Add UMA for the kernel's TCP RTT estimate.

net/socket/tcp_socket_libevent.h

Index: net/socket/tcp_socket_libevent.h
diff --git a/net/socket/tcp_socket_libevent.h b/net/socket/tcp_socket_libevent.h
index 78b035d2d8d892385aeb8037b30b3c41ea437726..e608942360de2159bd5505f22cb3f4e265a67937 100644
--- a/net/socket/tcp_socket_libevent.h
+++ b/net/socket/tcp_socket_libevent.h
@@ -9,6 +9,7 @@
 #include "base/callback.h"
 #include "base/compiler_specific.h"
 #include "base/memory/scoped_ptr.h"
+#include "base/time/time.h"
 #include "net/base/address_family.h"
 #include "net/base/completion_callback.h"
 #include "net/base/net_export.h"
@@ -63,6 +64,10 @@ class NET_EXPORT TCPSocketLibevent {
   bool SetKeepAlive(bool enable, int delay);
   bool SetNoDelay(bool no_delay);
 
+  // Gets the estimated RTT. Returns false if the RTT is
+  // unavailable. May also return false when estimated RTT is 0.
+  bool GetEstimatedRoundTripTime(base::TimeDelta* out_rtt) const;

[Jana, 2015/07/08 12:33:29]

Since 0 is a special case that indicates failure, why not simply return 0 on
failure and non-zero values for all legit out values?

[Bryan McQuade, 2015/07/08 13:26:07]

Thanks Jana! I'll defer to Matt on the decision here, but my thought is that
with a signature like 'RetVal Fn();' it's easy for callers to miss that zero
indicates failure, which means they end up incorrectly using the zero value as
the estimated RTT. This is really undesirable and could lead to bugs.

We can document this behavior in a comment in the header file, but devs don't
always consult headers - many times they see a method used elsewhere in the code
and start using it without consulting the header, so I think it's important for
the method's signature alone to clearly explain method behavior as much as
possible.

The 'bool Fn(Param* out)' signature is a little more cumbersome, but I claim
it's much harder for callers to mistakenly miss the failure case with a
signature like this.

I'm actually inclined to go a step further and add WARN_UNUSED_RESULT to the
signature, to make sure callers always observe the return value. I see this
pattern used elsewhere in net:
https://code.google.com/p/chromium/codesearch#search/&q=bool%5C%20.*WARN_UNUS...

What do you think?

+
   void Close();
 
   // Setter/Getter methods for TCP FastOpen socket option.