[Maxima-bugs] [maxima:bugs] #4619 limit(inf = inf) causes stack overflow

[Barton W. <wil...@us...>]

Maybe not a fix, but the bug vanishes by inserting a trap for when `x` and `e` are syntactically equal at the top of `infsimp2`.   

The argument names to `infsimp2` are misleading, I think. The first argument to `infsimp2` is `expand(e,1,1)`; it is not a limit variable as the name `x` suggests. When `x` and `e` are syntactically equal, the call `(setq x ($limit x))` in `infsimp2` causes an infinite loop.  

A revised function:
~~~
(defun infsimp2 (x e)
   (cond ((alike1 x e) e)
         (t
            (setq x ($limit x))
            (if (isinop x '%limit) e x))))
~~~
This change to `infsimp2` seems harmless, but I'm not sure it it is really getting at the bug.  

Possibly the check `(alike1 x e)` should be moved up to `infsimp` but I didn't try that.

Also, possibly `infsimp` should send all  relational expressions directly to `infsimp2`, not just equal:

~~~
(defun infsimp (e)
  (let ((x ($expand e 1 1)))
    (cond ((or (not (free x '$ind)) (not (free x '$und))
	       (not (free x '$zeroa)) (not (free x '$zerob))
	       (not (free x '$infinity))
	       (mbagp x)
          (mrelationp x)) ;; <=== New
	   (infsimp2 x e))
	  ((and (free x '$inf) (free x '$minf)) x)
	  (t (infsimp1 x e)))))
~~~




---

**[bugs:#4619] limit(inf = inf) causes stack overflow**

**Status:** open
**Group:** None
**Labels:** limit 
**Created:** Thu Oct 09, 2025 06:22 PM UTC by Robert Dodier
**Last Updated:** Thu Oct 09, 2025 06:22 PM UTC
**Owner:** nobody


Looking at bug report #4603, looks like it is triggered by a bug in `limit`. Here is the simplest case I found:
```
(%i1) limit(inf=inf);
INFO: Binding stack guard page unprotected
Binding stack guard page temporarily disabled: proceed with caution

debugger invoked on a SB-KERNEL::BINDING-STACK-EXHAUSTED in thread
#<THREAD tid=4380 "main thread" RUNNING {1001760003}>:
  Binding stack exhausted.
```
This other example also causes a stack overflow: `limit(minf = minf)`.

Bug #4603 is triggered by `limit(foo - inf = -inf)`, where `foo` is a gensym (and the bug is also triggered when `foo` is just `foo`).

Looks like the stack overflow originates in INFSIMP2. I didn't investigate beyond that.

When this bug gets fixed, we should go back to #4603 and verify that bug is also fixed.


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