Re: [PATCH 10/14] dns: poll card between each dns request

[Josef Bacik <jbacik@fb.com>]

On 02/15/2016 01:45 AM, Andrei Borzenkov wrote:
> On Thu, Feb 11, 2016 at 12:21 AM, Josef Bacik <jbacik@fb.com> wrote:
>> If we have dns servers that we prefer to get AAAA records from we'll send a
>> packet and immediately check data.naddresses to see if we got a response.  If we
>> didn't we'll then send a request for an A record, and _then_ we'll poll the
>> card.  So if the DNS server doesn't respond between us sending the packet and
>> checking data.naddresses we'll send a request for the A record and then poll the
>> card.  Instead we need to make sure we poll after we issue each request to make
>> sure we give the server enough time to respond to our initial request.
>>
>> Signed-off-by: Josef Bacik <jbacik@fb.com>
>> ---
>>   grub-core/net/dns.c | 2 +-
>>   1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/grub-core/net/dns.c b/grub-core/net/dns.c
>> index 82a3307..86e609b 100644
>> --- a/grub-core/net/dns.c
>> +++ b/grub-core/net/dns.c
>> @@ -587,12 +587,12 @@ grub_net_dns_lookup (const char *name,
>>                     grub_errno = GRUB_ERR_NONE;
>>                     err = err2;
>>                   }
>> +             grub_net_poll_cards (200, &data.stop);
>
> One consideration is that it will increase timeouts in case of
> non-responsive servers, as now they are processed sequentially.
>
> But more importantly, this is still hit and miss - we rely on delivery
> order which is non-deterministic. We really need to ask for all and
> filter on receiving side. Two possible implementations are
>
> 1. Keep track of IPv4 and IPv6 answers separately; if non-preferred
> answer is received, continue to wait for preferred one until timeout.
>
> 2. Queries for both A and AAAA in the same packet and filter out answers.
>
> The 2 looks better. It avoids extra timeouts (at least if we assume
> that all DNS servers are equally authoritative) because as soon as we
> get any response we can stop polling.
>
> This will also indirectly fix another reported issue as we now can
> ignore any duplicate packet.
>
> Would you consider implementing it?
>

Yeah that sounds reasonable, I'll take a crack at it in a few days.  Thanks,

Josef