Unit test for media packet reordering in ForwardErrorCorrection.

webrtc/modules/rtp_rtcp/source/forward_error_correction.cc

Index: webrtc/modules/rtp_rtcp/source/forward_error_correction.cc
diff --git a/webrtc/modules/rtp_rtcp/source/forward_error_correction.cc b/webrtc/modules/rtp_rtcp/source/forward_error_correction.cc
index b60e3119f4145c262ef27bbc65c88fbf9a696b51..5d61a10499739352e09dee50321f2571ffe9e7a5 100644
--- a/webrtc/modules/rtp_rtcp/source/forward_error_correction.cc
+++ b/webrtc/modules/rtp_rtcp/source/forward_error_correction.cc
@@ -521,6 +521,7 @@ void ForwardErrorCorrection::InsertFecPacket(
         protected_packet->seq_num =
             static_cast<uint16_t>(seq_num_base + (byte_idx << 3) + bit_idx);
         protected_packet->pkt = nullptr;
+        // Note that |protected_packets| is sorted by construction.
         fec_packet->protected_packets.push_back(std::move(protected_packet));
       }
     }
@@ -532,6 +533,10 @@ void ForwardErrorCorrection::InsertFecPacket(
     AssignRecoveredPackets(fec_packet.get(), recovered_packets);
     // TODO(holmer): Consider replacing this with a binary search for the right
     // position, and then just insert the new packet. Would get rid of the sort.
+    //
+    // For correct decoding, |received_fec_packets_| does not necessarily
+    // need to be sorted (see decoding algorithm in AttemptRecover()), but
+    // by keeping it sorted we try to recover the oldest lost packets first.
     received_fec_packets_.push_back(std::move(fec_packet));
     received_fec_packets_.sort(SortablePacket::LessThan());
     if (received_fec_packets_.size() > kMaxFecPackets) {