Trace stats in RTCStatsCollector.

webrtc/pc/rtcstatscollector.cc

Index: webrtc/pc/rtcstatscollector.cc
diff --git a/webrtc/pc/rtcstatscollector.cc b/webrtc/pc/rtcstatscollector.cc
index f5b8c8a936c1b569826ff20d98741e62ddb5e550..3bd5a13bcfca1db09a6ac061e29ff3dae447a974 100644
--- a/webrtc/pc/rtcstatscollector.cc
+++ b/webrtc/pc/rtcstatscollector.cc
@@ -25,11 +25,17 @@
 #include "webrtc/pc/webrtcsession.h"
 #include "webrtc/rtc_base/checks.h"
 #include "webrtc/rtc_base/timeutils.h"
+#include "webrtc/rtc_base/trace_event.h"
 
 namespace webrtc {
 
 namespace {
 
+std::string TraceNameFromStatsTypeAndMemberName(const char* stats_type,
+                                                const char* member_name) {
+  return std::string(stats_type) + '.' + member_name;

[tommi, 2017/07/12 15:34:20]

'.' + member_name

by itself is the same as doing |46 + member_name|, so are you counting on
operator order and that the first operator yields a std::string, which will take
override the last + operator?

If that's the case, I would prefer to make it more explicit.  Other than that,
the general idea seems right.

[ehmaldonado_webrtc, 2017/07/12 15:40:32]

Yes, I'm counting on that.
How would I make it more explicit?
Maybe (std::string(stats_type) + '.') + member_name?

[tommi, 2017/07/12 15:44:52]

Yes, that's one way.

Alternatively you could use rtc::sprintfn and reduce the number of heap
allocations to 1 (instead of possibly 3):

char buffer[1024];  // assuming this would always be big enough?
rtc::sprintfn(&buffer[0], sizeof(buffer),
    "%s.%s", stats_type, member_name);
return buffer;

[ehmaldonado_webrtc, 2017/07/12 15:58:27]

Ok, I see :)

Something it's not clear to me, Is it still valid to reference buffer once the
function ends? Isn't it allocated for the duration of the function?

[hbos, 2017/07/12 16:20:15]

It would indeed not be valid to reference it once the function ends. But because
of the return type of our helper function "return buffer;" is implicitly the
same as "return std::string(buffer);". It would copy the characters up to the
first null terminating character into the std::string object that is returned.

Stats dictionaries and member names are quite short and would combined always
fit on much less than a single line. An 80 char buffer would have plenty of
margin.

[ehmaldonado_webrtc, 2017/07/12 16:36:07]

Makes sense.
Done.

+}
+
 std::string RTCCertificateIDFromFingerprint(const std::string& fingerprint) {
   return "RTCCertificate_" + fingerprint;
 }
@@ -751,6 +757,17 @@ void RTCStatsCollector::AddPartialResults_s(
     channel_name_pairs_.reset();
     track_media_info_map_.reset();
     track_to_id_.clear();
+    for (const RTCStats& stats : *cached_report_) {
+      for (const RTCStatsMemberInterface* member : stats.Members()) {
+        if (member->is_defined()) {
+          TRACE_EVENT_INSTANT2("webrtc_stats",
+                               TraceNameFromStatsTypeAndMemberName(
+                                   stats.type(), member->name()).c_str(),
+                               "value", member->ValueToString(),
+                               "id", stats.id());
+        }
+      }
+    }
     DeliverCachedReport();
   }
 }