NetEq: Fixing a corner case with depleted sync buffer

webrtc/modules/audio_coding/neteq/neteq_impl.cc

 /*
  *  Copyright (c) 2012 The WebRTC project authors. All Rights Reserved.
  *
  *  Use of this source code is governed by a BSD-style license
  *  that can be found in the LICENSE file in the root of the source
  *  tree. An additional intellectual property rights grant can be found
  *  in the file PATENTS.  All contributing project authors may
  *  be found in the AUTHORS file in the root of the source tree.
  */
 
@@ skipping 817 matching lines @@
     num_output_samples = max_length;
     num_output_samples_per_channel = max_length / sync_buffer_->Channels();
   }
   const size_t samples_from_sync =
       sync_buffer_->GetNextAudioInterleaved(num_output_samples_per_channel,
                                             output);
   *num_channels = static_cast<int>(sync_buffer_->Channels());
   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_
                   << ")";
     // TODO(minyue): treatment of under-run, filling zeros
     memset(output, 0, num_output_samples * sizeof(int16_t));
     *samples_per_channel = output_size_samples_;
     return kSampleUnderrun;
   }
   *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);
   }
 
   // Update the background noise parameters if last operation wrote data
   // straight from the decoder to the |sync_buffer_|. That is, none of the
   // operations that modify the signal can be followed by a parameter update.
   if ((last_mode_ == kModeNormal) ||
       (last_mode_ == kModeAccelerateFail) ||
@@ skipping 1124 matching lines @@
 
 void NetEqImpl::CreateDecisionLogic() {
   decision_logic_.reset(DecisionLogic::Create(fs_hz_, output_size_samples_,
                                               playout_mode_,
                                               decoder_database_.get(),
                                               *packet_buffer_.get(),
                                               delay_manager_.get(),
                                               buffer_level_filter_.get()));
 }
 }  // namespace webrtc