Log how often DTLS negotiation failed because of incompatible ciphersuites.

webrtc/base/sslstreamadapter.h

Index: webrtc/base/sslstreamadapter.h
diff --git a/webrtc/base/sslstreamadapter.h b/webrtc/base/sslstreamadapter.h
index ba60ce3da09b32ce28b832597784cdaa78e0866a..341510fa00f4890794f1f53ed9c2b9ee02906b9c 100644
--- a/webrtc/base/sslstreamadapter.h
+++ b/webrtc/base/sslstreamadapter.h
@@ -76,6 +76,9 @@ enum SSLProtocolVersion {
 // Errors for Read -- in the high range so no conflict with OpenSSL.
 enum { SSE_MSG_TRUNC = 0xff0001 };
 
+// Used to back UMA histogram value.

[honghaiz3, 2016/07/22 21:35:26]

=> "Used to send back UMA histogram values." or something like that. 

[Zhi Huang, 2016/07/25 17:24:28]

Done.

+enum class SSLHandshakeError { ERR_INCOMPATIBLE_CIPHERSUITE, ERR_OTHER };

[skvlad, 2016/07/22 01:25:14]

Would it be better to make ERR_OTHER the first value of the enum?
The UMA methods take a maximum value but I'm not sure if they can handle
reordering of enum values correctly. E.g. if subsequently we add another value
ERR_NEW before ERR_OTHER - it would be hard to interpret if a value of "1" means
ERR_OTHER (for older versions) or ERR_NEW (for newer ones).

To maximize forward compatibility, I'd rather define it this way:
enum class SSLHandshakeError { UNKNOWN, INCOMPATIBLE_CIPHERSUITE, MAX_VALUE }
and use MAX_VALUE in calls to UMA.

Furthermore, you don't need prefixes (ERR_...) for enum class types as they are
already in their own namespace unlike regular enums.

[Zhi Huang, 2016/07/25 17:24:28]

Yes, this is better! 

+
 class SSLStreamAdapter : public StreamAdapterInterface {
  public:
   // Instantiate an SSLStreamAdapter wrapping the given stream,
@@ -207,6 +210,8 @@ class SSLStreamAdapter : public StreamAdapterInterface {
   // depending on specific SSL implementation.
   static std::string SslCipherSuiteToName(int cipher_suite);
 
+  sigslot::signal1<SSLHandshakeError> SignalSSLHandshakeError;
+
  private:
   // If true, the server certificate need not match the configured
   // server_name, and in fact missing certificate authority and other