[Sbcl-devel] uncleared stack locations cause garbage retention?

[Ken O. <kd...@co...>]

I'm having a problem where my garbage is not getting collected after
the function that was using it returns.  I've attempted to isolate the
problem into a simple test case.  It seems to me that some location on
the stack is not getting cleared, so that things used by
previously-called functions are retained.  I understand that the
conservative GC can sometimes retain your garbage, for example because
of integer-pointer confusions, but I don't think it should happen this
easily.

I put the following in the file gc-bug.lisp:

----------------------------------------------------------------------
(defvar marker (gensym))		;A unique object we can find later

;;Recursively create a list of our objects of the given length.
(defun make-markers-1 (count)
  (if (plusp count)
      (cons marker (make-markers-1 (1- count)))
    nil))

;;Make a list, but don't return it or store it anywhere
(defun make-markers (count)
  (make-markers-1 count)
  nil)

;;Look through dynamic space to see if any of the cons cells
;;of our list are still there.
(defun find-markers (when)
  (let ((count 0))
    (sb-vm::map-allocated-objects
     #'(lambda (object type size)
	 (declare (ignore type size))
	 (when (and (consp object) (eq (car object) marker))
	   (incf count)))
     :dynamic)
    (format t "~&~A, there are ~D markers~%" when count)))

(defun cons-markers ()
  (make-markers 10) 		        ;Create (but don't return) the list
  (gc :full t)				;This should free it
  (find-markers "Inside cons-markers")	;See if it is still there
  )

(defun test ()
  (gc :full t)				;GC first just to make sure
  (find-markers "After initial GC")	;Is anything there beforehand?
  (cons-markers)			;Now create, GC, check once
  (gc :full t)				;GC again
  (find-markers "After final GC")	;check again
  )

----------------------------------------------------------------------

I get:

This is SBCL 1.0.25, an implementation of ANSI Common Lisp.
More information about SBCL is available at <http://www.sbcl.org/>....

* (progn (load "gc-bug") (test))

After initial GC, there are 0 markers
Inside cons-markers, there are 10 markers
After final GC, there are 0 markers

So inside CONS-MARKERS, there's still a pointer to this list, even
though MAKE-MARKERS did not return it.  But outside, in TEST, the
pointer is no longer active.

                                        Ken