Set SO_REUSEPORT on multicast input sockets for Apple platforms

OpenAndBindInputSocket() only enabled SO_REUSEPORT for QNX (#272, #350).
On Darwin, SO_REUSEADDR alone is not enough for two processes to share a
UDP multicast port: the second bind fails with EADDRINUSE, the failure is
not surfaced, and that DomainParticipant never receives SPDP multicast.

Extend the existing QNX conditional to __APPLE__ in UDPv4 and UDPv6
transports. Linux intentionally unchanged: SO_REUSEPORT there load-balances
datagrams instead of duplicating delivery.

Signed-off-by: Onur Keskin <keskinonur@me.com>

Author: Onur Keskin

src/cpp/rtps/transport/UDPv4Transport.cpp

@@ -426,10 +426,14 @@ eProsimaUDPSocket UDPv4Transport::OpenAndBindInputSocket(
     if (is_multicast)
     {
         getSocketPtr(socket)->set_option(ip::udp::socket::reuse_address(true));
-#if defined(__QNX__)
+        // QNX and Apple require SO_REUSEPORT for multiple processes to all
+        // receive the same multicast datagrams; SO_REUSEADDR alone makes the
+        // second bind fail (EADDRINUSE) on Darwin. Linux deliberately omitted:
+        // there SO_REUSEPORT load-balances instead of duplicating delivery.
+#if defined(__QNX__) || defined(__APPLE__)
         getSocketPtr(socket)->set_option(asio::detail::socket_option::boolean<
                     ASIO_OS_DEF(SOL_SOCKET), SO_REUSEPORT>(true));
-#endif // if defined(__QNX__)
+#endif // if defined(__QNX__) || defined(__APPLE__)
     }
     else
     {

src/cpp/rtps/transport/UDPv6Transport.cpp

@@ -427,10 +427,14 @@ eProsimaUDPSocket UDPv6Transport::OpenAndBindInputSocket(
     if (is_multicast)
     {
         getSocketPtr(socket)->set_option(ip::udp::socket::reuse_address(true));
-#if defined(__QNX__)
+        // QNX and Apple require SO_REUSEPORT for multiple processes to all
+        // receive the same multicast datagrams; SO_REUSEADDR alone makes the
+        // second bind fail (EADDRINUSE) on Darwin. Linux deliberately omitted:
+        // there SO_REUSEPORT load-balances instead of duplicating delivery.
+#if defined(__QNX__) || defined(__APPLE__)
         getSocketPtr(socket)->set_option(asio::detail::socket_option::boolean<
                     ASIO_OS_DEF(SOL_SOCKET), SO_REUSEPORT>(true));
-#endif // if defined(__QNX__)
+#endif // if defined(__QNX__) || defined(__APPLE__)
     }
     else
     {