From 07f452125c9795a9a36da2dee4f9e465ea9b6bb9 Mon Sep 17 00:00:00 2001
From: Soni L <EnderMoneyMod@gmail.com>
Date: Sun, 31 Jan 2016 14:46:52 -0200
Subject: [PATCH] Fix __gc and use raw metatable when it's disabled

This fixes a small __gc abuse I found, and also uses the raw metatable in a safe way when __gc is disabled. Both were tested.
---
 .../assets/opencomputers/lua/machine.lua      | 40 +++++++++++--------
 1 file changed, 23 insertions(+), 17 deletions(-)

diff --git a/src/main/resources/assets/opencomputers/lua/machine.lua b/src/main/resources/assets/opencomputers/lua/machine.lua
index a2617aa1b..baf883485 100644
--- a/src/main/resources/assets/opencomputers/lua/machine.lua
+++ b/src/main/resources/assets/opencomputers/lua/machine.lua
@@ -683,7 +683,7 @@ sandbox = {
     end
     local result = getmetatable(t)
     -- check if we have a wrapped __gc using mt
-    if type(result) == "table" and rawget(result, "__gc") == sgc then
+    if type(result) == "table" and system.allowGC() and rawget(result, "__gc") == sgc then
       result = rawget(result, "mt")
     end
     return result
@@ -713,26 +713,32 @@ sandbox = {
     if type(mt) ~= "table" then
       return setmetatable(t, mt)
     end
-    if type(rawget(mt, "__gc")) == "function" then
-      -- For all user __gc functions we enforce a much tighter deadline.
-      -- This is because these functions may be called from the main
-      -- thread under certain circumstanced (such as when saving the world),
-      -- which can lead to noticeable lag if the __gc function behaves badly.
-      local sbmt = {} -- sandboxed metatable. only for __gc stuff, so it's
-                      -- kinda ok to have a shallow copy instead... meh.
-      for k, v in next, mt do
-        sbmt[k] = v
-      end
-      sbmt.mt = mt
+    if rawget(mt, "__gc") ~= nil then -- If __gc is set to ANYTHING not `nil`, we're gonna have issues
       -- Garbage collector callbacks apparently can't be sandboxed after
       -- all, because hooks are disabled while they're running. So we just
       -- disable them altogether by default.
-      if not system.allowGC() then
-        sbmt.__gc = nil -- Silent fail for backwards compat. TODO error in OC 1.7
-      else
+      if system.allowGC() then
+        -- For all user __gc functions we enforce a much tighter deadline.
+        -- This is because these functions may be called from the main
+        -- thread under certain circumstanced (such as when saving the world),
+        -- which can lead to noticeable lag if the __gc function behaves badly.
+        local sbmt = {} -- sandboxed metatable. only for __gc stuff, so it's
+                        -- kinda ok to have a shallow copy instead... meh.
+        for k, v in next, mt do
+          sbmt[k] = v
+        end
         sbmt.__gc = sgc
+        sbmt.mt = mt
+        mt = sbmt
+      else
+        -- Don't allow marking for finalization, but use the raw metatable.
+        local gc = rawget(mt, "__gc")
+        rawset(mt, "__gc", nil) -- remove __gc
+        local ret = table.pack(pcall(setmetatable, t, mt))
+        rawset(mt, "__gc", gc) -- restore __gc
+        if not ret[1] then error(ret[2], 0) end
+        return table.unpack(ret, 1, ret.n)
       end
-      mt = sbmt
     end
     return setmetatable(t, mt)
   end,
@@ -1442,4 +1448,4 @@ end
 
 -- JNLua converts the coroutine to a string immediately, so we can't get the
 -- traceback later. Because of that we have to do the error handling here.
-return pcall(main)
\ No newline at end of file
+return pcall(main)