Add a data structure QuicMultipathTransmissionsMap which is not in use now.

net/quic/quic_multipath_transmissions_map.cc

+#include "net/quic/quic_multipath_transmissions_map.h"
+
+namespace net_quic {
+
+QuicMultipathTransmissionsMap::QuicMultipathTransmissionsMap() {}
+
+QuicMultipathTransmissionsMap::~QuicMultipathTransmissionsMap() {
+  for (std::pair<QuicPathIdPacketNumber, MultipathTransmissionsList*>
+           packet_transmissions : transmission_map_) {
+    packet_transmissions.second->pop_front();
+    if (packet_transmissions.second->empty()) {
+      delete packet_transmissions.second;
+    }
+  }
+}
+
+void QuicMultipathTransmissionsMap::OnPacketRetransmittedOnDifferentPath(
+    QuicPathIdPacketNumber original_path_id_packet_number,
+    QuicPathIdPacketNumber path_id_packet_number) {
+  MultipathTransmissionsList* across_paths_transmission_list = nullptr;
+  MultipathTransmissionsMap::iterator it =
+      transmission_map_.find(original_path_id_packet_number);
+  if (it != transmission_map_.end()) {
+    across_paths_transmission_list = it->second;
+  } else {
+    across_paths_transmission_list = new MultipathTransmissionsList();
+    across_paths_transmission_list->push_back(original_path_id_packet_number);
+    transmission_map_[original_path_id_packet_number] =
+        across_paths_transmission_list;
+  }
+
+  across_paths_transmission_list->push_back(path_id_packet_number);
+  transmission_map_[path_id_packet_number] = across_paths_transmission_list;
+}
+
+const QuicMultipathTransmissionsMap::MultipathTransmissionsList*
+QuicMultipathTransmissionsMap::MaybeGetTransmissionsOnOtherPaths(
+    QuicPathIdPacketNumber path_id_packet_number) const {
+  MultipathTransmissionsMap::const_iterator it =
+      transmission_map_.find(path_id_packet_number);
+  if (it == transmission_map_.end()) {
+    return nullptr;
+  }
+
+  return it->second;
+}
+
+void QuicMultipathTransmissionsMap::OnPacketHandled(
+    QuicPathIdPacketNumber path_id_packet_number) {
+  MultipathTransmissionsMap::iterator it =
+      transmission_map_.find(path_id_packet_number);
+  if (it == transmission_map_.end()) {
+    return;
+  }
+
+  MultipathTransmissionsList* transmission_list = it->second;
+  MultipathTransmissionsList::iterator transmission_it;
+  // Remove all across paths transmissions of this packet from the map.
+  for (QuicPathIdPacketNumber path_id_packet_number : *transmission_list) {
+    transmission_map_.erase(path_id_packet_number);
+  }
+  // Remove the multipath transmissions list.
+  delete transmission_list;
+}
+
+}  // namespace net_quic