Add UMA for the kernel's TCP RTT estimate.

net/socket/tcp_socket_libevent.cc

Index: net/socket/tcp_socket_libevent.cc
diff --git a/net/socket/tcp_socket_libevent.cc b/net/socket/tcp_socket_libevent.cc
index 6a6417b1ba12151a291b896312b93b9d27aeaa16..db339e2ced7f58b87a814954289ec4c68796377b 100644
--- a/net/socket/tcp_socket_libevent.cc
+++ b/net/socket/tcp_socket_libevent.cc
@@ -746,4 +746,25 @@ void TCPSocketLibevent::UpdateTCPFastOpenStatusAfterRead() {
   }
 }
 
+bool TCPSocketLibevent::GetEstimatedRoundTripTime(base::TimeDelta* out_rtt) {

[mmenke, 2015/07/07 19:53:50]

How do you intend to use this to inform future code decisions?  Just wondering
what role localhost requests and the like play here, and if we care.

[Bryan McQuade, 2015/07/07 21:03:33]

I'm mostly interested in understanding TCP RTT in UMA, but the team working on
the network quality estimator is also interested in using this to improve their
estimation. I imagine they'd want to ignore connections to localhost.

+  DCHECK(out_rtt);
+  if (!socket_ || !socket_->IsConnected()) {

[mmenke, 2015/07/07 19:53:50]

The IsConnected() check seems concerning - if the server hangs up on us, it will
return false, but I think it may still have stats we can get?

[Bryan McQuade, 2015/07/07 21:03:32]

Yes, good point. I had added that to prevent logging the RTT when the kernel
doesn't have an estimate yet, but I think it's better for me to heck if the
kernel estimate is zero and discard if so. I suppose that technically a
connection could have a zero-microsecond RTT but I'm inclined to interpret zero
values as indicating the kernel doesn't yet have an estimate.

+    return false;
+  }

[mmenke, 2015/07/07 19:53:49]

nit:  Remove braces.

[Bryan McQuade, 2015/07/07 21:03:33]

Done.

+
+  bool success = false;
+#if defined(TCP_INFO)
+  tcp_info info;
+  socklen_t info_len = sizeof(tcp_info);
+  success = getsockopt(socket_->socket_fd(), IPPROTO_TCP, TCP_INFO, &info,
+                       &info_len) == 0 &&
+            info_len == sizeof(tcp_info);
+  if (success) {
+    *out_rtt = base::TimeDelta::FromMicroseconds(info.tcpi_rtt);

[mmenke, 2015/07/07 19:53:49]

Suggest getting rid of success variable, and just returning true here.

[Bryan McQuade, 2015/07/07 21:03:33]

Done.

+  }

[mmenke, 2015/07/07 19:53:50]

nit:  Remove braces.

[Bryan McQuade, 2015/07/07 21:03:32]

Done.

+#endif

[mmenke, 2015/07/07 19:53:49]

nit:  #endif  // defined(TCP_INFO)

(Not done in most places in this file, but it's the preferred style).

[Bryan McQuade, 2015/07/07 21:03:32]

Done.

+
+  return success;
+}
+
 }  // namespace net