Subject: bug#58601: 29.0.50; Infinite loop in byte-compile--first-symbol-with-pos

From: Alan Mackenzie <[email-masked]>
Date: Wed, 19 Oct 2022 08:52:10 +0000

On Tue, Oct 18, 2022 at 15:19:35 -0400, Stefan Monnier wrote:
> >> That's not the problem, because it's just for illustrative purposes.
> >> Instead of 'arg' being passed unchanged as the FORM argument, it could
> >> just as well have been `(my-frobnicate ,arg).

> > That would not loop, since there is a symbol with position there.

> BTE, the real-world use case is a compiler macro which takes the list of
> args and tucks the name of another function in front.  So the generated
> code, passed to `macroexp-warn-and-return` has the shape

>     (somefun somearg1 somearg2)

> where `somefun` does *not* come from the source but from the
> compiler-macro instead and hence doesn't have any sympos, and then
> `somearg1` was an expression of the form

>    (quote #1=(1 2 3 . #1#))

> So, there was no cycle in the code part, the cycle is only inside the
> data embedded in the code.

I think somebody who writes something like that _deserves_ to go into an
infinite loop.  Have you ever seen anything at all like this in real
life?

You have fixed this "problem" by arbitrarily limiting the size of valid
form stacks that can be searched, and the depth to which they can be
searched.  This limit, 10, seems too small.  There can easily be form
stacks with more than 10 elements.  Do you have any objection if I
change that 10 to something closer to infinity?  Something which will
encompass any form stack likely to be encountered in practice?  I would
suggest the number 300.

>         Stefan

-- 
Alan Mackenzie (Nuremberg, Germany).