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 ecaa86e5d2f38fc26ccf382cd623949226a915b3..64a846fd65c26bcad4f7565fa967b4d4e48552ad 100644
--- a/webrtc/modules/rtp_rtcp/source/forward_error_correction.cc
+++ b/webrtc/modules/rtp_rtcp/source/forward_error_correction.cc
@@ -544,11 +544,13 @@ void ForwardErrorCorrection::InsertFecPacket(
     for (uint16_t bit_idx = 0; bit_idx < 8; ++bit_idx) {
       if (packet_mask & (1 << (7 - bit_idx))) {
         ProtectedPacket* protected_packet = new ProtectedPacket();
-        fec_packet->protected_pkt_list.push_back(protected_packet);
         // This wraps naturally with the sequence number.
         protected_packet->seq_num =
             static_cast<uint16_t>(seq_num_base + (byte_idx << 3) + bit_idx);
         protected_packet->pkt = nullptr;
+        // Note that |protected_pkt_list| is sorted (according to sequence
+        // number) by construction.
+        fec_packet->protected_pkt_list.push_back(protected_packet);
       }
     }
   }
@@ -560,6 +562,11 @@ void ForwardErrorCorrection::InsertFecPacket(
     AssignRecoveredPackets(fec_packet, recovered_packet_list);
     // 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, |fec_packet_list_| does not necessarily
+    // need to be sorted by sequence number (see decoding algorithm in
+    // AttemptRecover()), but by keeping it sorted we try to recover the
+    // oldest lost packets first.
     fec_packet_list_.push_back(fec_packet);
     fec_packet_list_.sort(SortablePacket::LessThan);
     if (fec_packet_list_.size() > kMaxFecPackets) {