Re: Dynamic modules: MODULE_HANDLE_SIGNALS etc.

[Paul Eggert]

Eli Zaretskii wrote:

> Are you
> now saying something different from what you said back then, i.e. that
> we cannot rely on any function/macro from lisp.h to be signal-safe?

Yes and no. As I understood it, that old conversation was about functions that 
explicitly signal or throw, and it's safe to assume that EQ, NILP, etc. won't do 
that. The new conversation is about running out of memory, which is a different 
form of non-local exit. There may be other forms, such as operating-system 
signals (I don't recall exactly).

> If so, we should add the necessary protection, in the form of calls to
> MODULE_FUNCTION_BEGIN, to emacs-module.c functions that until now
> relied on those lisp.h functions/macros to be safe.

This wouldn't suffice for these other non-local exits, I think; at least, not as 
currently constructed.

> AFAIK, proper C++ exception handling
> requires non-trivial amounts of stack space that is not available when
> there's stack overflow, where you have at most a single guard page to
> work with.

There should be workarounds for that. Surely the C++ community has run into this 
problem and has solutions. If we want to support C++ modules, we need to employ 
them.

> I think there is some misunderstanding here, or some confusion,
> perhaps mine: emacs-module.c is not supposed to deal with any C++
> exceptions.  C++ exceptions are supposed to be caught at the C++
> level, below emacs-module.c, and handled there.  An exception that
> isn't caught will be recorded and will cause all the subsequent calls
> to Lisp or to emacs-module.c function to fail,

Why bother? If C++ exceptions are supposed to be caught by the C++ module in 
question, why does Emacs need to worry about C++ exceptions that are not caught?

> What emacs-module.c does with non-local exits of _any_ kind is record
> the first occurrence of such an exit, and silently return to the
> caller, thus allowing the C++ objects on the stack to be destroyed
> normally.  IOW, it defers the exit until internal--module-call is
> about to return.  What problems do you see with that which cause you
> to think it's error-prone, let alone dysfunctional?

It uses a different model at the C level from what one sees in Elisp, or from 
what one normally sees in C for that matter.  I don't feel that I will really 
understand the model unless I see some actual modules that do function calls and 
exception handling; but it's hard to believe that a model that does silent 
returns and that defers returns until later and that records some returns but 
not others will be problem-free. Wouldn't it be simpler to have a module invoke 
analogs of 'condition-case' and/or 'catch', and to dispense with the 
funcall_exit stuff?