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> __gc is called asynchronously at rather unpredictable points in the
> program.  A C extension writer can take that into account (and can
> actually do some useful work without invoking the garbage collector),
> but it's more difficult to do this in Lua code.  It's also not
> possible to create a critical section which locks out the garbage
> collector.
> It might be better if there was some sort of __gcqueue metatable slot
> which referred to an array.  When the object becomes unreachable, it
> is added to the __gcqueue array.  Lua functions which interact with
> these objects would process the queue.  This way, the side effect
> would occur at well-defined points in the program.

We thought about that. (Alexandra Barros, who did the first
implementation of ephemeron tables, also did a prototype implementation
of this feature.) One problem is memory allocation. What the collector
should do if there is a memory allocation error when resizing __gcqueue?
It is tricky to handle this situation.

-- Roberto