refactor(tvix/eval): flatten call stack of VM using generators r/5964

Warning: This is probably the biggest refactor in tvix-eval history,
so far.

This replaces all instances of trampolines and recursion during
evaluation of the VM loop with generators. A generator is an
asynchronous function that can be suspended to yield a message (in our
case, vm::generators::GeneratorRequest) and receive a
response (vm::generators::GeneratorResponsee).

The `genawaiter` crate provides an interpreter for generators that can
drive their execution and lets us move control flow between the VM and
suspended generators.

To do this, massive changes have occured basically everywhere in the
code. On a high-level:

1. The VM is now organised around a frame stack. A frame is either a
   call frame (execution of Tvix bytecode) or a generator frame (a
   running or suspended generator).

   The VM has an outer loop that pops a frame off the frame stack, and
   then enters an inner loop either driving the execution of the
   bytecode or the execution of a generator.

   Both types of frames have several branches that can result in the
   frame re-enqueuing itself, and enqueuing some other work (in the
   form of a different frame) on top of itself. The VM will eventually
   resume the frame when everything "above" it has been suspended.

   In this way, the VM's new frame stack takes over much of the work
   that was previously achieved by recursion.

2. All methods previously taking a VM have been refactored into async
   functions that instead emit/receive generator messages for
   communication with the VM.

   Notably, this includes *all* builtins.

This has had some other effects:

- Some test have been removed or commented out, either because they
  tested code that was mostly already dead (nix_eq) or because they
  now require generator scaffolding which we do not have in place for
  tests (yet).

- Because generator functions are technically async (though no async
  IO is involved), we lose the ability to use much of the Rust
  standard library e.g. in builtins. This has led to many algorithms
  being unrolled into iterative versions instead of iterator
  combinations, and things like sorting had to be implemented from scratch.

- Many call sites that previously saw a `Result<..., ErrorKind>`
  bubble up now only see the result value, as the error handling is
  encapsulated within the generator loop.

  This reduces number of places inside of builtin implementations
  where error context can be attached to calls that can fail.
  Currently what we gain in this tradeoff is significantly more
  detailed span information (which we still need to bubble up, this
  commit does not change the error display).

  We'll need to do some analysis later of how useful the errors turn
  out to be and potentially introduce some methods for attaching
  context to a generator frame again.

This change is very difficult to do in stages, as it is very much an
"all or nothing" change that affects huge parts of the codebase. I've
tried to isolate changes that can be isolated into the parent CLs of
this one, but this change is still quite difficult to wrap one's mind
and I'm available to discuss it and explain things to any reviewer.

Fixes: b/238, b/237, b/251 and potentially others.
Change-Id: I39244163ff5bbecd169fe7b274df19262b515699
Reviewed-on: https://cl.tvl.fyi/c/depot/+/8104
Reviewed-by: raitobezarius <tvl@lahfa.xyz>
Reviewed-by: Adam Joseph <adam@westernsemico.com>
Tested-by: BuildkiteCI

2 files changed, 95 insertions, 76 deletions

diff --git a/tvix/eval/src/compiler/import.rs b/tvix/eval/src/compiler/import.rs
index 3a8847f2cbb4..467fc256afc6 100644
--- a/tvix/eval/src/compiler/import.rs
+++ b/tvix/eval/src/compiler/import.rs
@@ -5,21 +5,98 @@
 //! compiler and VM state (such as the [`crate::SourceCode`]
 //! instance, or observers).
 
+use super::GlobalsMap;
+use genawaiter::rc::Gen;
 use std::rc::Weak;
 
 use crate::{
+    builtins::coerce_value_to_path,
+    generators::pin_generator,
     observer::NoOpObserver,
-    value::{Builtin, BuiltinArgument, Thunk},
-    vm::VM,
+    value::{Builtin, Thunk},
+    vm::generators::{self, GenCo},
     ErrorKind, SourceCode, Value,
 };
 
-use super::GlobalsMap;
-use crate::builtins::coerce_value_to_path;
+async fn import_impl(
+    co: GenCo,
+    globals: Weak<GlobalsMap>,
+    source: SourceCode,
+    mut args: Vec<Value>,
+) -> Result<Value, ErrorKind> {
+    let mut path = coerce_value_to_path(&co, args.pop().unwrap()).await?;
+
+    if path.is_dir() {
+        path.push("default.nix");
+    }
+
+    if let Some(cached) = generators::request_import_cache_lookup(&co, path.clone()).await {
+        return Ok(cached);
+    }
+
+    // TODO(tazjin): make this return a string directly instead
+    let contents = generators::request_read_to_string(&co, path.clone())
+        .await
+        .to_str()?
+        .as_str()
+        .to_string();
+
+    let parsed = rnix::ast::Root::parse(&contents);
+    let errors = parsed.errors();
+    let file = source.add_file(path.to_string_lossy().to_string(), contents);
+
+    if !errors.is_empty() {
+        return Err(ErrorKind::ImportParseError {
+            path,
+            file,
+            errors: errors.to_vec(),
+        });
+    }
+
+    let result = crate::compiler::compile(
+        &parsed.tree().expr().unwrap(),
+        Some(path.clone()),
+        file,
+        // The VM must ensure that a strong reference to the globals outlives
+        // any self-references (which are weak) embedded within the globals. If
+        // the expect() below panics, it means that did not happen.
+        globals
+            .upgrade()
+            .expect("globals dropped while still in use"),
+        &mut NoOpObserver::default(),
+    )
+    .map_err(|err| ErrorKind::ImportCompilerError {
+        path: path.clone(),
+        errors: vec![err],
+    })?;
+
+    if !result.errors.is_empty() {
+        return Err(ErrorKind::ImportCompilerError {
+            path,
+            errors: result.errors,
+        });
+    }
+
+    // TODO: emit not just the warning kind, hmm
+    // for warning in result.warnings {
+    //     vm.push_warning(warning);
+    // }
+
+    // Compilation succeeded, we can construct a thunk from whatever it spat
+    // out and return that.
+    let res = Value::Thunk(Thunk::new_suspended(
+        result.lambda,
+        generators::request_span(&co).await,
+    ));
+
+    generators::request_import_cache_put(&co, path, res.clone()).await;
+
+    Ok(res)
+}
 
-/// Constructs and inserts the `import` builtin. This builtin is special in that
-/// it needs to capture the [crate::SourceCode] structure to correctly track
-/// source code locations while invoking a compiler.
+/// Constructs the `import` builtin. This builtin is special in that
+/// it needs to capture the [crate::SourceCode] structure to correctly
+/// track source code locations while invoking a compiler.
 // TODO: need to be able to pass through a CompilationObserver, too.
 // TODO: can the `SourceCode` come from the compiler?
 pub(super) fn builtins_import(globals: &Weak<GlobalsMap>, source: SourceCode) -> Builtin {
@@ -31,75 +108,10 @@ pub(super) fn builtins_import(globals: &Weak<GlobalsMap>, source: SourceCode) ->
 
     Builtin::new(
         "import",
-        &[BuiltinArgument {
-            strict: true,
-            name: "path",
-        }],
-        None,
-        move |mut args: Vec<Value>, vm: &mut VM| {
-            let mut path = coerce_value_to_path(&args.pop().unwrap(), vm)?;
-            if path.is_dir() {
-                path.push("default.nix");
-            }
-
-            let current_span = vm.current_light_span();
-
-            if let Some(cached) = vm.import_cache.get(&path) {
-                return Ok(cached.clone());
-            }
-
-            let contents = vm.io().read_to_string(path.clone())?;
-
-            let parsed = rnix::ast::Root::parse(&contents);
-            let errors = parsed.errors();
-
-            let file = source.add_file(path.to_string_lossy().to_string(), contents);
-
-            if !errors.is_empty() {
-                return Err(ErrorKind::ImportParseError {
-                    path,
-                    file,
-                    errors: errors.to_vec(),
-                });
-            }
-
-            let result = crate::compiler::compile(
-                &parsed.tree().expr().unwrap(),
-                Some(path.clone()),
-                file,
-                // The VM must ensure that a strong reference to the
-                // globals outlives any self-references (which are
-                // weak) embedded within the globals.  If the
-                // expect() below panics, it means that did not
-                // happen.
-                globals
-                    .upgrade()
-                    .expect("globals dropped while still in use"),
-                &mut NoOpObserver::default(),
-            )
-            .map_err(|err| ErrorKind::ImportCompilerError {
-                path: path.clone(),
-                errors: vec![err],
-            })?;
-
-            if !result.errors.is_empty() {
-                return Err(ErrorKind::ImportCompilerError {
-                    path,
-                    errors: result.errors,
-                });
-            }
-
-            // Compilation succeeded, we can construct a thunk from whatever it spat
-            // out and return that.
-            let res = Value::Thunk(Thunk::new_suspended(result.lambda, current_span));
-
-            vm.import_cache.insert(path, res.clone());
-
-            for warning in result.warnings {
-                vm.push_warning(warning);
-            }
-
-            Ok(res)
+        Some("Import the given file and return the Nix value it evaluates to"),
+        1,
+        move |args| {
+            Gen::new(|co| pin_generator(import_impl(co, globals.clone(), source.clone(), args)))
         },
     )
 }
diff --git a/tvix/eval/src/compiler/mod.rs b/tvix/eval/src/compiler/mod.rs
index 69c232926b41..80e1cd27c953 100644
--- a/tvix/eval/src/compiler/mod.rs
+++ b/tvix/eval/src/compiler/mod.rs
@@ -1021,6 +1021,12 @@ impl Compiler<'_> {
         // lambda as a constant.
         let mut compiled = self.contexts.pop().unwrap();
 
+        // Emit an instruction to inform the VM that the chunk has ended.
+        compiled
+            .lambda
+            .chunk
+            .push_op(OpCode::OpReturn, self.span_for(node));
+
         // Capturing the with stack counts as an upvalue, as it is
         // emitted as an upvalue data instruction.
         if compiled.captures_with_stack {
@@ -1433,6 +1439,7 @@ pub fn compile(
     // unevaluated state (though in practice, a value *containing* a
     // thunk might be returned).
     c.emit_force(expr);
+    c.push_op(OpCode::OpReturn, &root_span);
 
     let lambda = Rc::new(c.contexts.pop().unwrap().lambda);
     c.observer.observe_compiled_toplevel(&lambda);