The rctx_chaseds() function calls dns_resolver_createfetch(), passing
fctx->task as the target task to run resume_dslookup() from. This
breaks task-based serialization of events as fctx->task is the task that
the dns_resolver_createfetch() caller wants to receive its fetch
completion event in; meanwhile, intermediate fetches started by the
resolver itself (e.g. related to QNAME minimization) must use
res->buckets[bucketnum].task instead. This discrepancy may cause
trouble if the resume_dslookup() callback happens to be run concurrently
with e.g. fctx_doshutdown().
Fix by passing the correct task to dns_resolver_createfetch() in
rctx_chaseds().
Ondřej Surý
741a7096fc