fix(tvix/eval): add stack depth assertion to OpReturn r/7271

I'm still trying to work out the exact stack invariants for tvix.
We really should add assertions for them; getting the stack messed
up is no fun.  This commit adds one simple assertion.  It also adds
a missing stack-push (my mistake) in one place, which was uncovered
by the assertion.

Change-Id: I9d8b4bd1702d954e325832c5935b0d7e3eb68422
Reviewed-on: https://cl.tvl.fyi/c/depot/+/10369
Reviewed-by: tazjin <tazjin@tvl.su>
Tested-by: BuildkiteCI

1 files changed, 7 insertions, 0 deletions

diff --git a/tvix/eval/src/vm/mod.rs b/tvix/eval/src/vm/mod.rs
index 0ac5024d6b..c3c0b259bc 100644
--- a/tvix/eval/src/vm/mod.rs
+++ b/tvix/eval/src/vm/mod.rs
@@ -486,6 +486,9 @@ impl<'o> VM<'o> {
 
                 // Discard the current frame.
                 OpCode::OpReturn => {
+                    // TODO(amjoseph): I think this should assert `==` rather
+                    // than `<=` but it fails with the stricter condition.
+                    debug_assert!(self.stack.len() - 1 <= frame.stack_offset);
                     return Ok(true);
                 }
 
@@ -1081,6 +1084,10 @@ impl<'o> VM<'o> {
             }
 
             val @ Value::Catchable(_) => {
+                // the argument that we tried to apply a catchable to
+                self.stack.pop();
+                // applying a `throw` to anything is still a `throw`, so we just
+                // push it back on the stack.
                 self.stack.push(val);
                 Ok(())
             }