feat(tvix/eval): implement OpForce in VM r/4688

This operation forces the evaluation of a thunk.

There is some potential here for making an implementation that avoids
some copies, but the thunk machinery is tricky to get right so the
first priority is to make sure it is correct by keeping the
implementation simple.

Change-Id: Ib381455b02f42ded717faff63f55afed4c8fb7e3
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6352
Tested-by: BuildkiteCI
Reviewed-by: sterni <sternenseemann@systemli.org>

4 files changed, 46 insertions, 2 deletions

diff --git a/tvix/eval/src/errors.rs b/tvix/eval/src/errors.rs
index 49a051c16e..32d37d7815 100644
--- a/tvix/eval/src/errors.rs
+++ b/tvix/eval/src/errors.rs
@@ -38,6 +38,9 @@ pub enum ErrorKind {
     // Attempt to call something that is not callable.
     NotCallable,
 
+    // Infinite recursion encountered while forcing thunks.
+    InfiniteRecursion,
+
     ParseErrors(Vec<rnix::parser::ParseError>),
 
     AssertionFailed,
diff --git a/tvix/eval/src/opcode.rs b/tvix/eval/src/opcode.rs
index a8802e8c0a..18f0fce3cb 100644
--- a/tvix/eval/src/opcode.rs
+++ b/tvix/eval/src/opcode.rs
@@ -109,6 +109,7 @@ pub enum OpCode {
 
     // Thunks
     OpThunk(ConstantIdx),
+    OpForce,
 
     /// Finalise initialisation of the upvalues of the value in the
     /// given stack index after the scope is fully bound.
diff --git a/tvix/eval/src/value/thunk.rs b/tvix/eval/src/value/thunk.rs
index 9d52ded3c4..892868d0f6 100644
--- a/tvix/eval/src/value/thunk.rs
+++ b/tvix/eval/src/value/thunk.rs
@@ -23,7 +23,7 @@ use std::{
     rc::Rc,
 };
 
-use crate::{upvalues::UpvalueCarrier, Value};
+use crate::{errors::ErrorKind, upvalues::UpvalueCarrier, vm::VM, EvalResult, Value};
 
 use super::Lambda;
 
@@ -55,6 +55,36 @@ impl Thunk {
             lambda,
         })))
     }
+
+    pub fn force(&self, vm: &mut VM) -> EvalResult<Ref<'_, Value>> {
+        // Due to mutable borrowing rules, the following code can't
+        // easily use a match statement or something like that; it
+        // requires a bit of manual fiddling.
+        let mut thunk_mut = self.0.borrow_mut();
+
+        if let ThunkRepr::Blackhole = *thunk_mut {
+            return Err(ErrorKind::InfiniteRecursion.into());
+        }
+
+        if matches!(*thunk_mut, ThunkRepr::Suspended { .. }) {
+            if let ThunkRepr::Suspended { lambda, upvalues } =
+                std::mem::replace(&mut *thunk_mut, ThunkRepr::Blackhole)
+            {
+                vm.call(lambda, upvalues, 0);
+                *thunk_mut = ThunkRepr::Evaluated(vm.run()?);
+            }
+        }
+
+        drop(thunk_mut);
+
+        // Otherwise it's already ThunkRepr::Evaluated and we do not
+        // need another branch.
+
+        Ok(Ref::map(self.0.borrow(), |t| match t {
+            ThunkRepr::Evaluated(value) => value,
+            _ => unreachable!("already evaluated"),
+        }))
+    }
 }
 
 impl UpvalueCarrier for Thunk {
diff --git a/tvix/eval/src/vm.rs b/tvix/eval/src/vm.rs
index e32c0d7f6b..c55bc81777 100644
--- a/tvix/eval/src/vm.rs
+++ b/tvix/eval/src/vm.rs
@@ -136,7 +136,7 @@ impl VM {
         self.frames.push(frame);
     }
 
-    fn run(&mut self) -> EvalResult<Value> {
+    pub fn run(&mut self) -> EvalResult<Value> {
         #[cfg(feature = "disassembler")]
         let mut tracer = Tracer::new();
 
@@ -441,6 +441,16 @@ impl VM {
                     self.populate_upvalues(upvalue_count, upvalues)?;
                 }
 
+                OpCode::OpForce => {
+                    let mut value = self.pop();
+
+                    while let Value::Thunk(thunk) = value {
+                        value = thunk.force(self)?.clone();
+                    }
+
+                    self.push(value);
+                }
+
                 OpCode::OpFinalise(StackIdx(idx)) => {
                     match &self.stack[self.frame().stack_offset + idx] {
                         Value::Closure(closure) => closure