Add API for configuring a custom HostsFileEntriesResolver

[violetagg]

Fixes #1822

[violetagg]

@KimJejun Once this PR is merged you will be able to do the following:

HttpClient.create()
          .resolver(spec ->
              spec.hostsFileEntriesResolver((inetHost, resolvedAddressTypes) -> {
                  HostsFileEntriesProvider provider = HostsFileEntriesProvider.parser().parseSilently();
                  List<InetAddress> addresses = provider.ipv4Entries().get(inetHost);
                  return addresses != null && !addresses.isEmpty() ? addresses.get(0) : null;
              }));

This example returns the first resolved address, but it is not limited to the first entry only.

[simonbasle]

I was confused for a bit by Netty's DefaultHostsFileEntriesResolver having an adresses method. The DnsNameResolver using HostsFileEntriesResolver does check if it is the default instance in which case it returns the whole list, while user-provided resolvers can only return a single address...

DefaultHostsFileEntriesResolver is final, and thus can't be extended by users.

That's a bummer, but a limitation of the Netty API.

Note that in the provided example in comment above, the resolvedAddressTypes is ignored.
An alternative code doing the same but honoring the type could easily be done using DefaultHostsFileEntriesResolver:

DefaultHostsFileEntriesResolver defaultResolver = new DefaultHostsFileEntriesResolver();
List<InetAddress> addresses = defaultResolver.addresses(inetHost, resolvedAddressTypes);
return addresses != null && !addresses.isEmpty() ? addresses.get(0) : null;

[simonbasle]

So this actually fixes the issue because the DnsNameResolver won't see a DefaultHostsFileEntriesResolver and thus won't return multiple addresses for the purpose of resolveAll 👍

[simonbasle]

DnsNameResolver using HostsFileEntriesResolver does check if it is the default instance in which case it returns the whole list, while user-provided resolvers can only return a single address...

I wonder: should this and the snippet you provided (or my edit) be mentioned in the reference doc @violetagg ?

[violetagg]

I wonder: should this and the snippet you provided (or my edit) be mentioned in the reference doc @violetagg ?

This is a rare case and for the moment I do not intend to provide an example in the ref documentation.

[KimJejun]

@violetagg Thanks. I will try this when released