tree 3da6357680ee5e1211c3a737a7efcb752b075b11
parent 0c0a1dfe57cd9ecd3533fcde7f27d35a4757cc98
author Aleksei Bavshin <a.bavshin@f5.com> 1654139843 -0700
committer Aleksei Bavshin <a.bavshin@f5.com> 1654139843 -0700

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.