Request for comment for async resolution of DNS names

[Volker Lendecke]

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,

Volker