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-rw-r--r--tvix/eval/src/value/mod.rs2
-rw-r--r--tvix/eval/src/vm.rs2
-rw-r--r--tvix/eval/src/vm/generators.rs538
3 files changed, 541 insertions, 1 deletions
diff --git a/tvix/eval/src/value/mod.rs b/tvix/eval/src/value/mod.rs
index ede93e7dddd2..07ce83020613 100644
--- a/tvix/eval/src/value/mod.rs
+++ b/tvix/eval/src/value/mod.rs
@@ -32,7 +32,7 @@ pub use path::canon_path;
 pub use string::NixString;
 pub use thunk::Thunk;
 
-use self::thunk::ThunkSet;
+pub use self::thunk::{SharedThunkSet, ThunkSet};
 
 use lazy_static::lazy_static;
 
diff --git a/tvix/eval/src/vm.rs b/tvix/eval/src/vm.rs
index 10966aca811b..3e3052555639 100644
--- a/tvix/eval/src/vm.rs
+++ b/tvix/eval/src/vm.rs
@@ -1,6 +1,8 @@
 //! This module implements the virtual (or abstract) machine that runs
 //! Tvix bytecode.
 
+pub mod generators;
+
 use serde_json::json;
 use std::{cmp::Ordering, collections::HashMap, ops::DerefMut, path::PathBuf, rc::Rc};
 
diff --git a/tvix/eval/src/vm/generators.rs b/tvix/eval/src/vm/generators.rs
new file mode 100644
index 000000000000..3b822b086346
--- /dev/null
+++ b/tvix/eval/src/vm/generators.rs
@@ -0,0 +1,538 @@
+//! This module implements generator logic for the VM. Generators are functions
+//! used during evaluation which can suspend their execution during their
+//! control flow, and request that the VM do something.
+//!
+//! This is used to keep the VM's stack size constant even when evaluating
+//! deeply nested recursive data structures.
+//!
+//! We implement generators using the [`genawaiter`] crate.
+
+use core::pin::Pin;
+use genawaiter::rc::Co;
+pub use genawaiter::rc::Gen;
+use smol_str::SmolStr;
+use std::fmt::Display;
+use std::future::Future;
+
+use crate::value::SharedThunkSet;
+use crate::warnings::WarningKind;
+use crate::FileType;
+use crate::NixString;
+
+use super::*;
+
+/// Dummy type, before the actual implementation is in place.
+#[derive(Debug)]
+pub struct PointerEquality {}
+
+// -- Implementation of generic generator logic.
+
+/// States that a generator can be in while being driven by the VM.
+pub(crate) enum GeneratorState {
+    /// Normal execution of the generator.
+    Running,
+
+    /// Generator is awaiting the result of a forced value.
+    AwaitingValue,
+}
+
+/// Messages that can be sent from generators to the VM. In most
+/// cases, the VM will suspend the generator when receiving a message
+/// and enter some other frame to process the request.
+///
+/// Responses are returned to generators via the [`GeneratorResponse`] type.
+pub enum GeneratorRequest {
+    /// Request that the VM forces this value. This message is first sent to the
+    /// VM with the unforced value, then returned to the generator with the
+    /// forced result.
+    ForceValue(Value),
+
+    /// Request that the VM deep-forces the value.
+    DeepForceValue(Value, SharedThunkSet),
+
+    /// Request the value at the given index from the VM's with-stack, in forced
+    /// state.
+    ///
+    /// The value is returned in the `ForceValue` message.
+    WithValue(usize),
+
+    /// Request the value at the given index from the *captured* with-stack, in
+    /// forced state.
+    CapturedWithValue(usize),
+
+    /// Request that the two values be compared for Nix equality. The result is
+    /// returned in the `ForceValue` message.
+    NixEquality(Box<(Value, Value)>, PointerEquality),
+
+    /// Push the given value to the VM's stack. This is used to prepare the
+    /// stack for requesting a function call from the VM.
+    ///
+    /// The VM does not respond to this request, so the next message received is
+    /// a `NoOp`.
+    StackPush(Value),
+
+    /// Pop a value from the stack and return it to the generator.
+    StackPop,
+
+    /// Request that the VM coerces this value to a string.
+    StringCoerce(Value, CoercionKind),
+
+    /// Request that the VM calls the given value, with arguments already
+    /// prepared on the stack. Value must already be forced.
+    Call(Value),
+
+    /// Request a call frame entering the given lambda immediately. This can be
+    /// used to force thunks.
+    EnterLambda {
+        lambda: Rc<Lambda>,
+        upvalues: Rc<Upvalues>,
+        light_span: LightSpan,
+    },
+
+    /// Emit a runtime warning through the VM. Receives a NoOp-response.
+    EmitWarning(WarningKind),
+
+    /// Request a lookup in the VM's import cache, which tracks the
+    /// thunks yielded by previously imported files.
+    ImportCacheLookup(PathBuf),
+
+    /// Provide the VM with an imported value for a given path, which
+    /// it can populate its input cache with.
+    ImportCachePut(PathBuf, Value),
+
+    /// Request that the VM imports the given path through its I/O interface.
+    PathImport(PathBuf),
+
+    /// Request that the VM reads the given path to a string.
+    ReadToString(PathBuf),
+
+    /// Request that the VM checks whether the given path exists.
+    PathExists(PathBuf),
+
+    /// Request that the VM reads the given path.
+    ReadDir(PathBuf),
+
+    /// Request a reasonable span from the VM.
+    Span,
+
+    /// Request evaluation of `builtins.tryEval` from the VM. See
+    /// [`VM::catch_result`] for an explanation of how this works.
+    TryForce(Value),
+}
+
+/// Human-readable representation of a generator message, used by observers.
+impl Display for GeneratorRequest {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        match self {
+            GeneratorRequest::ForceValue(v) => write!(f, "force_value({})", v),
+            GeneratorRequest::DeepForceValue(v, _) => write!(f, "deep_force_value({})", v),
+            GeneratorRequest::WithValue(_) => write!(f, "with_value"),
+            GeneratorRequest::CapturedWithValue(_) => write!(f, "captured_with_value"),
+            GeneratorRequest::NixEquality(values, ptr_eq) => {
+                write!(
+                    f,
+                    "nix_eq({}, {}, PointerEquality::{:?})",
+                    values.0, values.1, ptr_eq
+                )
+            }
+            GeneratorRequest::StackPush(v) => write!(f, "stack_push({})", v),
+            GeneratorRequest::StackPop => write!(f, "stack_pop"),
+            GeneratorRequest::StringCoerce(v, kind) => match kind {
+                CoercionKind::Weak => write!(f, "weak_string_coerce({})", v),
+                CoercionKind::Strong => write!(f, "strong_string_coerce({})", v),
+                CoercionKind::ThunksOnly => todo!("remove this branch (not live)"),
+            },
+            GeneratorRequest::Call(v) => write!(f, "call({})", v),
+            GeneratorRequest::EnterLambda { lambda, .. } => {
+                write!(f, "enter_lambda({:p})", *lambda)
+            }
+            GeneratorRequest::EmitWarning(_) => write!(f, "emit_warning"),
+            GeneratorRequest::ImportCacheLookup(p) => {
+                write!(f, "import_cache_lookup({})", p.to_string_lossy())
+            }
+            GeneratorRequest::ImportCachePut(p, _) => {
+                write!(f, "import_cache_put({})", p.to_string_lossy())
+            }
+            GeneratorRequest::PathImport(p) => write!(f, "path_import({})", p.to_string_lossy()),
+            GeneratorRequest::ReadToString(p) => {
+                write!(f, "read_to_string({})", p.to_string_lossy())
+            }
+            GeneratorRequest::PathExists(p) => write!(f, "path_exists({})", p.to_string_lossy()),
+            GeneratorRequest::ReadDir(p) => write!(f, "read_dir({})", p.to_string_lossy()),
+            GeneratorRequest::Span => write!(f, "span"),
+            GeneratorRequest::TryForce(v) => write!(f, "try_force({})", v),
+        }
+    }
+}
+
+/// Responses returned to generators from the VM.
+pub enum GeneratorResponse {
+    /// Empty message. Passed to the generator as the first message,
+    /// or when return values were optional.
+    Empty,
+
+    /// Value produced by the VM and returned to the generator.
+    Value(Value),
+
+    /// Path produced by the VM in response to some IO operation.
+    Path(PathBuf),
+
+    /// VM response with the contents of a directory.
+    Directory(Vec<(SmolStr, FileType)>),
+
+    /// VM response with a span to use at the current point.
+    Span(LightSpan),
+
+    /// Message returned by the VM when a catchable error is encountered during
+    /// the evaluation of `builtins.tryEval`.
+    ForceError,
+}
+
+impl Display for GeneratorResponse {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        match self {
+            GeneratorResponse::Empty => write!(f, "empty"),
+            GeneratorResponse::Value(v) => write!(f, "value({})", v),
+            GeneratorResponse::Path(p) => write!(f, "path({})", p.to_string_lossy()),
+            GeneratorResponse::Directory(d) => write!(f, "dir(len = {})", d.len()),
+            GeneratorResponse::Span(_) => write!(f, "span"),
+            GeneratorResponse::ForceError => write!(f, "force_error"),
+        }
+    }
+}
+
+pub(crate) type Generator = Gen<
+    GeneratorRequest,
+    GeneratorResponse,
+    Pin<Box<dyn Future<Output = Result<Value, ErrorKind>>>>,
+>;
+
+/// Helper function to provide type annotations which are otherwise difficult to
+/// infer.
+pub fn pin_generator(
+    f: impl Future<Output = Result<Value, ErrorKind>> + 'static,
+) -> Pin<Box<dyn Future<Output = Result<Value, ErrorKind>>>> {
+    Box::pin(f)
+}
+
+pub type GenCo = Co<GeneratorRequest, GeneratorResponse>;
+
+// -- Implementation of concrete generator use-cases.
+
+/// Request that the VM place the given value on its stack.
+pub async fn request_stack_push(co: &GenCo, val: Value) {
+    match co.yield_(GeneratorRequest::StackPush(val)).await {
+        GeneratorResponse::Empty => {}
+        msg => panic!(
+            "Tvix bug: VM responded with incorrect generator message: {}",
+            msg
+        ),
+    }
+}
+
+/// Request that the VM pop a value from the stack and return it to the
+/// generator.
+pub async fn request_stack_pop(co: &GenCo) -> Value {
+    match co.yield_(GeneratorRequest::StackPop).await {
+        GeneratorResponse::Value(value) => value,
+        msg => panic!(
+            "Tvix bug: VM responded with incorrect generator message: {}",
+            msg
+        ),
+    }
+}
+
+/// Force any value and return the evaluated result from the VM.
+pub async fn request_force(co: &GenCo, val: Value) -> Value {
+    if let Value::Thunk(_) = val {
+        match co.yield_(GeneratorRequest::ForceValue(val)).await {
+            GeneratorResponse::Value(value) => value,
+            msg => panic!(
+                "Tvix bug: VM responded with incorrect generator message: {}",
+                msg
+            ),
+        }
+    } else {
+        val
+    }
+}
+
+/// Force a value, but inform the caller (by returning `None`) if a catchable
+/// error occured.
+pub(crate) async fn request_try_force(co: &GenCo, val: Value) -> Option<Value> {
+    if let Value::Thunk(_) = val {
+        match co.yield_(GeneratorRequest::TryForce(val)).await {
+            GeneratorResponse::Value(value) => Some(value),
+            GeneratorResponse::ForceError => None,
+            msg => panic!(
+                "Tvix bug: VM responded with incorrect generator message: {}",
+                msg
+            ),
+        }
+    } else {
+        Some(val)
+    }
+}
+
+/// Call the given value as a callable. The argument(s) must already be prepared
+/// on the stack.
+pub async fn request_call(co: &GenCo, val: Value) -> Value {
+    let val = request_force(co, val).await;
+    match co.yield_(GeneratorRequest::Call(val)).await {
+        GeneratorResponse::Value(value) => value,
+        msg => panic!(
+            "Tvix bug: VM responded with incorrect generator message: {}",
+            msg
+        ),
+    }
+}
+
+/// Helper function to call the given value with the provided list of arguments.
+/// This uses the StackPush and Call messages under the hood.
+pub async fn request_call_with<I>(co: &GenCo, mut callable: Value, args: I) -> Value
+where
+    I: IntoIterator<Item = Value>,
+    I::IntoIter: DoubleEndedIterator,
+{
+    let mut num_args = 0_usize;
+    for arg in args.into_iter().rev() {
+        num_args += 1;
+        request_stack_push(co, arg).await;
+    }
+
+    debug_assert!(num_args > 0, "call_with called with an empty list of args");
+
+    while num_args > 0 {
+        callable = request_call(co, callable).await;
+        num_args -= 1;
+    }
+
+    callable
+}
+
+pub async fn request_string_coerce(co: &GenCo, val: Value, kind: CoercionKind) -> NixString {
+    match val {
+        Value::String(s) => s,
+        _ => match co.yield_(GeneratorRequest::StringCoerce(val, kind)).await {
+            GeneratorResponse::Value(value) => value
+                .to_str()
+                .expect("coerce_to_string always returns a string"),
+            msg => panic!(
+                "Tvix bug: VM responded with incorrect generator message: {}",
+                msg
+            ),
+        },
+    }
+}
+
+/// Deep-force any value and return the evaluated result from the VM.
+pub async fn request_deep_force(co: &GenCo, val: Value, thunk_set: SharedThunkSet) -> Value {
+    match co
+        .yield_(GeneratorRequest::DeepForceValue(val, thunk_set))
+        .await
+    {
+        GeneratorResponse::Value(value) => value,
+        msg => panic!(
+            "Tvix bug: VM responded with incorrect generator message: {}",
+            msg
+        ),
+    }
+}
+
+/// Fetch and force a value on the with-stack from the VM.
+async fn fetch_forced_with(co: &GenCo, idx: usize) -> Value {
+    match co.yield_(GeneratorRequest::WithValue(idx)).await {
+        GeneratorResponse::Value(value) => value,
+        msg => panic!(
+            "Tvix bug: VM responded with incorrect generator message: {}",
+            msg
+        ),
+    }
+}
+
+/// Fetch and force a value on the *captured* with-stack from the VM.
+async fn fetch_captured_with(co: &GenCo, idx: usize) -> Value {
+    match co.yield_(GeneratorRequest::CapturedWithValue(idx)).await {
+        GeneratorResponse::Value(value) => value,
+        msg => panic!(
+            "Tvix bug: VM responded with incorrect generator message: {}",
+            msg
+        ),
+    }
+}
+
+/// Ask the VM to compare two values for equality.
+pub(crate) async fn check_equality(
+    co: &GenCo,
+    a: Value,
+    b: Value,
+    ptr_eq: PointerEquality,
+) -> Result<bool, ErrorKind> {
+    match co
+        .yield_(GeneratorRequest::NixEquality(Box::new((a, b)), ptr_eq))
+        .await
+    {
+        GeneratorResponse::Value(value) => value.as_bool(),
+        msg => panic!(
+            "Tvix bug: VM responded with incorrect generator message: {}",
+            msg
+        ),
+    }
+}
+
+/// Emit a runtime warning.
+pub(crate) async fn emit_warning(co: &GenCo, kind: WarningKind) {
+    match co.yield_(GeneratorRequest::EmitWarning(kind)).await {
+        GeneratorResponse::Empty => {}
+        msg => panic!(
+            "Tvix bug: VM responded with incorrect generator message: {}",
+            msg
+        ),
+    }
+}
+
+/// Request that the VM enter the given lambda.
+pub(crate) async fn request_enter_lambda(
+    co: &GenCo,
+    lambda: Rc<Lambda>,
+    upvalues: Rc<Upvalues>,
+    light_span: LightSpan,
+) -> Value {
+    let msg = GeneratorRequest::EnterLambda {
+        lambda,
+        upvalues,
+        light_span,
+    };
+
+    match co.yield_(msg).await {
+        GeneratorResponse::Value(value) => value,
+        msg => panic!(
+            "Tvix bug: VM responded with incorrect generator message: {}",
+            msg
+        ),
+    }
+}
+
+/// Request a lookup in the VM's import cache.
+pub(crate) async fn request_import_cache_lookup(co: &GenCo, path: PathBuf) -> Option<Value> {
+    match co.yield_(GeneratorRequest::ImportCacheLookup(path)).await {
+        GeneratorResponse::Value(value) => Some(value),
+        GeneratorResponse::Empty => None,
+        msg => panic!(
+            "Tvix bug: VM responded with incorrect generator message: {}",
+            msg
+        ),
+    }
+}
+
+/// Request that the VM populate its input cache for the given path.
+pub(crate) async fn request_import_cache_put(co: &GenCo, path: PathBuf, value: Value) {
+    match co
+        .yield_(GeneratorRequest::ImportCachePut(path, value))
+        .await
+    {
+        GeneratorResponse::Empty => {}
+        msg => panic!(
+            "Tvix bug: VM responded with incorrect generator message: {}",
+            msg
+        ),
+    }
+}
+
+/// Request that the VM import the given path.
+pub(crate) async fn request_path_import(co: &GenCo, path: PathBuf) -> PathBuf {
+    match co.yield_(GeneratorRequest::PathImport(path)).await {
+        GeneratorResponse::Path(path) => path,
+        msg => panic!(
+            "Tvix bug: VM responded with incorrect generator message: {}",
+            msg
+        ),
+    }
+}
+
+pub(crate) async fn request_read_to_string(co: &GenCo, path: PathBuf) -> Value {
+    match co.yield_(GeneratorRequest::ReadToString(path)).await {
+        GeneratorResponse::Value(value) => value,
+        msg => panic!(
+            "Tvix bug: VM responded with incorrect generator message: {}",
+            msg
+        ),
+    }
+}
+
+pub(crate) async fn request_path_exists(co: &GenCo, path: PathBuf) -> Value {
+    match co.yield_(GeneratorRequest::PathExists(path)).await {
+        GeneratorResponse::Value(value) => value,
+        msg => panic!(
+            "Tvix bug: VM responded with incorrect generator message: {}",
+            msg
+        ),
+    }
+}
+
+pub(crate) async fn request_read_dir(co: &GenCo, path: PathBuf) -> Vec<(SmolStr, FileType)> {
+    match co.yield_(GeneratorRequest::ReadDir(path)).await {
+        GeneratorResponse::Directory(dir) => dir,
+        msg => panic!(
+            "Tvix bug: VM responded with incorrect generator message: {}",
+            msg
+        ),
+    }
+}
+
+pub(crate) async fn request_span(co: &GenCo) -> LightSpan {
+    match co.yield_(GeneratorRequest::Span).await {
+        GeneratorResponse::Span(span) => span,
+        msg => panic!(
+            "Tvix bug: VM responded with incorrect generator message: {}",
+            msg
+        ),
+    }
+}
+
+pub(crate) async fn neo_resolve_with(
+    co: GenCo,
+    ident: String,
+    vm_with_len: usize,
+    upvalue_with_len: usize,
+) -> Result<Value, ErrorKind> {
+    for with_stack_idx in (0..vm_with_len).rev() {
+        // TODO(tazjin): is this branch still live with the current with-thunking?
+        let with = fetch_forced_with(&co, with_stack_idx).await;
+
+        match with.to_attrs()?.select(&ident) {
+            None => continue,
+            Some(val) => return Ok(val.clone()),
+        }
+    }
+
+    for upvalue_with_idx in (0..upvalue_with_len).rev() {
+        let with = fetch_captured_with(&co, upvalue_with_idx).await;
+
+        match with.to_attrs()?.select(&ident) {
+            None => continue,
+            Some(val) => return Ok(val.clone()),
+        }
+    }
+
+    Err(ErrorKind::UnknownDynamicVariable(ident))
+}
+
+/// Call the given value as if it was an attribute set containing a functor. The
+/// arguments must already be prepared on the stack when a generator frame from
+/// this function is invoked.
+///
+pub(crate) async fn call_functor(co: GenCo, value: Value) -> Result<Value, ErrorKind> {
+    let attrs = value.to_attrs()?;
+
+    match attrs.select("__functor") {
+        None => Err(ErrorKind::NotCallable("set without `__functor_` attribute")),
+        Some(functor) => {
+            // The functor receives the set itself as its first argument and
+            // needs to be called with it.
+            let functor = request_force(&co, functor.clone()).await;
+            let primed = request_call_with(&co, functor, [value]).await;
+            Ok(request_call(&co, primed).await)
+        }
+    }
+}