Stepping is not GC-safe

[Stas Boukarev]

The function which is arranged to be called by the stepper gets moved by the GC:
(defun test-step-out/2 ()
  (let* ((count 0)
         (*stepper-hook* (lambda (condition)
                           (typecase condition
                             (step-form-condition
                              (if (>= (incf count) 3)
                                  (invoke-restart 'step-out)
                                  (invoke-restart 'step-into)))))))
    (step (out))))

(defun z (x)
  x)

(defun in ()
  (declare (optimize debug))
  (z 1)
  (z 2)
  (z 3)
  (z 4))

(defun out ()
  (declare (optimize debug))
  (in))

(sb-thread:make-thread
 (lambda ()
   (loop
    (handler-bind ((step-condition #'sb-impl::invoke-stepper))
      (test-step-out/2)))))

(sb-thread:make-thread
 (lambda ()
   (sb-ext:gc :full t)))

* CORRUPTION WARNING in SBCL pid 27887(tid 140736519829248):
Memory fault at (nil) (pc=(nil), sp=0x7fffc6456580)
The integrity of this image is possibly compromised.
Continuing with fingers crossed.

debugger invoked on a SB-SYS:MEMORY-FAULT-ERROR in thread
#<THREAD RUNNING {10034C1653}>:
  Unhandled memory fault at #x0.

[Stas Boukarev (stassats)]

It needs with-pinned-objects around the call to (setf (context-register context callee-register-offset) (get-lisp-obj-address new-callee)), since %set-context-register is a function and get-lisp-obj-address passes a fixnum representation of the function, which doesn't hold it. context is registered by fake_foreign_function_call, so once it's there, it's pinned down.
Will commit after the freeze.

[Stas Boukarev (stassats)]

In 433d08123bdcff2bb1868c2884beab8b431bdb75.