Re: coroutine library can cause panic with low memory

[Patrick Donnelly <batrick@...>]

On Mon, Mar 2, 2009 at 12:07 PM, M Joonas Pihlaja
<jpihlaja@cc.helsinki.fi> wrote:
>
> Hey,
>
> On Mon, 2 Mar 2009, Patrick Donnelly wrote:
>
>> Using the attached script one can cause Lua to panic. The problem is
>> the API call lua_checkstack which may throw a memory error.
>
> [snip]
>
>> lua_checkstack should probably not throw memory errors because it
>> cannot knowingly throw the error on the currently running thread. The
>> thread you are checking stack space for may not actually be running!
>
> Uhm.. sorry if I'm being dense, but isn't growing stack and possibly
> throwing a memory error the very reason for calling lua_checkstack in
> the first place?  What on earth is it supposed to do if it can't throw
> memory errors when it can't grow the stack?

Because, in preparation for lua_xmove, you will want to ensure stack
space on the coroutine you are moving values to. However, there is no
method of checking the stack space on that coroutine without
_possibly_ causing a panic due to an out of memory error.


On Mon, Mar 2, 2009 at 12:19 PM, Roberto Ierusalimschy
<roberto@inf.puc-rio.br> wrote:
>> Uhm.. sorry if I'm being dense, but isn't growing stack and possibly
>> throwing a memory error the very reason for calling lua_checkstack in
>> the first place?  What on earth is it supposed to do if it can't throw
>> memory errors when it can't grow the stack?
>
> Probably a better correction would be to throw memory errors on the
> main thread, if the current thread does not have one.

I don't feel this is the right way to go either. It bypasses, to the
bewilderment of the programmer, all the error protection measures, a
potentially large chain of pcalls and resumes, and may still cause a
panic if the main thread has no error handler.

-- 
-Patrick Donnelly

"One of the lessons of history is that nothing is often a good thing
to do and always a clever thing to say."