Preserve receiving host port in peer connect responses

The host pool answers connect requests on more than one UDP port, but every
connect response named the primary port. The client locates its connecting
channel by the address in that response, so a request that reached a secondary
pool port was answered with an address it had no channel for. Name the port the
request arrived on, keeping the configured advertised IPv4 address and falling
back to the primary port when the receiving port is zero, as the transport's
send path already does.

Sunrise/src/server/gameplay/peer/peer_out_of_band.cpp

@@ -29,11 +29,17 @@ constexpr unsigned kByteBits = 8;
 /** Sequence the first packet to a peer carries, because the head advances before it is written. */
 constexpr std::uint16_t kFirstPacketSequence = 1;
 
-/** Fills the address blob that names this host on the direct path. */
-void local_address(std::array<std::byte, wire::kAddressBlobSize>& output) noexcept {
+/**
+ * Fills the address blob that names this host on the direct path.
+ * @param receivingPort Host pool port the request arrived on. Zero names the primary port, as it
+ * does on the transport's send path.
+ * @param output Receives the direct-path address blob.
+ */
+void local_address(std::uint16_t receivingPort,
+                   std::array<std::byte, wire::kAddressBlobSize>& output) noexcept {
     const gp::Endpoint advertised = endpoint::advertised();
     middleware::gameplay::descriptor::write_direct_net_addr(
-        advertised.address, advertised.port, output);
+        advertised.address, receivingPort != 0 ? receivingPort : advertised.port, output);
 }
 
 /** @return A random 32-bit sequence, or zero when Windows refused. */
@@ -98,7 +104,9 @@ void answer_connect(const gp::Endpoint& from,
     // The peer checks both echoed fields and closes the connection on a wrong sequence.
     response.remoteChannelId = request.channelId;
     response.remoteSequence = request.sequence;
-    local_address(response.address);
+    // The client locates its connecting channel by this address, so it must name the host pool
+    // port the request reached rather than always the primary port.
+    local_address(from.localPort, response.address);
     DisplacedExternals displaced{};
     std::size_t displacedCount = 0;
     std::array<std::uint64_t, gp::kSessionsPerLink> resetSessions{};