From 738ea59e2367b1f24233fa1897554fecbbbcf185 Mon Sep 17 00:00:00 2001
From: Mark Thom <markjordanthom@gmail.com>
Date: Tue, 3 Dec 2019 22:01:38 -0700
Subject: [PATCH] fix copy_term/3 infinite looping on cyclic terms

---
 src/prolog/machine/copier.rs | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/src/prolog/machine/copier.rs b/src/prolog/machine/copier.rs
index 51a56dd2..13e21ee3 100644
--- a/src/prolog/machine/copier.rs
+++ b/src/prolog/machine/copier.rs
@@ -90,6 +90,11 @@ impl<T: CopierTarget> CopyTermState<T> {
             ra @ Addr::AttrVar(_) | ra @ Addr::HeapCell(..) | ra @ Addr::StackCell(..) => {
                 if ra == rd {
                     self.reinstantiate_var(ra, threshold);
+
+                    if let AttrVarPolicy::StripAttributes = self.attr_var_policy {
+                        self.trail.push((Ref::HeapCell(addr), self.target[addr].clone()));
+                        self.target[addr] = HeapCellValue::Addr(Addr::HeapCell(threshold));
+                    }
                 } else {
                     self.target[threshold] = HeapCellValue::Addr(ra);
                 }
-- 
2.54.0