asyn-ares.c and asyn-thread.c are two separate backends that implement
the same (internal) async resolver API for libcurl to use. Backend is
specified at build time.
The internal resolver API is defined in asyn.h for asynch resolvers.
When built IPv6-enabled, we could do Curl_done() with one of the two
resolves having returned already, so when ares_cancel() is called the
resolve callback ends up doing funny things (sometimes resulting in a
segfault) since it would try to actually store the previous resolve even
though we're shutting down the resolve.
This bug was introduced in commit 8ab137b2bc so it hasn't been
included in any public release.
Bug: http://curl.haxx.se/bug/view.cgi?id=3145445
Reported by: Pedro Larroy
Make the c-ares resolver code ask for both IPv4 and IPv6 addresses when
IPv6 is enabled.
This is a workaround for the missing ares_getaddrinfo() and is a lot
easier to implement.
Note that as long as c-ares returns IPv4 addresses when IPv6 addresses
were requested but missing, this will cause a host's IPv4 addresses to
occur twice in the DNS cache.
URL: http://curl.haxx.se/mail/lib-2010-12/0041.html
(http://curl.haxx.se/bug/view.cgi?id=1481217), with follow-ups by Michele Bini
and David Byron. libcurl previously wrongly used GetLastError() on windows to
get error details after socket-related function calls, when it really should
use WSAGetLastError() instead.
When changing to this, the former function Curl_ourerrno() is now instead
called Curl_sockerrno() as it is necessary to only use it to get errno from
socket-related functions as otherwise it won't work as intended on Windows.
