Regression test for issue where Opus DTX status was being forgotten.

webrtc/modules/audio_coding/acm2/audio_coding_module.cc

Index: webrtc/modules/audio_coding/acm2/audio_coding_module.cc
diff --git a/webrtc/modules/audio_coding/acm2/audio_coding_module.cc b/webrtc/modules/audio_coding/acm2/audio_coding_module.cc
index b394c591567926b151b00a62c861f7f7bc35bbd1..3f404f736f31364f2b120c874efd3e8695b5a15d 100644
--- a/webrtc/modules/audio_coding/acm2/audio_coding_module.cc
+++ b/webrtc/modules/audio_coding/acm2/audio_coding_module.cc
@@ -48,6 +48,8 @@ class AudioCodingModuleImpl final : public AudioCodingModule {
   void ModifyEncoder(
       FunctionView<void(std::unique_ptr<AudioEncoder>*)> modifier) override;
 
+  void QueryEncoder(FunctionView<void(const AudioEncoder*)> query) override;
+
   // Get current send codec.
   rtc::Optional<CodecInst> SendCodec() const override;
 
@@ -596,6 +598,12 @@ void AudioCodingModuleImpl::ModifyEncoder(
   modifier(&encoder_stack_);
 }
 
+void AudioCodingModuleImpl::QueryEncoder(

[minyue-webrtc, 2016/07/26 09:37:01]

yes, I like it. but preferrably a std::unique_ptr<const AudioEncoder> as
argument

[ivoc, 2016/07/26 12:28:31]

I don't think it's possible to cast a unique_ptr<X> to unique_ptr<const X>. The
only way to do this is to use std::move, but then the new pointer would take the
ownership of the object, which means that the function has to return it as well
(which would not be a very nice solution in my opinion).

[ivoc, 2016/07/26 13:55:20]

I thought about this a little more, and I think we should stick to a const
pointer. I think unique_ptr is good for managing object ownership/lifetime, but
in this function we don't really want to change anything about the ownership of
the object, just provide access to its const functions. 

I also found this in the style guide: "If dynamic allocation is necessary,
prefer to keep ownership with the code that allocated it. If other code needs
access to the object, consider passing it a copy, or passing a pointer or
reference without transferring ownership. Prefer to use std::unique_ptr to make
ownership transfer explicit."
Let me know if you disagree, we can also ask another reviewer to have a look at
this if you're not sure. 

+    FunctionView<void(const AudioEncoder*)> query) {
+  rtc::CritScope lock(&acm_crit_sect_);
+  query(encoder_stack_.get());
+}
+
 // Get current send codec.
 rtc::Optional<CodecInst> AudioCodingModuleImpl::SendCodec() const {
   rtc::CritScope lock(&acm_crit_sect_);