openresty check upstreams status example

nginx.upstreams.lua

location = /status {
    default_type text/plain;

    content_by_lua '
        local ok, upstream = pcall(require, "ngx.upstream")
        if not ok then
            error("ngx_upstream_lua module required")
        end

        local get_primary_peers = upstream.get_primary_peers
        local get_backup_peers = upstream.get_backup_peers
        local get_upstreams = upstream.get_upstreams

        local function is_good(peers)
            local npeers = #peers
            for i = 1, npeers do
                local peer = peers[i]
                if not peer.down then
                    return true
                end
            end
            return false
        end

        local us, err = get_upstreams()
        if not us then
            return "failed to get upstream names: " .. err
        end

        local n = #us
        for i = 1, n do
            local primary = get_primary_peers(us[i])
            local backup = get_backup_peers(us[i])
            if not is_good(primary) and not is_good(backup) then
                ngx.status = ngx.HTTP_SERVICE_UNAVAILABLE
                ngx.say("unavailable")
                return
            end
        end

        ngx.status = ngx.HTTP_OK
        ngx.say("ok")
    ';

It seems that the line 18 statement "if not peer.down then" is forever true whatever the server is up or down because the down element is always nil .

I donnot know why?