Fixes two bugs related to padding in the jitter buffer.

- Pad packets (empty) were often NACKed even though they were received.
- Padding only frames (empty) didn't properly update the decoding state,
  and would therefore be NACKed even though they were received.

TEST=trybots

BUG=1150

Review URL: https://webrtc-codereview.appspot.com/966026

git-svn-id: http://webrtc.googlecode.com/svn/trunk@3181 4adac7df-926f-26a2-2b94-8c16560cd09d

webrtc/modules/video_coding/main/source/session_info.cc

@@ -357,38 +357,44 @@ int VCMSessionInfo::BuildHardNackList(int* seq_num_list,
   if (NULL == seq_num_list || seq_num_list_length < 1) {
     return -1;
   }
-  if (packets_.empty()) {
+  if (packets_.empty() && empty_seq_num_low_ == -1) {
     return 0;
   }
 
   // Find end point (index of entry equals the sequence number of the first
   // packet).
   int index = 0;
+  int low_seq_num = (packets_.empty()) ? empty_seq_num_low_:
+      packets_.front().seqNum;
   for (; index < seq_num_list_length; ++index) {
-    if (seq_num_list[index] == packets_.front().seqNum) {
+    if (seq_num_list[index] == low_seq_num) {
       seq_num_list[index] = -1;
       ++index;
       break;
     }
   }
 
-  // Zero out between the first entry and the end point.
-  PacketIterator it = packets_.begin();
-  PacketIterator prev_it = it;
-  ++it;
-  while (it != packets_.end() && index < seq_num_list_length) {
-    if (!InSequence(it, prev_it)) {
-      // Found a sequence number gap due to packet loss.
-      index += PacketsMissing(it, prev_it);
-      session_nack_ = true;
-    }
-    seq_num_list[index] = -1;
-    ++index;
-    prev_it = it;
+  if (!packets_.empty()) {
+    // Zero out between the first entry and the end point.
+    PacketIterator it = packets_.begin();
+    PacketIterator prev_it = it;
     ++it;
+    while (it != packets_.end() && index < seq_num_list_length) {
+      if (!InSequence(it, prev_it)) {
+        // Found a sequence number gap due to packet loss.
+        index += PacketsMissing(it, prev_it);
+        session_nack_ = true;
+      }
+      seq_num_list[index] = -1;
+      ++index;
+      prev_it = it;
+      ++it;
+    }
+    if (!packets_.front().isFirstPacket)
+        session_nack_ = true;
   }
-  if (!packets_.front().isFirstPacket)
-    session_nack_ = true;
+  index = ClearOutEmptyPacketSequenceNumbers(seq_num_list, seq_num_list_length,
+                                             index);
   return 0;
 }
 
@@ -485,6 +491,17 @@ int VCMSessionInfo::BuildSoftNackList(int* seq_num_list,
     }
   }
 
+  index = ClearOutEmptyPacketSequenceNumbers(seq_num_list, seq_num_list_length,
+                                             index);
+
+  session_nack_ = allow_nack;
+  return 0;
+}
+
+int VCMSessionInfo::ClearOutEmptyPacketSequenceNumbers(
+    int* seq_num_list,
+    int seq_num_list_length,
+    int index) const {
   // Empty packets follow the data packets, and therefore have a higher
   // sequence number. We do not want to NACK empty packets.
   if ((empty_seq_num_low_ != -1) && (empty_seq_num_high_ != -1) &&
@@ -497,15 +514,14 @@ int VCMSessionInfo::BuildSoftNackList(int* seq_num_list,
     }
 
     // Mark empty packets.
-    while (seq_num_list[index] <= empty_seq_num_high_ &&
+    while (seq_num_list[index] >= 0 &&
+        seq_num_list[index] <= empty_seq_num_high_ &&
         index < seq_num_list_length) {
       seq_num_list[index] = -2;
       ++index;
     }
   }
-
-  session_nack_ = allow_nack;
-  return 0;
+  return index;
 }
 
 int VCMSessionInfo::PacketsMissing(const PacketIterator& packet_it,