Send empty snapshots to clients downloading using legacy protocol

Save bandwidth by excluding entity and player state changes from
snapshots sent to clients downloading using the legacy protocol. The
snapshots are mainly needed to update the reliableAcknowledge.

The idea is from Noah "Chomenor" Metzger's ioef-cmod project. Though I
implemented the dummy snapshot code closer to the real snapshot code.

code/server/server.h

@@ -401,7 +401,7 @@ void SV_SendMessageToClient( msg_t *msg, client_t *client );
 void SV_SendClientMessages( void );
 void SV_SendClientSnapshot( client_t *client );
 #ifdef ELITEFORCE
-void SV_WriteSnapshotToClient( client_t *client, msg_t *msg );
+void SV_WriteDummySnapshotToClient( client_t *client, msg_t *msg );
 #endif
 
 //

code/server/sv_client.c

@@ -1242,6 +1242,7 @@ int SV_SendDownloadMessages(void)
 			{
 				MSG_InitOOB(&msg, msgBuffer, sizeof(msgBuffer));
 				msg.compat = qtrue;
+				SV_WriteDummySnapshotToClient(cl, &msg);
 			}
 			else
 #endif
@@ -1255,12 +1256,7 @@ int SV_SendDownloadMessages(void)
 			if(retval)
 			{
 #ifdef ELITEFORCE
-				if(msg.compat)
-				{
-					// compat clients need svc_snapshot to update reliableAcknowledge
-					SV_WriteSnapshotToClient(cl, &msg);
-				}
-				else
+				if(!msg.compat)
 #endif
 					MSG_WriteByte(&msg, svc_EOF);
 				SV_Netchan_Transmit(cl, &msg);

code/server/sv_snapshot.c

@@ -119,10 +119,7 @@ static void SV_EmitPacketEntities( clientSnapshot_t *from, clientSnapshot_t *to,
 SV_WriteSnapshotToClient
 ==================
 */
-#ifndef ELITEFORCE
-static
-#endif
-void SV_WriteSnapshotToClient( client_t *client, msg_t *msg ) {
+static void SV_WriteSnapshotToClient( client_t *client, msg_t *msg ) {
 	clientSnapshot_t	*frame, *oldframe;
 	int					lastframe;
 	int					i;
@@ -216,6 +213,66 @@ void SV_WriteSnapshotToClient( client_t *client, msg_t *msg ) {
 }
 
 
+#ifdef ELITEFORCE
+/*
+==================
+SV_WriteDummySnapshotToClient
+
+While a client downloads a pk3 using the legacy protocol they need
+a snapshot to update the reliableAcknowledge. This dummy snapshot
+does not include areabits, entities, or player state updates.
+==================
+*/
+void SV_WriteDummySnapshotToClient( client_t *client, msg_t *msg ) {
+	int					snapFlags;
+	playerState_t		ps;
+
+	MSG_WriteByte (msg, svc_snapshot);
+
+	// NOTE, MRE: now sent at the start of every message from server to client
+	// let the client know which reliable clientCommands we have received
+	#ifdef ELITEFORCE
+	if(msg->compat)
+		MSG_WriteLong( msg, client->lastClientCommand );
+	#else
+	//MSG_WriteLong( msg, client->lastClientCommand );
+	#endif
+
+	// send over the current server time so the client can drift
+	// its view of time to try to match
+	if( client->oldServerTime ) {
+		MSG_WriteLong (msg, sv.time + client->oldServerTime);
+	} else {
+		MSG_WriteLong (msg, sv.time);
+	}
+
+	// what we are delta'ing from
+	MSG_WriteByte (msg, 0);
+
+	snapFlags = svs.snapFlagServerBit;
+	if ( client->rateDelayed ) {
+		snapFlags |= SNAPFLAG_RATE_DELAYED;
+	}
+	if ( client->state != CS_ACTIVE ) {
+		snapFlags |= SNAPFLAG_NOT_ACTIVE;
+	}
+
+	MSG_WriteByte (msg, snapFlags);
+
+	// send over the areabits
+	MSG_WriteByte (msg, 0);
+	MSG_WriteData (msg, NULL, 0);
+
+	// delta encode the playerstate
+	Com_Memset (&ps, 0, sizeof(ps));
+	MSG_WriteDeltaPlayerstate( msg, &ps, &ps );
+
+	// delta encode the entities
+	MSG_WriteBits( msg, (MAX_GENTITIES-1), GENTITYNUM_BITS );
+}
+#endif
+
+
 /*
 ==================
 SV_UpdateServerCommandsToClient