From fe35742026af80b5ac120dfb9e5966689a698040 Mon Sep 17 00:00:00 2001
From: kyren <kerriganw@gmail.com>
Date: Mon, 5 Feb 2018 14:39:01 -0500
Subject: [PATCH] Set the metatable of __gc'ed userdata to something more
 informative

---
 src/userdata.rs |  2 +-
 src/util.rs     | 73 ++++++++++++++++++++++++++++++++++++++++++++-----
 2 files changed, 67 insertions(+), 8 deletions(-)

diff --git a/src/userdata.rs b/src/userdata.rs
index eea63d5..2c395e4 100644
--- a/src/userdata.rs
+++ b/src/userdata.rs
@@ -548,7 +548,7 @@ mod tests {
     }
 
     #[test]
-    fn test_expired_userdata() {
+    fn test_gc_userdata() {
         struct MyUserdata {
             id: u8,
         }
diff --git a/src/util.rs b/src/util.rs
index a7b1c76..fa36028 100644
--- a/src/util.rs
+++ b/src/util.rs
@@ -250,13 +250,12 @@ pub unsafe fn get_userdata<T>(state: *mut ffi::lua_State, index: c_int) -> *mut
 
 pub unsafe extern "C" fn userdata_destructor<T>(state: *mut ffi::lua_State) -> c_int {
     callback_error(state, || {
-        // We clear the metatable of userdata on __gc so that it will not be double dropped, and
-        // also so that it cannot be used or identified as any particular userdata type after the
-        // first call to __gc.
-        gc_guard(state, || {
-            ffi::lua_newtable(state);
-            ffi::lua_setmetatable(state, -2);
-        });
+        // We set the metatable of userdata on __gc to a special table with no __gc method and with
+        // metamethods that trigger an error on access.  We do this so that it will not be double
+        // dropped, and also so that it cannot be used or identified as any particular userdata type
+        // after the first call to __gc.
+        get_gc_userdata_metatable(state);
+        ffi::lua_setmetatable(state, -2);
         let ud = &mut *(ffi::lua_touserdata(state, 1) as *mut T);
         mem::replace(ud, mem::uninitialized());
         Ok(0)
@@ -588,3 +587,63 @@ unsafe fn get_panic_metatable(state: *mut ffi::lua_State) -> c_int {
 
     ffi::LUA_TTABLE
 }
+
+unsafe fn get_gc_userdata_metatable(state: *mut ffi::lua_State) -> c_int {
+    static GC_USERDATA_METATABLE: u8 = 0;
+
+    unsafe extern "C" fn gc_error(state: *mut ffi::lua_State) -> c_int {
+        ffi::lua_pushstring(state, cstr!("userdata has been garbage collected"));
+        ffi::lua_error(state)
+    }
+
+    ffi::lua_pushlightuserdata(state, &GC_USERDATA_METATABLE as *const u8 as *mut c_void);
+    let t = ffi::lua_gettable(state, ffi::LUA_REGISTRYINDEX);
+
+    if t != ffi::LUA_TTABLE {
+        ffi::lua_pop(state, 1);
+
+        ffi::luaL_checkstack(state, 8, ptr::null());
+
+        gc_guard(state, || {
+            ffi::lua_newtable(state);
+            ffi::lua_pushlightuserdata(state, &GC_USERDATA_METATABLE as *const u8 as *mut c_void);
+            ffi::lua_pushvalue(state, -2);
+
+            for &method in &[
+                cstr!("__add"),
+                cstr!("__sub"),
+                cstr!("__mul"),
+                cstr!("__div"),
+                cstr!("__mod"),
+                cstr!("__pow"),
+                cstr!("__unm"),
+                cstr!("__idiv"),
+                cstr!("__band"),
+                cstr!("__bor"),
+                cstr!("__bxor"),
+                cstr!("__bnot"),
+                cstr!("__shl"),
+                cstr!("__shr"),
+                cstr!("__concat"),
+                cstr!("__len"),
+                cstr!("__eq"),
+                cstr!("__lt"),
+                cstr!("__le"),
+                cstr!("__index"),
+                cstr!("__newindex"),
+                cstr!("__call"),
+                cstr!("__tostring"),
+                cstr!("__pairs"),
+                cstr!("__ipairs"),
+            ] {
+                ffi::lua_pushstring(state, method);
+                ffi::lua_pushcfunction(state, gc_error);
+                ffi::lua_rawset(state, -3);
+            }
+
+            ffi::lua_rawset(state, ffi::LUA_REGISTRYINDEX);
+        });
+    }
+
+    ffi::LUA_TTABLE
+}