Avoids unnecessary calls to audio encoder.

As of this CL:
https://webrtc-review.googlesource.com/c/src/+/173704
...we now call AudioEncoder::OnReceivedOverhead() too often, since we
don't check if overhead has actually changed.
This CL rectifies that.

Bug: webrtc:10809
Change-Id: I0b86e0296a7860dde3e62e817f9b941fa82afe4a
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/175009
Reviewed-by: Per Åhgren <peah@webrtc.org>
Commit-Queue: Erik Språng <sprang@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#31279}

audio/audio_send_stream.cc

diff --git a/audio/audio_send_stream.cc b/audio/audio_send_stream.cc
index 8730c45..cdeea1a 100644
--- a/audio/audio_send_stream.cc
+++ b/audio/audio_send_stream.cc
@@ -544,10 +544,12 @@
 }
 
 void AudioSendStream::UpdateOverheadForEncoder() {
-  const size_t overhead_per_packet_bytes = GetPerPacketOverheadBytes();
-  if (overhead_per_packet_bytes == 0) {
-    return;  // Overhead is not known yet, do not tell the encoder.
+  size_t overhead_per_packet_bytes = GetPerPacketOverheadBytes();
+  if (overhead_per_packet_ == overhead_per_packet_bytes) {
+    return;
   }
+  overhead_per_packet_ = overhead_per_packet_bytes;
+
   channel_send_->CallEncoder([&](AudioEncoder* encoder) {
     encoder->OnReceivedOverhead(overhead_per_packet_bytes);
   });

audio/audio_send_stream.h

diff --git a/audio/audio_send_stream.h b/audio/audio_send_stream.h
index 92e9a7f..5f8a936 100644
--- a/audio/audio_send_stream.h
+++ b/audio/audio_send_stream.h
@@ -195,6 +195,7 @@
   static int TransportSeqNumId(const Config& config);
 
   rtc::CriticalSection overhead_per_packet_lock_;
+  size_t overhead_per_packet_ RTC_GUARDED_BY(overhead_per_packet_lock_) = 0;
 
   // Current transport overhead (ICE, TURN, etc.)
   size_t transport_overhead_per_packet_bytes_

audio/audio_send_stream_unittest.cc

diff --git a/audio/audio_send_stream_unittest.cc b/audio/audio_send_stream_unittest.cc
index 60655e9..89f94cd 100644
--- a/audio/audio_send_stream_unittest.cc
+++ b/audio/audio_send_stream_unittest.cc
@@ -794,7 +794,7 @@
     auto new_config = helper.config();
 
     // CallEncoder will be called on overhead change.
-    EXPECT_CALL(*helper.channel_send(), CallEncoder(::testing::_)).Times(1);
+    EXPECT_CALL(*helper.channel_send(), CallEncoder);
 
     const size_t transport_overhead_per_packet_bytes = 333;
     send_stream->SetTransportOverhead(transport_overhead_per_packet_bytes);
@@ -804,6 +804,27 @@
   }
 }
 
+TEST(AudioSendStreamTest, DoesntCallEncoderWhenOverheadUnchanged) {
+  for (bool use_null_audio_processing : {false, true}) {
+    ConfigHelper helper(false, true, use_null_audio_processing);
+    auto send_stream = helper.CreateAudioSendStream();
+    auto new_config = helper.config();
+
+    // CallEncoder will be called on overhead change.
+    EXPECT_CALL(*helper.channel_send(), CallEncoder);
+    const size_t transport_overhead_per_packet_bytes = 333;
+    send_stream->SetTransportOverhead(transport_overhead_per_packet_bytes);
+
+    // Set the same overhead again, CallEncoder should not be called again.
+    EXPECT_CALL(*helper.channel_send(), CallEncoder).Times(0);
+    send_stream->SetTransportOverhead(transport_overhead_per_packet_bytes);
+
+    // New overhead, call CallEncoder again
+    EXPECT_CALL(*helper.channel_send(), CallEncoder);
+    send_stream->SetTransportOverhead(transport_overhead_per_packet_bytes + 1);
+  }
+}
+
 TEST(AudioSendStreamTest, AudioOverheadChanged) {
   for (bool use_null_audio_processing : {false, true}) {
     ConfigHelper helper(false, true, use_null_audio_processing);