Keep reference to k_db abstract and avoid early finalizer call

[jonasmalacofilho]

In the SQLite CFFI library, connect returns an abstract Neko value, of k_db kind and with a finalizer set. If the k_result request value only keeps a pointer to the db struct, instead of the corresponding k_db value, the finalizer will be called as soon as no other external references remain to that connection, even if the request itself is still reachable.

The issue could manifest in code like (Haxe):

var rs = Sqlite.open("db.db").request("select * from tbl");
trace(rs.length);

Related: HaxeFoundation/haxe#8728

[ncannasse]

Should we should check if val_db(r->database) == NULL since the database might have been closed in the meanwhile

[jonasmalacofilho]

@ncannasse maybe, where exactly do you mean?

One thing I noticed is that free_db will set r->done early on, and that cleanly stops result_next. In fact, that's why we had no exception or segfault even though the db had been closed and freed.

[Simn]

I was told to merge this!