Let Lua scripts relink tables to their metatables when unarchiving

This is done through the new "registermetatable" function,
in a somewhat similar fashion to "freeslot" but for metatables:
it must be called at script load to tell SRB2 your metatable
can be automatically relinked during the unarchiving process.

src/lua_baselib.c

@@ -238,6 +238,33 @@ static int lib_userdataType(lua_State *L)
 		return luaL_typerror(L, 1, "userdata");
 }
 
+// Takes a metatable as first and only argument
+// Only callable during script loading
+static int lib_registermetatable(lua_State *L)
+{
+	static UINT32 nextid = 1;
+
+	if (!lua_lumploading)
+		return luaL_error(L, "This function cannot be called from within a hook or coroutine!");
+	luaL_checktype(L, 1, LUA_TTABLE);
+
+	lua_getfield(L, LUA_REGISTRYINDEX, LREG_METATABLES); // 2
+		// registry.metatables[metatable] = nextid
+		lua_pushvalue(L, 1); // 3
+			lua_pushnumber(L, nextid); // 4
+		lua_settable(L, 2);
+
+		// registry.metatables[nextid] = metatable
+		lua_pushnumber(L, nextid); // 3
+			lua_pushvalue(L, 1); // 4
+		lua_settable(L, 2);
+	lua_pop(L, 1);
+
+	nextid++;
+
+	return 0;
+}
+
 static int lib_isPlayerAdmin(lua_State *L)
 {
 	player_t *player = *((player_t **)luaL_checkudata(L, 1, META_PLAYER));
@@ -3433,6 +3460,7 @@ static luaL_Reg lib[] = {
 	{"chatprint", lib_chatprint},
 	{"chatprintf", lib_chatprintf},
 	{"userdataType", lib_userdataType},
+	{"registermetatable", lib_registermetatable},
 	{"IsPlayerAdmin", lib_isPlayerAdmin},
 	{"reserveLuabanks", lib_reserveLuabanks},
 

src/lua_libs.h

@@ -16,6 +16,7 @@ extern lua_State *gL;
 #define LREG_EXTVARS "LUA_VARS"
 #define LREG_STATEACTION "STATE_ACTION"
 #define LREG_ACTIONS "MOBJ_ACTION"
+#define LREG_METATABLES "METATABLES"
 
 #define META_STATE "STATE_T*"
 #define META_MOBJINFO "MOBJINFO_T*"

src/lua_script.c

@@ -437,6 +437,10 @@ static void LUA_ClearState(void)
 	lua_newtable(L);
 	lua_setfield(L, LUA_REGISTRYINDEX, LREG_VALID);
 
+	// make LREG_METATABLES table for all registered metatables
+	lua_newtable(L);
+	lua_setfield(L, LUA_REGISTRYINDEX, LREG_METATABLES);
+
 	// open srb2 libraries
 	for(i = 0; liblist[i]; i++) {
 		lua_pushcfunction(L, liblist[i]);
@@ -1269,8 +1273,22 @@ static void ArchiveTables(void)
 
 			lua_pop(gL, 1);
 		}
-		lua_pop(gL, 1);
 		WRITEUINT8(save_p, ARCH_TEND);
+
+		// Write metatable ID
+		if (lua_getmetatable(gL, -1))
+		{
+			// registry.metatables[metatable]
+			lua_getfield(gL, LUA_REGISTRYINDEX, LREG_METATABLES);
+			lua_pushvalue(gL, -2);
+			lua_gettable(gL, -2);
+			WRITEUINT16(save_p, lua_isnil(gL, -1) ? 0 : lua_tointeger(gL, -1));
+			lua_pop(gL, 3);
+		}
+		else
+			WRITEUINT16(save_p, 0);
+
+		lua_pop(gL, 1);
 	}
 }
 
@@ -1438,6 +1456,7 @@ static void UnArchiveTables(void)
 {
 	int TABLESINDEX;
 	UINT16 i, n;
+	UINT16 metatableid;
 
 	if (!gL)
 		return;
@@ -1462,6 +1481,19 @@ static void UnArchiveTables(void)
 			else
 				lua_rawset(gL, -3);
 		}
+
+		metatableid = READUINT16(save_p);
+		if (metatableid)
+		{
+			// setmetatable(table, registry.metatables[metatableid])
+			lua_getfield(gL, LUA_REGISTRYINDEX, LREG_METATABLES);
+				lua_rawgeti(gL, -1, metatableid);
+				if (lua_isnil(gL, -1))
+					I_Error("Unknown metatable ID %d\n", metatableid);
+				lua_setmetatable(gL, -3);
+			lua_pop(gL, 1);
+		}
+
 		lua_pop(gL, 1);
 	}
 }