Hooked up RtcEventLog. It lives in Voice Engine and pointers are propagated to ACM and Call.

webrtc/modules/audio_coding/main/acm2/audio_coding_module_impl.cc

Index: webrtc/modules/audio_coding/main/acm2/audio_coding_module_impl.cc
diff --git a/webrtc/modules/audio_coding/main/acm2/audio_coding_module_impl.cc b/webrtc/modules/audio_coding/main/acm2/audio_coding_module_impl.cc
index 9c3183271b75bcbdaca46d20cd915d8ecee05323..4535c868e8ac47d31720e954ac0e2208332558a9 100644
--- a/webrtc/modules/audio_coding/main/acm2/audio_coding_module_impl.cc
+++ b/webrtc/modules/audio_coding/main/acm2/audio_coding_module_impl.cc
@@ -27,6 +27,7 @@
 #include "webrtc/system_wrappers/interface/rw_lock_wrapper.h"
 #include "webrtc/system_wrappers/interface/trace.h"
 #include "webrtc/typedefs.h"
+#include "webrtc/video/rtc_event_log.h"
 
 namespace webrtc {
 
@@ -145,7 +146,9 @@ AudioCodingModuleImpl::AudioCodingModuleImpl(
       first_frame_(true),
       callback_crit_sect_(CriticalSectionWrapper::CreateCriticalSection()),
       packetization_callback_(NULL),
-      vad_callback_(NULL) {
+      vad_callback_(NULL),
+      event_log_ptr_crit_sect_(CriticalSectionWrapper::CreateCriticalSection()),

[terelius, 2015/08/05 10:35:42]

I got the following comment from pbos@ concering CriticalSectionWrapper
"Use a rtc::CriticalSection (this doesn't require additional heap allocation)."
"We're trying to move off webrtc::CriticalSectionWrapper."
What's the policy here?

Another question is whether we need a lock in the first place. If i understood
the argument, the point is to prevent anyone from changing the pointer itself
(and not the event log object) while someone else is reading it. Could this be
solved in some other way, e.g. by making the pointer itself const?

[hlundin-webrtc, 2015/08/05 11:02:06]

Yes, that was my concern. If you can make the design such that the pointer is
initialized in the constructors initialization list, and that it will never
change, then go ahead. But that means that you cannot deregister the event
logger by changing to a nullptr.

[ivoc, 2015/08/06 10:57:51]

So which option do we prefer? I would like to hear your opinions before
addressing this, since this would require a rewrite of about 70% of this CL and
also affects how I should address some of the other comments. I agree that it
would be nice to get rid of the lock here, on the other hand being able to
disable logging by setting a nullptr could also come in handy some day. 

+      event_log_(nullptr) {
   if (InitializeReceiverSafe() < 0) {
     WEBRTC_TRACE(webrtc::kTraceError, webrtc::kTraceAudioCoding, id_,
                  "Cannot initialize receiver");
@@ -736,6 +739,11 @@ int AudioCodingModuleImpl::PlayoutData10Ms(int desired_freq_hz,
                  "PlayoutData failed, RecOut Failed");
     return -1;
   }
+  {
+    CriticalSectionScoped lock(event_log_ptr_crit_sect_);
+    if (event_log_)

[terelius, 2015/08/05 10:35:42]

I thought the reason for having a no-op implementation of the RtcEventLog was so
that would not have to do these checks everywhere we want to log something.

[hlundin-webrtc, 2015/08/05 11:02:06]

Having a no-op implementation is not the same as not having a valid pointer.

[ivoc, 2015/08/06 10:57:51]

If I'm not mistaken, the ACM is also used standalone (without a corresponding
voice engine). So it's hard to avoid checking this pointer, since in the
standalone case there probably won't be a possibility to log.

[terelius, 2015/08/11 09:04:34]

Good point Ivo. It would still be able to point at the no-op implementation I
think, though perhaps that's not worth it.

[terelius, 2015/08/11 09:04:34]

Indeed. However, the proposed checks do not guarantee a "valid" pointer, merely
a non-null pointer. In other words, we test whether someone has explicitly
disabled logging by setting the pointer to null which (for most purposes) is
equivalent to pointing at the no-op implementation.

+      event_log_->LogDebugEvent(RtcEventLog::DebugEvent::kAudioPlayout);
+  }
 
   audio_frame->id_ = id_;
   return 0;
@@ -971,6 +979,11 @@ void AudioCodingModuleImpl::DisableNack() {
   receiver_.DisableNack();
 }
 
+void AudioCodingModuleImpl::SetEventLog(RtcEventLog* event_log) {
+  CriticalSectionScoped lock(event_log_ptr_crit_sect_);
+  event_log_ = event_log;
+}
+
 std::vector<uint16_t> AudioCodingModuleImpl::GetNackList(
     int64_t round_trip_time_ms) const {
   return receiver_.GetNackList(round_trip_time_ms);