Re: Dynamic modules: MODULE_HANDLE_SIGNALS etc.

[Paul Eggert]

Eli Zaretskii wrote:

> Is that practical?  One problem that I, as a module developer would
> face, is to estimate the amount of stack I'd need for unwinding.
> Where do I begin?

We could merely require that any module needing recursion must call a new 
stack-size-checking function at each level of recursion, in order to detect 
stack overflow.

Also, any module with an unbounded amount of computation should call the 
equivalent of QUIT every now and then. If the module API doesn't let (or 
ideally, require) modules to do that now, it should. Otherwise it's an Emacs 
freeze waiting to happen.

> Then there are all the stack frames below the module, which belong to
> some Lisp someone else wrote -- who will be responsible for ensuring
> those other unwinders don't need large amounts of stack space that
> might be unavailable at stack-overflow point?

Each stack-overflow unwinder should use only a small amount of stack space, in 
order to prevent an infinite loop. We could partially enforce this by detecting 
trivial infinite stack-overflow loops.

> Stack overflow detection on modern systems uses hardware assistance
> and processor exceptions to detect overflow with no runtime
> penalties.  Doing the equivalent in application code is bound to incur
> additional processing, which will slow down code, right?

Yes, but it's not a big deal.

> If you think
> about manipulating the guard pages to make them resizable, are we sure
> enough of the supported platforms allow that?

In platforms that don't support guard pages we'll need to have the run-time 
checks. Either approach should be adequate.

> Hitting stack overflow when some module runs is even rarer.
> Why is it a disaster to fail to invoke the unwinders in those cases?

Often I expect it wouldn't be a disaster; it'd just be a memory leak. I suppose 
there could be some modules where it would be a disaster. But perhaps we can 
just ask people to not write such modules, by making it a part of the Emacs API 
that unwinders might not be invoked.