From a2829904d02c5fb13b8441f93952c7925bbd0d9c Mon Sep 17 00:00:00 2001
From: John Hawthorn <john@hawthorn.email>
Date: Wed, 28 Jul 2021 16:46:42 -0700
Subject: [PATCH] Add assertions for bad backtrace locations

---
 vm_backtrace.c | 16 +++++++++++-----
 1 file changed, 11 insertions(+), 5 deletions(-)

diff --git a/vm_backtrace.c b/vm_backtrace.c
index 670f73d2a2..0cb829b7ef 100644
--- a/vm_backtrace.c
+++ b/vm_backtrace.c
@@ -191,12 +191,13 @@ location_lineno(rb_backtrace_location_t *loc)
 	return calc_lineno(loc->body.iseq.iseq, loc->body.iseq.pc);
       case LOCATION_TYPE_CFUNC:
 	if (loc->body.cfunc.prev_loc) {
-	    return location_lineno(loc->body.cfunc.prev_loc);
-	}
-	return 0;
+        VM_ASSERT(loc->body.cfunc.prev_loc != loc);
+        return location_lineno(loc->body.cfunc.prev_loc);
+    }
+    return 0;
       default:
-	rb_bug("location_lineno: unreachable");
-	UNREACHABLE;
+    rb_bug("location_lineno: unreachable");
+    UNREACHABLE;
     }
 }
 
@@ -294,6 +295,7 @@ location_path(rb_backtrace_location_t *loc)
 	return rb_iseq_path(loc->body.iseq.iseq);
       case LOCATION_TYPE_CFUNC:
 	if (loc->body.cfunc.prev_loc) {
+        VM_ASSERT(loc->body.cfunc.prev_loc != loc);
 	    return location_path(loc->body.cfunc.prev_loc);
 	}
 	return Qnil;
@@ -358,6 +360,7 @@ location_realpath(rb_backtrace_location_t *loc)
 	return rb_iseq_realpath(loc->body.iseq.iseq);
       case LOCATION_TYPE_CFUNC:
 	if (loc->body.cfunc.prev_loc) {
+		VM_ASSERT(loc->body.cfunc.prev_loc != loc);
 	    return location_realpath(loc->body.cfunc.prev_loc);
 	}
 	return Qnil;
@@ -411,6 +414,7 @@ location_to_str(rb_backtrace_location_t *loc)
 	break;
       case LOCATION_TYPE_CFUNC:
 	if (loc->body.cfunc.prev_loc) {
+			  VM_ASSERT(loc->body.cfunc.prev_loc != loc);
 	    file = rb_iseq_path(loc->body.cfunc.prev_loc->body.iseq.iseq);
 	    lineno = location_lineno(loc->body.cfunc.prev_loc);
 	}
@@ -764,6 +768,7 @@ bt_iter_cfunc(void *ptr, const rb_control_frame_t *cfp, ID mid)
     loc->type = LOCATION_TYPE_CFUNC;
     loc->body.cfunc.mid = mid;
     if (arg->prev_loc) {
+		VM_ASSERT(loc != arg->prev_loc);
         loc->body.cfunc.prev_loc = arg->prev_loc;
     }
     else if (arg->prev_cfp) {
@@ -772,6 +777,7 @@ bt_iter_cfunc(void *ptr, const rb_control_frame_t *cfp, ID mid)
         arg->init_loc->type = LOCATION_TYPE_ISEQ;
         arg->init_loc->body.iseq.iseq = iseq;
         arg->init_loc->body.iseq.pc = pc;
+		VM_ASSERT(loc != arg->init_loc);
         loc->body.cfunc.prev_loc = arg->prev_loc = arg->init_loc;
     }
     else {
-- 
2.32.0