Drop on impossible server command sequence nums

Have the client drop the connection in these cases:
- A gamestate that decreases the sequence number. This should be
  impossible because the netchan guarantees that packets can't be
  reordered, although they may be dropped.
- A server command which a sequence number higher than the next one.
  This should be impossible because the server re-sends all
  unacknowledged commands together each time. If the number jumped by
  more than one the client would end up re-executing old garbage from
  the buffer.

Author: slipher

src/engine/client/cl_cgame.cpp

@@ -137,7 +137,8 @@ void CL_ConfigstringModified( Cmd::Args& csCmd )
 /*
 ===================
 CL_HandleServerCommand
-CL_GetServerCommand
+
+Returns true if the command should be passed to the cgame
 ===================
 */
 bool CL_HandleServerCommand(Str::StringRef text, std::string& newText) {

src/engine/client/cl_parse.cpp

@@ -408,7 +408,14 @@ void CL_ParseGamestate( msg_t *msg )
 		CL_ClearState();
 
 		// a gamestate always marks a server command sequence
-		clc.serverCommandSequence = MSG_ReadLong( msg );
+		int commandNum = MSG_ReadLong( msg );
+
+		if ( commandNum < clc.serverCommandSequence )
+		{
+			Sys::Drop( "Gamestate moved serverCommandSequence backward" );
+		}
+
+		clc.serverCommandSequence = commandNum;
 
 		// trash any commands from previous game
 		clc.lastExecutedServerCommand = clc.serverCommandSequence;
@@ -499,6 +506,13 @@ void CL_ParseCommandString( msg_t *msg )
 		return;
 	}
 
+	if ( clc.serverCommandSequence + 1 != seq )
+	{
+		Sys::Drop( "Out-of-sequence server command: expected %d, got %d",
+		           clc.serverCommandSequence + 1, seq );
+		return;
+	}
+
 	clc.serverCommandSequence = seq;
 
 	index = seq & ( MAX_RELIABLE_COMMANDS - 1 );