- Prevent shutdown races: attach/detach to dns_resolver in dns_fetch_t
and fctx_t; delay destruction of fctx when finds are still active;
reference the fctx while canceling; reverse the order of
fctx_destroy() and empty_bucket().
- Don't resend queries if fetches have been canceled.
- It's possible for fctx_doshutdown() to run before a TCP connection has
completed. if the query is not on the queries list, then it is not
canceled, but the adbaddrinfo is freed. when tcp_connected() runs
later, the query is in an inconstent state. to fix this, we add the
query to queries before running dns_dispatch_connect(), instead of in
the connect callback.
- Combined the five fctx_cleanup* functions into a single one.
- Added comments and changed some names to make this code easier to
understand.
dc1203b426