feat(tvix/eval): nonrecursive deep_force()

This commit implements deep_force() nonrecursively, by maintaining
an explicit stack rather than using the call stack for recursion.

As an added bonus, we don't need to pass around the SharedThunkSet
anymore, and can in fact completely eliminate SharedThunkSet.

Change-Id: I7c4f59f37834d451a28bf6be317eb0a90eac4ee6
Reviewed-on: https://cl.tvl.fyi/c/depot/+/10252
Tested-by: BuildkiteCI
Reviewed-by: tazjin <tazjin@tvl.su>
Autosubmit: Adam Joseph <adam@westernsemico.com>

tvix/eval/src/value/mod.rs

@@ -34,7 +34,7 @@ pub use path::canon_path;
 pub use string::NixString;
 pub use thunk::Thunk;
 
-pub use self::thunk::{SharedThunkSet, ThunkSet};
+pub use self::thunk::ThunkSet;
 
 use lazy_static::lazy_static;
 
@@ -206,74 +206,87 @@ pub enum PointerEquality {
 }
 
 impl Value {
-    // TODO(amjoseph): de-asyncify this (when called directly by the VM)
     /// Deeply forces a value, traversing e.g. lists and attribute sets and forcing
     /// their contents, too.
     ///
     /// This is a generator function.
-    pub(super) async fn deep_force(
-        self,
-        co: GenCo,
-        thunk_set: SharedThunkSet,
-    ) -> Result<Value, ErrorKind> {
-        // Get rid of any top-level thunks, and bail out of self-recursive
-        // thunks.
-        let value = if let Value::Thunk(ref t) = &self {
-            if !thunk_set.insert(t) {
-                return Ok(self);
-            }
-            generators::request_force(&co, self).await
+    pub(super) async fn deep_force(self, co: GenCo, span: LightSpan) -> Result<Value, ErrorKind> {
+        if let Some(v) = Self::deep_force_(self.clone(), co, span).await? {
+            Ok(v)
         } else {
-            self
-        };
+            Ok(self)
+        }
+    }
 
-        match &value {
-            // Short-circuit on already evaluated values, or fail on internal values.
-            Value::Null
-            | Value::Bool(_)
-            | Value::Integer(_)
-            | Value::Float(_)
-            | Value::String(_)
-            | Value::Path(_)
-            | Value::Closure(_)
-            | Value::Builtin(_) => return Ok(value),
+    /// Returns Some(v) or None to indicate the returned value is myself
+    async fn deep_force_(
+        myself: Value,
+        co: GenCo,
+        span: LightSpan,
+    ) -> Result<Option<Value>, ErrorKind> {
+        // This is a stack of values which still remain to be forced.
+        let mut vals = vec![myself];
 
-            Value::List(list) => {
-                for val in list {
-                    if let c @ Value::Catchable(_) =
-                        generators::request_deep_force(&co, val.clone(), thunk_set.clone()).await
-                    {
-                        return Ok(c);
-                    }
+        let mut thunk_set: ThunkSet = Default::default();
+
+        loop {
+            let v = if let Some(v) = vals.pop() {
+                v
+            } else {
+                return Ok(None);
+            };
+
+            // Get rid of any top-level thunks, and bail out of self-recursive
+            // thunks.
+            let value = if let Value::Thunk(t) = &v {
+                if !thunk_set.insert(t) {
+                    continue;
                 }
-            }
+                Thunk::force_(t.clone(), &co, span.clone()).await?
+            } else {
+                v
+            };
 
-            Value::Attrs(attrs) => {
-                for (_, val) in attrs.iter() {
-                    if let c @ Value::Catchable(_) =
-                        generators::request_deep_force(&co, val.clone(), thunk_set.clone()).await
-                    {
-                        return Ok(c);
+            match value {
+                // Short-circuit on already evaluated values, or fail on internal values.
+                Value::Null
+                | Value::Bool(_)
+                | Value::Integer(_)
+                | Value::Float(_)
+                | Value::String(_)
+                | Value::Path(_)
+                | Value::Closure(_)
+                | Value::Builtin(_) => continue,
+
+                Value::List(list) => {
+                    for val in list.into_iter().rev() {
+                        vals.push(val);
                     }
+                    continue;
                 }
+
+                Value::Attrs(attrs) => {
+                    for (_, val) in attrs.into_iter().rev() {
+                        vals.push(val);
+                    }
+                    continue;
+                }
+
+                Value::Thunk(_) => panic!("Tvix bug: force_value() returned a thunk"),
+
+                Value::Catchable(_) => return Ok(Some(value)),
+
+                Value::AttrNotFound
+                | Value::Blueprint(_)
+                | Value::DeferredUpvalue(_)
+                | Value::UnresolvedPath(_)
+                | Value::Json(_)
+                | Value::FinaliseRequest(_) => panic!(
+                    "Tvix bug: internal value left on stack: {}",
+                    value.type_of()
+                ),
             }
-
-            Value::Thunk(_) => panic!("Tvix bug: force_value() returned a thunk"),
-
-            Value::Catchable(_) => return Ok(value),
-
-            Value::AttrNotFound
-            | Value::Blueprint(_)
-            | Value::DeferredUpvalue(_)
-            | Value::UnresolvedPath(_)
-            | Value::Json(_)
-            | Value::FinaliseRequest(_) => panic!(
-                "Tvix bug: internal value left on stack: {}",
-                value.type_of()
-            ),
-        };
-
-        Ok(value)
+        }
     }
 
     // TODO(amjoseph): de-asyncify this (when called directly by the VM)