Use Chromium BUILD to approximate Blimp protocol version, and check it.

blimp/net/engine_authentication_handler.cc

Index: blimp/net/engine_authentication_handler.cc
diff --git a/blimp/net/engine_authentication_handler.cc b/blimp/net/engine_authentication_handler.cc
index fe020083d54cca0debb14ad23222b94f9f331243..9d1f37659c3fd746f919fdbdb7cc6a28fba3d66f 100644
--- a/blimp/net/engine_authentication_handler.cc
+++ b/blimp/net/engine_authentication_handler.cc
@@ -9,8 +9,10 @@
 #include "base/callback_helpers.h"
 #include "base/logging.h"
 #include "base/timer/timer.h"
+#include "blimp/common/create_blimp_message.h"
 #include "blimp/common/logging.h"
 #include "blimp/common/proto/blimp_message.pb.h"
+#include "blimp/common/version_info.h"
 #include "blimp/net/blimp_connection.h"
 #include "blimp/net/blimp_message_processor.h"
 #include "blimp/net/blimp_transport.h"
@@ -111,23 +113,40 @@ void Authenticator::OnConnectionError(int error) {
 
 void Authenticator::ProcessMessage(std::unique_ptr<BlimpMessage> message,
                                    const net::CompletionCallback& callback) {
-  if (message->has_protocol_control() &&
-      message->protocol_control().has_start_connection()) {
-    bool token_match =
-        client_token_ ==
-        message->protocol_control().start_connection().client_token();
-    DVLOG(1) << "Authentication challenge received: "
-             << message->protocol_control().start_connection().client_token()
-             << ", and token "
-             << (token_match ? " matches" : " does not match");
-    OnConnectionAuthenticated(token_match);
-  } else {
-    DVLOG(1) << "Expected START_CONNECTION message, got " << *message
-             << " instead.";
+  base::ScopedClosureRunner run_callback(base::Bind(callback, net::OK));
+
+  if (!message->has_protocol_control() ||
+      !message->protocol_control().has_start_connection()) {
+    DVLOG(1) << "Expected PROTOCOL_CONTROL->START_CONNECTION, got " << *message;
+    OnConnectionAuthenticated(false);
+    return;
+  }
+
+  const StartConnectionMessage& start_connection =
+      message->protocol_control().start_connection();
+
+  // Verify that the protocol version is supported.
+  if (start_connection.protocol_version() != GetProtocolVersion()) {
+    DVLOG(1) << "Protocol version mismatch: "
+             << start_connection.protocol_version() << " vs "
+             << GetProtocolVersion();
+
+    // Inform the client of the mismatch before disconnecting it, so it can
+    // show the user an appropriate error.
+    connection_->GetOutgoingMessageProcessor()->ProcessMessage(
+        CreateEndConnectionMessage(EndConnectionMessage::PROTOCOL_MISMATCH),
+        net::CompletionCallback());
+
     OnConnectionAuthenticated(false);
+    return;
   }
 
-  callback.Run(net::OK);
+  // Verify that the authentication token matches.
+  bool token_match = client_token_ == start_connection.client_token();
+  DVLOG(1) << "Authentication challenge received: "
+           << start_connection.client_token() << ", and token "
+           << (token_match ? " matches" : " does not match");
+  OnConnectionAuthenticated(token_match);
 }
 
 }  // namespace