Fix fallback to plaintext

Summary:
Fallback to plaintext happens in handleSecureMessage, when the client
thinks the sasl handshake fails but the server thinks it succeeded. So the
server will receive a plaintext request with client type set to header and
protection state is still VALID. So simply skipping handleSecureMessage when
protection state is VALID is a bug.

Reviewed By: alandau

Differential Revision: D2907010

fb-gh-sync-id: e43907141d2b209d7225dbdfaade6a6e7002b405

thrift/lib/cpp2/async/SaslNegotiationHandler.cpp

@@ -20,8 +20,7 @@ namespace apache { namespace thrift {
 using ProtectionState = apache::thrift::ProtectionHandler::ProtectionState;
 
 void SaslNegotiationHandler::read(Context* ctx, BufAndHeader bufAndHeader) {
-  if (protectionHandler_->getProtectionState() == ProtectionState::NONE ||
-      protectionHandler_->getProtectionState() == ProtectionState::VALID) {
+  if (protectionHandler_->getProtectionState() == ProtectionState::NONE) {
     // This handler should be removed from the pipeline after sasl
     // negotiation is completed. If it is still installed, it should
     // do nothing.