[PATCH] Avoid 'frame-local-ref' errors when printing backtrace.

[Andrew Whatson <whatson@gmail.com>]

Workaround for <https://bugs.gnu.org/57948>.

* module/system/vm/frame.scm (frame-call-representation): Treat a
binding as "unspecified" if its slot exceeds 'frame-num-locals'.
---
 module/system/vm/frame.scm | 12 ++++++++++--
 1 file changed, 10 insertions(+), 2 deletions(-)

diff --git a/module/system/vm/frame.scm b/module/system/vm/frame.scm
index 3be73e29c..e36eb797c 100644
--- a/module/system/vm/frame.scm
+++ b/module/system/vm/frame.scm
@@ -381,8 +381,16 @@
         (frame-local-ref frame i 'scm))
        ((find-slot i bindings)
         => (lambda (binding)
-             (let ((val (frame-local-ref frame (binding-slot binding)
-                                         (binding-representation binding))))
+             (let* ((slot (binding-slot binding))
+                    ;; HACK: Avoid out-of-range from frame-local-ref.
+                    ;; Some frames have bindings beyond nlocals.  That
+                    ;; is probably a bug somewhere else, but at least
+                    ;; this workaround allows them to be printed.
+                    ;; See <https://bugs.gnu.org/57948>.
+                    (val (if (< slot nlocals)
+                             (frame-local-ref frame slot
+                                              (binding-representation binding))
+                             *unspecified*)))
                ;; It could be that there's a value that isn't clobbered
                ;; by a call but that isn't live after a call either.  In
                ;; that case, if GC runs during the call, the value will
-- 
2.37.3