Re: crash in Lua 5.1 and 5.2: calling C function from unloaded lib

[Josh Haberman <jhaberman@...>]

On Mon, Apr 9, 2012 at 1:07 AM, Robert G. Jakabosky
<bobby@sharedrealm.com> wrote:
> On Sunday 08, Josh Haberman wrote:
> To support safe unloading the Lua C API would need to be changed provide a way
> to tell the VM what module a C function is a part of.

If Lua can't unload modules safely, why is it unloading them at all?

There is no warning in the documentation of __gc that improper use can
SEGV the interpreter.  It seems counter to Lua's goal of being an
embeddable language that a Lua program can take down the whole
process.  Especially since it is not obvious how to trigger this
behavior.

>> Perhaps this is a GC-related bug where
>> the loaded library is collected prematurely?
>
> No, the collection order is correct.

I'm not sure how you can call the collection order "correct" when I
was able to reference a collected object.

> Try:
> local ext = require "ext"
> atexit(function() ext.nop() end)

How should a person who sees this SEGV in a large program know how to
fix it?  I ran into this problem when using lunitx
(https://github.com/dcurrie/lunit), and I had no idea why I was
getting SEGVs inside of Lua.  The atexit() function was implemented
and called from inside lunitx; I had no idea that a __gc metamethod
was involved at all.

Josh