Revert "VM: always clear the data and retainstack in general_error because they might contain uninitialized values (#1187)"

This reverts commit 378f208025.

vm/contexts.cpp

@@ -39,6 +39,16 @@ void context::reset() {
   reset_context_objects();
 }
 
+void context::fix_stacks() {
+  if (datastack + sizeof(cell) < datastack_seg->start ||
+      datastack + stack_reserved >= datastack_seg->end)
+    reset_datastack();
+
+  if (retainstack + sizeof(cell) < retainstack_seg->start ||
+      retainstack + stack_reserved >= retainstack_seg->end)
+    reset_retainstack();
+}
+
 context::~context() {
   delete datastack_seg;
   delete retainstack_seg;

vm/contexts.hpp

@@ -47,6 +47,7 @@ struct context {
   void reset_callstack();
   void reset_context_objects();
   void reset();
+  void fix_stacks();
   void scrub_stacks(gc_info* info, cell index);
 
   cell peek() { return *(cell*)datastack; }

vm/errors.cpp

@@ -44,19 +44,17 @@ void factor_vm::general_error(vm_error_type error, cell arg1_, cell arg2_) {
 
   faulting_p = true;
 
-  /* If we had an underflow or overflow, data or retain stack pointers
-     might be out of bounds, or some of their slots might be
-     uninitialized, so reset them before allocating anything. */
-  ctx->reset_datastack();
-  ctx->reset_retainstack();
+  /* If we had an underflow or overflow, data or retain stack
+     pointers might be out of bounds, so fix them before allocating
+     anything */
+  ctx->fix_stacks();
 
   /* If error was thrown during heap scan, we re-enable the GC */
   gc_off = false;
 
-  cell error_handler = special_objects[ERROR_HANDLER_QUOT];
   /* If the error handler is set, we rewind any C stack frames and
      pass the error to user-space. */
-  if (!current_gc && to_boolean(error_handler)) {
+  if (!current_gc && to_boolean(special_objects[ERROR_HANDLER_QUOT])) {
 #ifdef FACTOR_DEBUG
     /* Doing a GC here triggers all kinds of funny errors */
     primitive_compact_gc();
@@ -74,7 +72,8 @@ void factor_vm::general_error(vm_error_type error, cell arg1_, cell arg2_) {
 
     /* The unwind-native-frames subprimitive will clear faulting_p
        if it was successfully reached. */
-    unwind_native_frames(error_handler, ctx->callstack_top);
+    unwind_native_frames(special_objects[ERROR_HANDLER_QUOT],
+                         ctx->callstack_top);
   } /* Error was thrown in early startup before error handler is set, so just
        crash. */
   else {