RUBY-986 Fallback to IPv4 if IPv6 isn't supported

lib/mongo/address.rb

@@ -146,12 +146,12 @@ def to_s
     def initialize_resolver!(timeout, ssl_options)
       family = (host == 'localhost') ? ::Socket::AF_INET : ::Socket::AF_UNSPEC
       error = nil
-      ::Socket.getaddrinfo(host, nil, family, ::Socket::SOCK_STREAM).detect do |info|
+      ::Socket.getaddrinfo(host, nil, family, ::Socket::SOCK_STREAM).each do |info|
         begin
-          return FAMILY_MAP[info[4]].new(info[3], port, host).tap do |res|
-            res.socket(timeout, ssl_options).connect!
-          end
-        rescue IOError, SystemCallError => e
+          res = FAMILY_MAP[info[4]].new(info[3], port, host)
+          res.socket(timeout, ssl_options).connect!
+          return res
+        rescue IOError, SystemCallError, Error::SocketError => e
           error = e
         end
       end

spec/mongo/address_spec.rb

@@ -203,4 +203,23 @@
       end
     end
   end
+
+  describe "#socket" do
+    context 'when providing a DNS entry that resolves to both IPv6 and IPv4' do
+      let(:address) do
+        described_class.new('localhost:27017')
+      end
+
+      before do
+        allow(::Socket).to receive(:getaddrinfo).and_return(
+          [ ["AF_INET6", 0, "::1", "::1", ::Socket::AF_INET6, 1, 6],
+            ["AF_INET", 0, "127.0.0.1", "127.0.0.1", ::Socket::AF_INET, 1, 6]]
+        )
+      end
+
+      it "attempts to use IPv6 and fallbacks to IPv4" do
+        expect(address.socket(0.0)).not_to be_nil
+      end
+    end
+  end
 end