windows: issue with ipv6 address detection

I'm currently seeing an issue where libevent is misidentifying ip6 addresses (i.e ::1) as ipv4. This is only happening on Windows, and was preventing our RPC server from binding to an ipv6 address. The PR were with the project-specific fix and explanation is here: https://github.com/bitcoin/bitcoin/pull/19375.

We are using evhttp_bind_socket_with_handle() to create an RPC server, and try and bind to 127.0.0.1 and ::1 by default. This works for all platforms, except for Windows, which would always fail to bind to ::1. You'd see lines like:

2020-06-24T01:49:04Z libevent: getaddrinfo: nodename nor servname provided, or not known

in our debug log.

Our call into libevent starts with evhttp_bind_socket_with_handle():

evhttp_bind_socket_with_handle()
	bind_socket()
		make_addrinfo()
			evutil_getaddrinfo()
				if #USE_NATIVE_GETADDRINFO
					#ifndef AI_ADDRCONFIG
						evutil_adjust_hints_for_addrconfig_()
							evutil_check_interfaces()
								evutil_check_ifaddrs()
									evutil_found_ifaddr()
										// miss identifies ipv6 as ipv4?
					#endif
					evutil_getaddrinfo_common_()

The problem seems to be falling into "#ifndef AI_ADDRCONFIG" and calling evutil_adjust_hints_for_addrconfig_():

#ifndef AI_ADDRCONFIG
	/* Not every system has AI_ADDRCONFIG, so fake it. */
	if (hints.ai_family == PF_UNSPEC &&
	    (hints.ai_flags & EVUTIL_AI_ADDRCONFIG)) {
		evutil_adjust_hints_for_addrconfig_(&hints);
	}
#endif

At this point, hints gets adjusted in such a way, that by the time you get to evutil_found_ifaddr() ipv6 addresses are mis-identified as ipv4. My knowledge of libevent is not good enough to know exactly why this is happening.

These AI_ definitions are available on Windows, however, only from Vista (0x0600) onwards, see here.

Our fix is just to set -D_WIN32_WINNT=0x0601 when building libevent (we require >= Windows 7 at runtime), which means the AI_ definitions are available at compile time, as they are included from ws2tcpip.h before WIN32_WINNT is later undef'd and set to 0x501 (XP) in evutil.c.

I'm not sure what the supported versions of Windows are for libevent, so obviously changing those defines isn't necessarily a solution here, but I'm happy to help improve this if it's deemed an issue.

Start in evutil.c at the WIN32_WINNT definition, the #ifndef AI_ADDRCONFIG path, evutil_adjust_hints_for_addrconfig(), and evutil_found_ifaddr(). Compare the Windows build configuration with the reported Windows 7 workaround and trace how ::1 is classified; done means the Windows path no longer treats IPv6 as IPv4 and evhttp_bind_socket_with_handle() can bind to ::1.