- avoid double linking to and unlinking from the world for actors.

Some people apparently have to toy around with the engine's innards without fully understanding them. :(
When properly used the saveguards should never be triggered.

src/playsim/actor.h

@@ -414,6 +414,7 @@ enum ActorFlag8
 	MF8_STOPRAILS		= 0x00000200,	// [MC] Prevent rails from going further if an actor has this flag.
 	MF8_ABSVIEWANGLES	= 0x00000400,	// [MC] By default view angle/pitch/roll is an offset. This will make it absolute instead.
 	MF8_FALLDAMAGE		= 0x00000800,	// Monster will take fall damage regardless of map settings.
+	MF8_LINKEDTOWORLD	= 0x00010000,	// actor is currently linked to the blockmap and sector chain.
 
 };
 

src/playsim/p_maputl.cpp

@@ -286,6 +286,12 @@ void P_LineOpening (FLineOpening &open, AActor *actor, const line_t *linedef, co
 void AActor::UnlinkFromWorld (FLinkContext *ctx)
 {
 	if (ctx != nullptr) ctx->sector_list = nullptr;
+	if (!(flags8 & MF8_LINKEDTOWORLD))
+	{
+		//assert(false);
+		return;
+	}
+	flags8 &= ~MF8_LINKEDTOWORLD;
 	if (!(flags & MF_NOSECTOR))
 	{
 		// invisible things don't need to be in sector list
@@ -443,6 +449,12 @@ bool AActor::FixMapthingPos()
 
 void AActor::LinkToWorld(FLinkContext *ctx, bool spawningmapthing, sector_t *sector)
 {
+	if (flags8 & MF8_LINKEDTOWORLD) 
+	{
+		//assert(false);
+		return;
+	}
+
 	bool spawning = spawningmapthing;
 
 	if (spawning)
@@ -468,6 +480,7 @@ void AActor::LinkToWorld(FLinkContext *ctx, bool spawningmapthing, sector_t *sec
 	Sector = sector;
 	subsector = Level->PointInRenderSubsector(Pos());	// this is from the rendering nodes, not the gameplay nodes!
 	section = subsector->section;
+	flags8 |= MF8_LINKEDTOWORLD;
 
 	if (!(flags & MF_NOSECTOR))
 	{

src/playsim/p_mobj.cpp

@@ -371,7 +371,7 @@ void AActor::Serialize(FSerializer &arc)
 
 		SerializeTerrain(arc, "floorterrain", floorterrain, &def->floorterrain);
 		SerializeArgs(arc, "args", args, def->args, special);
-
+		if (arc.isReading()) flags8 &= ~MF8_LINKEDTOWORLD;
 }
 
 #undef A