Request for comment for async resolution of DNS names

[Matthieu Patou]

On 02/09/2017 11:30 AM, Jeremy Allison wrote:
> On Thu, Feb 09, 2017 at 11:18:37AM -0800, Matthieu Patou wrote:
>> 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.
> interpret_string_addr_internal() calls strchr_m() which calls
> get_iconv_handle(). I am 100% sure get_iconv_handle() is not
> thread-safe.
I'm fine, I guess we also don't want to have any DEBUG statement in the
thread ?

-- 
Matthieu Patou
Samba Team
http://samba.org