[PATCH] Resolver: make TCP write timer event cancelable.

[Maxim Dounin]

Hello!

On Thu, Jun 02, 2022 at 03:49:50AM +0000, Aleksei Bavshin via nginx-devel wrote:

> Updated patch, with maybe too verbose description.
> For the record, I did check a few similar modules I'm aware of (nginx-upstream-dynamic-servers, ngx_upstream_jdomain) and haven't found any affected code. So, the effect on third-party modules is hypothetical.
> 
> # HG changeset patch
> # User Aleksei Bavshin <a.bavshin at f5.com>
> # Date 1654139843 25200
> #      Wed Jun 01 20:17:23 2022 -0700
> # Node ID 488f7a926ba54303c930b0b1e19c2b27e0af5079
> # Parent  40926829be83986a3a5a5f941d2014000a0acd2e
> Resolver: make TCP write timer event cancelable.
> 
> Similar to 70e65bf8dfd7, the change is made to ensure that the ability to
> cancel resolver tasks is fully controlled by the caller. As mentioned in the
> referenced commit, it is safe to make this timer cancelable because resolve
> tasks can have their own timeouts that are not cancelable.
> 
> The scenario where this may become a problem is a periodic background resolve
> task (not tied to a specific request or a client connection), which receives a
> response with short TTL, large enough to warrant fallback to a TCP query.
> With each event loop wakeup, we either have a previously set write timer
> instance or schedule a new one. The non-cancelable write timer can delay or
> block graceful shutdown of a worker even if the ngx_resolver_ctx_t->cancelable
> flag is set by the API user, and there are no other tasks or connections.
> 
> We use the resolver API in this way to maintain the list of upstream server
> addresses specified with the 'resolve' parameter, and there could be third-party
> modules implementing similar logic.
> 
> diff --git a/src/core/ngx_resolver.c b/src/core/ngx_resolver.c
> --- a/src/core/ngx_resolver.c
> +++ b/src/core/ngx_resolver.c
> @@ -1389,6 +1389,7 @@ ngx_resolver_send_tcp_query(ngx_resolver
>  
>          rec->tcp->data = rec;
>          rec->tcp->write->handler = ngx_resolver_tcp_write;
> +        rec->tcp->write->cancelable = 1;
>          rec->tcp->read->handler = ngx_resolver_tcp_read;
>          rec->tcp->read->resolver = 1;
> 
> 

Looks good.

-- 
Maxim Dounin
http://mdounin.ru/