Request for comment for async resolution of DNS names

[Matthieu Patou]

Volker,
On 02/09/2017 12:33 AM, Volker Lendecke wrote:
> On Wed, Feb 08, 2017 at 11:10:11AM -0800, Matthieu Patou wrote:
>> +static void resolve_one_name(void *private_data)
>> +{
>> +	struct dns_rr_srv *rr = private_data;
>> +	struct addrinfo *res = NULL;
>> +	struct addrinfo *p;
>> +	int num_ips = 0;
>> +	if (!interpret_string_addr_internal(&res, rr->hostname, 0)) {
>> +		return;
>> +	}
>> +	/* Add in every IP from the lookup. How many is that ? */
>> +	for (p = res; p; p = p->ai_next) {
>> +		if (num_ips != 0) {
>> +			if (NULL == (rr->ss_s = talloc_realloc(rr->ss_s, rr->ss_s,
>> +					struct sockaddr_storage, num_ips + 1)))
>> +			{
>> +				return;
>> +			}
>> +		}
>> +		memcpy(&rr->ss_s[num_ips], p->ai_addr, p->ai_addrlen);
>> +		if (is_zero_addr(&rr->ss_s[num_ips])) {
>> +			continue;
>> +		}
>> +		num_ips++;
>> +	}
>> +	rr->num_ips = num_ips;
>> +	if (res) {
>> +		freeaddrinfo(res);
>> +	}
>> +}
> For my taste, this is much too complex for a thread job function. It
> calls minterpret_string_addr_internal, talloc_realloc and is_zero_addr
> where I'd have to think very long about whether they are safe to be
> called in a thread. With this change, interpret_string_addr_internal
> gets a completely fresh requirement that we would have to document
> very clearly: Thread safety. I would not hold my breath for us being
> good enough in our review process to always be alert that potentially
> thread-unsafe changes will be blocked for interpret_string_addr_internal.
>
> That's what I meant with my comment to do only the absolute bare minimum
> in a threaded function to make sure it's thread-safe.
>
> Can you restructure the code to only call getaddrinfo and nothing else
> in the threaded job.
Thanks for the feedback, I wasn't sure that if I called getaddrinfo
directly someone wouldn't argue that interpret_string_addr_internal was
meant for that and that I should use it.

I'll adapt the code,  the is_zero_addr can be removed as it's also in
the main thread.


Matthieu.