fix(tvix/eval): calling a catchable is catchable r/7187

When attempting to call a Value, if it is a Value::Catchable we must
not cause an uncatchable failure.  This commit simply reuses the
Value::Catchable as the result of attempting to call it.  This is
safe because nix is designed so that nix code cannot distinguish
between different catchable failures -- they all look the same to
the interpreted code.

This fixes b/351.

Change-Id: Ibf763a08753e541843626182ff59fdbf15ea2959
Reviewed-on: https://cl.tvl.fyi/c/depot/+/10300
Autosubmit: Adam Joseph <adam@westernsemico.com>
Tested-by: BuildkiteCI
Reviewed-by: tazjin <tazjin@tvl.su>

3 files changed, 8 insertions, 0 deletions

diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-attempt-to-call-catchable.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-attempt-to-call-catchable.exp
new file mode 100644
index 0000000000..c508d5366f
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-attempt-to-call-catchable.exp
@@ -0,0 +1 @@
+false
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-attempt-to-call-catchable.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-attempt-to-call-catchable.nix
new file mode 100644
index 0000000000..f4ef72a88b
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-attempt-to-call-catchable.nix
@@ -0,0 +1 @@
+(builtins.tryEval (throw "fred" 5)).success
diff --git a/tvix/eval/src/vm/mod.rs b/tvix/eval/src/vm/mod.rs
index 3d3b47be23..d57bd21e44 100644
--- a/tvix/eval/src/vm/mod.rs
+++ b/tvix/eval/src/vm/mod.rs
@@ -1056,6 +1056,12 @@ impl<'o> VM<'o> {
                 self.enqueue_generator("__functor call", span, |co| call_functor(co, val));
                 Ok(())
             }
+
+            val @ Value::Catchable(_) => {
+                self.stack.push(val);
+                Ok(())
+            }
+
             v => Err(ErrorKind::NotCallable(v.type_of())).with_span(&span, self),
         }
     }