Log how often DTLS negotiation failed because of incompatible ciphersuites.

webrtc/base/opensslstreamadapter.cc

Index: webrtc/base/opensslstreamadapter.cc
diff --git a/webrtc/base/opensslstreamadapter.cc b/webrtc/base/opensslstreamadapter.cc
index e04eb04d67c8d05eee38938f406484ccbe5d5247..257e172065c4a7a192b85341558cffb0afff4f01 100644
--- a/webrtc/base/opensslstreamadapter.cc
+++ b/webrtc/base/opensslstreamadapter.cc
@@ -854,6 +854,12 @@ int OpenSSLStreamAdapter::ContinueSSL() {
     case SSL_ERROR_ZERO_RETURN:
     default:
       LOG(LS_VERBOSE) << " -- error " << code;
+      SSLHandshakeError ssl_handshake_err = SSLHandshakeError::UNKNOWN;
+      int err_code = ERR_peek_last_error();
+      if (err_code != 0 && ERR_GET_REASON(err_code) == SSL_R_NO_SHARED_CIPHER) {
+        ssl_handshake_err = SSLHandshakeError::INCOMPATIBLE_CIPHERSUITE;
+      }
+      SignalSSLHandshakeError(ssl_handshake_err);

[skvlad, 2016/07/27 00:29:18]

I wonder if it could be better to directly log the ERR_GET_REASON values instead
of translating it into our own enums? That way we won't need to extend it to
other types of errors; on the other hand we'll need OpenSSL headers to interpret
this data correctly.

[honghaiz3, 2016/07/27 17:11:48]

Are we potentially interested in knowing other types of SSL errors?
If yes, we may want to use the return value from ERR_GET_REASON. The range of
that is pretty big though, up to 2^12. 

[Zhi Huang, 2016/07/27 21:04:13]

I will remove the new defined enum and use the error code directly from
ERR_GET_REASON(). Since in this issue we are more interested in
incompatible-ciphersuite, I will just log that for now. It won't be too hard to
extend the histogram if we become interested in other types.

       return (ssl_error != 0) ? ssl_error : -1;
   }