fix(tvix/eval): Propagate catchables in NixAttrs::construct

Correctly propagate the case where the *key* of an attrset is a
Value::Catchable (eg { "${builtins.throw "c"}" = "b"; }) in
`NixAttrs::construct`, by converting the return type to
`Result<Result<Self, CatchableErrorKind>, ErrorKind>` (ugh!!) and
correctly handling that everywhere (including an `expect` in the
Deserialize impl for NixAttrs, since afaict this is impossible to hit
when deserializing from stuff like JSON).

Change-Id: Ic4bc611fbfdab27c0bd8a40759689a87c4004a17
Reviewed-on: https://cl.tvl.fyi/c/depot/+/10786
Reviewed-by: raitobezarius <tvl@lahfa.xyz>
Tested-by: BuildkiteCI

tvix/eval/src/tests/tvix_tests/eval-okay-attr-key-catchable.exp

@@ -0,0 +1 @@
+false

tvix/eval/src/tests/tvix_tests/eval-okay-attr-key-catchable.nix

@@ -0,0 +1 @@
+(builtins.tryEval { "${builtins.throw "a"}" = "b"; }).success

tvix/eval/src/value/attrs.rs

@@ -19,6 +19,7 @@ use super::thunk::ThunkSet;
 use super::TotalDisplay;
 use super::Value;
 use crate::errors::ErrorKind;
+use crate::CatchableErrorKind;
 
 lazy_static! {
     static ref NAME_S: NixString = "name".into();
@@ -170,7 +171,9 @@ impl<'de> Deserialize<'de> for NixAttrs {
                     stack_array.push(value);
                 }
 
-                NixAttrs::construct(stack_array.len() / 2, stack_array).map_err(A::Error::custom)
+                Ok(NixAttrs::construct(stack_array.len() / 2, stack_array)
+                    .map_err(A::Error::custom)?
+                    .expect("Catchable values are unreachable here"))
             }
         }
 
@@ -333,7 +336,10 @@ impl NixAttrs {
 
     /// Implement construction logic of an attribute set, to encapsulate
     /// logic about attribute set optimisations inside of this module.
-    pub fn construct(count: usize, mut stack_slice: Vec<Value>) -> Result<Self, ErrorKind> {
+    pub fn construct(
+        count: usize,
+        mut stack_slice: Vec<Value>,
+    ) -> Result<Result<Self, CatchableErrorKind>, ErrorKind> {
         debug_assert!(
             stack_slice.len() == count * 2,
             "construct_attrs called with count == {}, but slice.len() == {}",
@@ -343,13 +349,13 @@ impl NixAttrs {
 
         // Optimisation: Empty attribute set
         if count == 0 {
-            return Ok(NixAttrs(AttrsRep::Empty));
+            return Ok(Ok(NixAttrs(AttrsRep::Empty)));
         }
 
         // Optimisation: KV pattern
         if count == 2 {
             if let Some(kv) = attempt_optimise_kv(&mut stack_slice) {
-                return Ok(kv);
+                return Ok(Ok(kv));
             }
         }
 
@@ -369,11 +375,13 @@ impl NixAttrs {
                     continue;
                 }
 
+                Value::Catchable(err) => return Ok(Err(err)),
+
                 other => return Err(ErrorKind::InvalidAttributeName(other)),
             }
         }
 
-        Ok(attrs)
+        Ok(Ok(attrs))
     }
 
     /// Construct an optimized "KV"-style attribute set given the value for the

tvix/eval/src/value/attrs/tests.rs

@@ -4,7 +4,9 @@ use super::*;
 
 #[test]
 fn test_empty_attrs() {
-    let attrs = NixAttrs::construct(0, vec![]).expect("empty attr construction should succeed");
+    let attrs = NixAttrs::construct(0, vec![])
+        .expect("empty attr construction should succeed")
+        .unwrap();
 
     assert!(
         matches!(attrs, NixAttrs(AttrsRep::Empty)),
@@ -15,7 +17,8 @@ fn test_empty_attrs() {
 #[test]
 fn test_simple_attrs() {
     let attrs = NixAttrs::construct(1, vec![Value::from("key"), Value::from("value")])
-        .expect("simple attr construction should succeed");
+        .expect("simple attr construction should succeed")
+        .unwrap();
 
     assert!(
         matches!(attrs, NixAttrs(AttrsRep::Im(_))),
@@ -39,7 +42,8 @@ fn test_kv_attrs() {
             meaning_val.clone(),
         ],
     )
-    .expect("constructing K/V pair attrs should succeed");
+    .expect("constructing K/V pair attrs should succeed")
+    .unwrap();
 
     match kv_attrs {
         NixAttrs(AttrsRep::KV { name, value })
@@ -55,7 +59,7 @@ fn test_kv_attrs() {
 
 #[test]
 fn test_empty_attrs_iter() {
-    let attrs = NixAttrs::construct(0, vec![]).unwrap();
+    let attrs = NixAttrs::construct(0, vec![]).unwrap().unwrap();
     assert!(attrs.iter().next().is_none());
 }
 
@@ -75,7 +79,8 @@ fn test_kv_attrs_iter() {
             meaning_val.clone(),
         ],
     )
-    .expect("constructing K/V pair attrs should succeed");
+    .expect("constructing K/V pair attrs should succeed")
+    .unwrap();
 
     let mut iter = kv_attrs.iter().collect::<Vec<_>>().into_iter();
     let (k, v) = iter.next().unwrap();
@@ -90,7 +95,8 @@ fn test_kv_attrs_iter() {
 #[test]
 fn test_map_attrs_iter() {
     let attrs = NixAttrs::construct(1, vec![Value::from("key"), Value::from("value")])
-        .expect("simple attr construction should succeed");
+        .expect("simple attr construction should succeed")
+        .unwrap();
 
     let mut iter = attrs.iter().collect::<Vec<_>>().into_iter();
     let (k, v) = iter.next().unwrap();

tvix/eval/src/vm/mod.rs

@@ -932,9 +932,11 @@ where
 
     fn run_attrset(&mut self, frame: &CallFrame, count: usize) -> EvalResult<()> {
         let attrs = NixAttrs::construct(count, self.stack.split_off(self.stack.len() - count * 2))
-            .with_span(frame, self)?;
+            .with_span(frame, self)?
+            .map(Value::attrs)
+            .into();
 
-        self.stack.push(Value::attrs(attrs));
+        self.stack.push(attrs);
         Ok(())
     }