NetEq: Fixing a corner case with depleted sync buffer

webrtc/modules/audio_coding/neteq/neteq_impl.cc

Index: webrtc/modules/audio_coding/neteq/neteq_impl.cc
diff --git a/webrtc/modules/audio_coding/neteq/neteq_impl.cc b/webrtc/modules/audio_coding/neteq/neteq_impl.cc
index 00f854b8db0b6f5ca794a98fd939540115419499..76726fd38529a8c28a8277d3c7c2eb0765ac3dc6 100644
--- a/webrtc/modules/audio_coding/neteq/neteq_impl.cc
+++ b/webrtc/modules/audio_coding/neteq/neteq_impl.cc
@@ -835,6 +835,15 @@ int NetEqImpl::GetAudioInternal(size_t max_length,
   LOG(LS_VERBOSE) << "Sync buffer (" << *num_channels << " channel(s)):" <<
       " insert " << algorithm_buffer_->Size() << " samples, extract " <<
       samples_from_sync << " samples";
+  if (sync_buffer_->FutureLength() < expand_->overlap_length()) {
+    // The sync buffer should always contain |overlap_length| samples, but now
+    // too many samples have been extracted. Reinstall the |overlap_length|
+    // lookahead by moving the index.
+    const size_t missing_lookahead_samples =
+        expand_->overlap_length() - sync_buffer_->FutureLength();
+    sync_buffer_->set_next_index(sync_buffer_->next_index() -
+                                 missing_lookahead_samples);

[minyue-webrtc, 2015/08/30 21:50:27]

How do you know that next_index_ >= missing_lookahead_samples

[hlundin-webrtc, 2015/08/31 08:08:25]

next_index_ is by definition FutureLength() from the end of sync_buffer, that
is, next_index_ = sync_buffer_->Size() - sync_buffer_->FutureLength().
Meanwhile, missing_lookahead_samples = expand_->overlap_length() -
sync_buffer_->FutureLength().

So, next_index_ >= missing_lookahead_samples holds true if sync_buffer_->Size()
>= expand_->overlap_length().

sync_buffer_->Size() is 120 ms and overlap_length() is 0.625 ms (5 samples in
8000 Hz). So, it holds true. But I'll add a DCHECK.

[minyue-webrtc, 2015/09/01 07:36:07]

Acknowledged.

+  }
   if (samples_from_sync != output_size_samples_) {
     LOG(LS_ERROR) << "samples_from_sync (" << samples_from_sync
                   << ") != output_size_samples_ (" << output_size_samples_
@@ -847,7 +856,7 @@ int NetEqImpl::GetAudioInternal(size_t max_length,
   *samples_per_channel = output_size_samples_;
 
   // Should always have overlap samples left in the |sync_buffer_|.
-  assert(sync_buffer_->FutureLength() >= expand_->overlap_length());
+  DCHECK_GE(sync_buffer_->FutureLength(), expand_->overlap_length());
 
   if (play_dtmf) {
     return_value = DtmfOverdub(dtmf_event, sync_buffer_->Channels(), output);