECONNREFUSED
            if $VERBOSE
              warn "#{self} is unable to find memcached server."
              warn $!.inspect
            end
            raise
          ensure
            @mutex.unlock if @mutex.locked?
          end