Enable mDNS on Darwin

We have to disable IPv6 to get this going, but nobody was yet using
IPv6, so that should be fine for now.

Test: Verified mDNS discovery on a MacBook
Bug: 31042497
(cherry picked from 9ae65de087171072f3890a81b074ae27db319508)

Change-Id: I628f0a1e9c4784db42fa56f5faf2904b6e1cf548

adb/Android.mk

@@ -151,7 +151,7 @@ LOCAL_SRC_FILES_linux := $(LIBADB_linux_SRC_FILES)
 LOCAL_SRC_FILES_windows := $(LIBADB_windows_SRC_FILES)
 
 LOCAL_SRC_FILES_linux += transport_mdns.cpp
-LOCAL_SRC_FILES_darwin += transport_mdns_unsupported.cpp
+LOCAL_SRC_FILES_darwin += transport_mdns.cpp
 LOCAL_SRC_FILES_windows += transport_mdns_unsupported.cpp
 
 LOCAL_SANITIZE := $(adb_host_sanitize)

adb/transport_mdns.cpp

@@ -85,10 +85,17 @@ class ResolvedService : public AsyncServiceRef {
                     const char* hosttarget, uint16_t port) :
             name_(name),
             port_(port) {
+
+        /* TODO: We should be able to get IPv6 support by adding
+         * kDNSServiceProtocol_IPv6 to the flags below. However, when we do
+         * this, we get served link-local addresses that are usually useless to
+         * connect to. What's more, we seem to /only/ get those and nothing else.
+         * If we want IPv6 in the future we'll have to figure out why.
+         */
         DNSServiceErrorType ret =
             DNSServiceGetAddrInfo(
                 &sdRef_, 0, interfaceIndex,
-                kDNSServiceProtocol_IPv6|kDNSServiceProtocol_IPv4, hosttarget,
+                kDNSServiceProtocol_IPv4, hosttarget,
                 register_service_ip, reinterpret_cast<void*>(this));
 
         if (ret != kDNSServiceErr_NoError) {