* Fix blackholing. If evaluation fails due to an assertion failure,

  then the blackhole has to be removed to ensure that repeated
  evaluation of the same value gives an assertion failure again rather
  than an "infinite recursion" error.

1 files changed, 8 insertions, 2 deletions

diff --git a/src/libexpr/eval.cc b/src/libexpr/eval.cc
index 98b6b7bdd150..e72538e60874 100644
--- a/src/libexpr/eval.cc
+++ b/src/libexpr/eval.cc
@@ -681,8 +681,14 @@ bool EvalState::evalBool(Env & env, Expr e)
 void EvalState::forceValue(Value & v)
 {
     if (v.type == tThunk) {
-        //v.type = tBlackhole;
-        eval(*v.thunk.env, v.thunk.expr, v);
+        ValueType saved = v.type;
+        try {
+            v.type = tBlackhole;
+            eval(*v.thunk.env, v.thunk.expr, v);
+        } catch (Error & e) {
+            v.type = saved;
+            throw;
+        }
     }
     else if (v.type == tCopy) {
         forceValue(*v.val);