C++ TSSLSocket shutdown delay/vulnerability

In the C++ implementation of TSSLSocket, during close() the following code sequence is found:

void TSSLSocket::close() {
  if (ssl_ != NULL) {
    int rc = SSL_shutdown(ssl_);
    if (rc == 0) {
      rc = SSL_shutdown(ssl_);
    }

According to the OpenSSL documentation, while it is "nice" to attempt to shutdown SSL in this way, it is not required when the code is performing a unilateral shutdown (meaning the underlying connection will be closed). It is only necessary to call SSL_Shutdown twice like this if the socket (and configured SSL therein) is going to be reused.

It is possible to have a misbehaving client that does not handle this part of the shutdown process properly and fail to reply, and also fail to close. The most likely embodiment would be a program designed specifically to fail to reply to the "close notify" and hold the socket open in order to produce a denial-of-service attack. Another possibility is a poorly written client.

Given the OpenSSL documentation states that calling SSL_Shutdown once is sufficient when the socket is going to be closed, I recommend that we remove the second SSL_Shutdown call and prevent something that can block the close() call. We have seen this happen, and it interacts with THRIFT-2441.