Don't run GC finalizers on ref thread.

If this happen then we don't have full access to the ref thread stack.
Fixes #491

src/state.rs

@@ -1043,8 +1043,8 @@ impl Lua {
         let state = lua.state();
         unsafe {
             if lua.unlikely_memory_error() {
-                crate::util::push_buffer(lua.ref_thread(), buf.as_ref(), false)?;
-                return Ok(Buffer(lua.pop_ref_thread()));
+                crate::util::push_buffer(state, buf.as_ref(), false)?;
+                return Ok(Buffer(lua.pop_ref()));
             }
 
             let _sg = StackGuard::new(state);

src/state/raw.rs

@@ -432,8 +432,8 @@ impl RawLua {
     pub(crate) unsafe fn create_string(&self, s: impl AsRef<[u8]>) -> Result<String> {
         let state = self.state();
         if self.unlikely_memory_error() {
-            push_string(self.ref_thread(), s.as_ref(), false)?;
-            return Ok(String(self.pop_ref_thread()));
+            push_string(state, s.as_ref(), false)?;
+            return Ok(String(self.pop_ref()));
         }
 
         let _sg = StackGuard::new(state);
@@ -444,12 +444,12 @@ impl RawLua {
 
     /// See [`Lua::create_table_with_capacity`]
     pub(crate) unsafe fn create_table_with_capacity(&self, narr: usize, nrec: usize) -> Result<Table> {
+        let state = self.state();
         if self.unlikely_memory_error() {
-            push_table(self.ref_thread(), narr, nrec, false)?;
-            return Ok(Table(self.pop_ref_thread()));
+            push_table(state, narr, nrec, false)?;
+            return Ok(Table(self.pop_ref()));
         }
 
-        let state = self.state();
         let _sg = StackGuard::new(state);
         check_stack(state, 3)?;
         push_table(state, narr, nrec, true)?;
@@ -734,6 +734,10 @@ impl RawLua {
 
     pub(crate) unsafe fn drop_ref(&self, vref: &ValueRef) {
         let ref_thread = self.ref_thread();
+        mlua_debug_assert!(
+            ffi::lua_gettop(ref_thread) >= vref.index,
+            "GC finalizer is not allowed in ref_thread"
+        );
         ffi::lua_pushnil(ref_thread);
         ffi::lua_replace(ref_thread, vref.index);
         (*self.extra.get()).ref_free.push(vref.index);

tests/tests.rs

@@ -1377,3 +1377,21 @@ fn test_exec_raw() -> Result<()> {
 
     Ok(())
 }
+
+#[test]
+fn test_gc_drop_ref_thread() -> Result<()> {
+    let lua = Lua::new();
+
+    let t = lua.create_table()?;
+    lua.create_function(move |_, ()| {
+        _ = &t;
+        Ok(())
+    })?;
+
+    for _ in 0..10000 {
+        // GC will run eventually to collect the function and the table above
+        lua.create_table()?;
+    }
+
+    Ok(())
+}