//! 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 std::fmt::Display;
use std::future::Future;
use crate::value::{PointerEquality, SharedThunkSet};
use crate::warnings::{EvalWarning, WarningKind};
use crate::FileType;
use crate::NixString;
use super::*;
// -- 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 VMRequest {
/// 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
/// `Empty`.
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 (already containing a span) through the VM.
EmitWarning(EvalWarning),
/// Emit a runtime warning through the VM. The span of the current generator
/// is used for the final warning.
EmitWarningKind(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),
/// Request serialisation of a value to JSON, according to the
/// slightly odd Nix evaluation rules.
ToJson(Value),
}
/// Human-readable representation of a generator message, used by observers.
impl Display for VMRequest {
fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
match self {
VMRequest::ForceValue(v) => write!(f, "force_value({})", v.type_of()),
VMRequest::DeepForceValue(v, _) => {
write!(f, "deep_force_value({})", v.type_of())
}
VMRequest::WithValue(_) => write!(f, "with_value"),
VMRequest::CapturedWithValue(_) => write!(f, "captured_with_value"),
VMRequest::NixEquality(values, ptr_eq) => {
write!(
f,
"nix_eq({}, {}, PointerEquality::{:?})",
values.0.type_of(),
values.1.type_of(),
ptr_eq
)
}
VMRequest::StackPush(v) => write!(f, "stack_push({})", v.type_of()),
VMRequest::StackPop => write!(f, "stack_pop"),
VMRequest::StringCoerce(v, kind) => match kind {
CoercionKind::Weak => write!(f, "weak_string_coerce({})", v.type_of()),
CoercionKind::Strong => write!(f, "strong_string_coerce({})", v.type_of()),
},
VMRequest::Call(v) => write!(f, "call({})", v),
VMRequest::EnterLambda { lambda, .. } => {
write!(f, "enter_lambda({:p})", *lambda)
}
VMRequest::EmitWarning(_) => write!(f, "emit_warning"),
VMRequest::EmitWarningKind(_) => write!(f, "emit_warning_kind"),
VMRequest::ImportCacheLookup(p) => {
write!(f, "import_cache_lookup({})", p.to_string_lossy())
}
VMRequest::ImportCachePut(p, _) => {
write!(f, "import_cache_put({})", p.to_string_lossy())
}
VMRequest::PathImport(p) => write!(f, "path_import({})", p.to_string_lossy()),
VMRequest::ReadToString(p) => {
write!(f, "read_to_string({})", p.to_string_lossy())
}
VMRequest::PathExists(p) => write!(f, "path_exists({})", p.to_string_lossy()),
VMRequest::ReadDir(p) => write!(f, "read_dir({})", p.to_string_lossy()),
VMRequest::Span => write!(f, "span"),
VMRequest::TryForce(v) => write!(f, "try_force({})", v.type_of()),
VMRequest::ToJson(v) => write!(f, "to_json({})", v.type_of()),
}
}
}
/// Responses returned to generators *from* the VM.
pub enum VMResponse {
/// 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<(Vec<u8>, 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 VMResponse {
fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
match self {
VMResponse::Empty => write!(f, "empty"),
VMResponse::Value(v) => write!(f, "value({})", v),
VMResponse::Path(p) => write!(f, "path({})", p.to_string_lossy()),
VMResponse::Directory(d) => write!(f, "dir(len = {})", d.len()),
VMResponse::Span(_) => write!(f, "span"),
VMResponse::ForceError => write!(f, "force_error"),
}
}
}
pub(crate) type Generator =
Gen<VMRequest, VMResponse, 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)
}
impl<'o> VM<'o> {
/// Helper function to re-enqueue the current generator while it
/// is awaiting a value.
fn reenqueue_generator(&mut self, name: &'static str, span: LightSpan, generator: Generator) {
self.frames.push(Frame::Generator {
name,
generator,
span,
state: GeneratorState::AwaitingValue,
});
}
/// Helper function to enqueue a new generator.
pub(super) fn enqueue_generator<F, G>(&mut self, name: &'static str, span: LightSpan, gen: G)
where
F: Future<Output = Result<Value, ErrorKind>> + 'static,
G: FnOnce(GenCo) -> F,
{
self.frames.push(Frame::Generator {
name,
span,
state: GeneratorState::Running,
generator: Gen::new(|co| pin_generator(gen(co))),
});
}
/// Run a generator frame until it yields to the outer control loop, or runs
/// to completion.
///
/// The return value indicates whether the generator has completed (true),
/// or was suspended (false).
pub(crate) fn run_generator(
&mut self,
name: &'static str,
span: LightSpan,
frame_id: usize,
state: GeneratorState,
mut generator: Generator,
initial_message: Option<VMResponse>,
) -> EvalResult<bool> {
// Determine what to send to the generator based on its state.
let mut message = match (initial_message, state) {
(Some(msg), _) => msg,
(_, GeneratorState::Running) => VMResponse::Empty,
// If control returned here, and the generator is
// awaiting a value, send it the top of the stack.
(_, GeneratorState::AwaitingValue) => VMResponse::Value(self.stack_pop()),
};
loop {
match generator.resume_with(message) {
// If the generator yields, it contains an instruction
// for what the VM should do.
genawaiter::GeneratorState::Yielded(request) => {
self.observer.observe_generator_request(name, &request);
match request {
VMRequest::StackPush(value) => {
self.stack.push(value);
message = VMResponse::Empty;
}
VMRequest::StackPop => {
message = VMResponse::Value(self.stack_pop());
}
// Generator has requested a force, which means that
// this function prepares the frame stack and yields
// back to the outer VM loop.
VMRequest::ForceValue(value) => {
self.reenqueue_generator(name, span.clone(), generator);
self.enqueue_generator("force", span.clone(), |co| {
value.force(co, span)
});
return Ok(false);
}
// Generator has requested a deep-force.
VMRequest::DeepForceValue(value, thunk_set) => {
self.reenqueue_generator(name, span.clone(), generator);
self.enqueue_generator("deep_force", span, |co| {
value.deep_force(co, thunk_set)
});
return Ok(false);
}
// Generator has requested a value from the with-stack.
// Logic is similar to `ForceValue`, except with the
// value being taken from that stack.
VMRequest::WithValue(idx) => {
self.reenqueue_generator(name, span.clone(), generator);
let value = self.stack[self.with_stack[idx]].clone();
self.enqueue_generator("force", span.clone(), |co| {
value.force(co, span)
});
return Ok(false);
}
// Generator has requested a value from the *captured*
// with-stack. Logic is same as above, except for the
// value being from that stack.
VMRequest::CapturedWithValue(idx) => {
self.reenqueue_generator(name, span.clone(), generator);
let call_frame = self.last_call_frame()
.expect("Tvix bug: generator requested captured with-value, but there is no call frame");
let value = call_frame.upvalues.with_stack().unwrap()[idx].clone();
self.enqueue_generator("force", span.clone(), |co| {
value.force(co, span)
});
return Ok(false);
}
VMRequest::NixEquality(values, ptr_eq) => {
let values = *values;
self.reenqueue_generator(name, span.clone(), generator);
self.enqueue_generator("nix_eq", span, |co| {
values.0.nix_eq(values.1, co, ptr_eq)
});
return Ok(false);
}
VMRequest::StringCoerce(val, kind) => {
self.reenqueue_generator(name, span.clone(), generator);
self.enqueue_generator("coerce_to_string", span, |co| {
val.coerce_to_string(co, kind)
});
return Ok(false);
}
VMRequest::Call(callable) => {
self.reenqueue_generator(name, span.clone(), generator);
self.call_value(span, None, callable)?;
return Ok(false);
}
VMRequest::EnterLambda {
lambda,
upvalues,
light_span,
} => {
self.reenqueue_generator(name, span, generator);
self.frames.push(Frame::CallFrame {
span: light_span,
call_frame: CallFrame {
lambda,
upvalues,
ip: CodeIdx(0),
stack_offset: self.stack.len(),
},
});
return Ok(false);
}
VMRequest::EmitWarning(warning) => {
self.push_warning(warning);
message = VMResponse::Empty;
}
VMRequest::EmitWarningKind(kind) => {
self.emit_warning(kind);
message = VMResponse::Empty;
}
VMRequest::ImportCacheLookup(path) => {
if let Some(cached) = self.import_cache.get(path) {
message = VMResponse::Value(cached.clone());
} else {
message = VMResponse::Empty;
}
}
VMRequest::ImportCachePut(path, value) => {
self.import_cache.insert(path, value);
message = VMResponse::Empty;
}
VMRequest::PathImport(path) => {
let imported = self
.io_handle
.import_path(&path)
.map_err(|e| ErrorKind::IO {
path: Some(path),
error: e.into(),
})
.with_span(&span, self)?;
message = VMResponse::Path(imported);
}
VMRequest::ReadToString(path) => {
let content = self
.io_handle
.read_to_string(&path)
.map_err(|e| ErrorKind::IO {
path: Some(path),
error: e.into(),
})
.with_span(&span, self)?;
message = VMResponse::Value(Value::String(content.into()))
}
VMRequest::PathExists(path) => {
let exists = self
.io_handle
.path_exists(&path)
.map_err(|e| ErrorKind::IO {
path: Some(path),
error: e.into(),
})
.map(Value::Bool)
.with_span(&span, self)?;
message = VMResponse::Value(exists);
}
VMRequest::ReadDir(path) => {
let dir = self
.io_handle
.read_dir(&path)
.map_err(|e| ErrorKind::IO {
path: Some(path),
error: e.into(),
})
.with_span(&span, self)?;
message = VMResponse::Directory(dir);
}
VMRequest::Span => {
message = VMResponse::Span(self.reasonable_light_span());
}
VMRequest::TryForce(value) => {
self.try_eval_frames.push(frame_id);
self.reenqueue_generator(name, span.clone(), generator);
debug_assert!(
self.frames.len() == frame_id + 1,
"generator should be reenqueued with the same frame ID"
);
self.enqueue_generator("force", span.clone(), |co| {
value.force(co, span)
});
return Ok(false);
}
VMRequest::ToJson(value) => {
self.reenqueue_generator(name, span.clone(), generator);
self.enqueue_generator("to_json", span, |co| {
value.to_json_generator(co)
});
return Ok(false);
}
}
}
// Generator has completed, and its result value should
// be left on the stack.
genawaiter::GeneratorState::Complete(result) => {
let value = result.with_span(&span, self)?;
self.stack.push(value);
return Ok(true);
}
}
}
}
}
pub type GenCo = Co<VMRequest, VMResponse>;
// -- 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_(VMRequest::StackPush(val)).await {
VMResponse::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_(VMRequest::StackPop).await {
VMResponse::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_(VMRequest::ForceValue(val)).await {
VMResponse::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_(VMRequest::TryForce(val)).await {
VMResponse::Value(value) => Some(value),
VMResponse::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_(VMRequest::Call(val)).await {
VMResponse::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_(VMRequest::StringCoerce(val, kind)).await {
VMResponse::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_(VMRequest::DeepForceValue(val, thunk_set)).await {
VMResponse::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_(VMRequest::NixEquality(Box::new((a, b)), ptr_eq))
.await
{
VMResponse::Value(value) => value.as_bool(),
msg => panic!(
"Tvix bug: VM responded with incorrect generator message: {}",
msg
),
}
}
/// Emit a fully constructed runtime warning.
pub(crate) async fn emit_warning(co: &GenCo, warning: EvalWarning) {
match co.yield_(VMRequest::EmitWarning(warning)).await {
VMResponse::Empty => {}
msg => panic!(
"Tvix bug: VM responded with incorrect generator message: {}",
msg
),
}
}
/// Emit a runtime warning with the span of the current generator.
pub async fn emit_warning_kind(co: &GenCo, kind: WarningKind) {
match co.yield_(VMRequest::EmitWarningKind(kind)).await {
VMResponse::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 = VMRequest::EnterLambda {
lambda,
upvalues,
light_span,
};
match co.yield_(msg).await {
VMResponse::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_(VMRequest::ImportCacheLookup(path)).await {
VMResponse::Value(value) => Some(value),
VMResponse::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_(VMRequest::ImportCachePut(path, value)).await {
VMResponse::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_(VMRequest::PathImport(path)).await {
VMResponse::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_(VMRequest::ReadToString(path)).await {
VMResponse::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_(VMRequest::PathExists(path)).await {
VMResponse::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<(Vec<u8>, FileType)> {
match co.yield_(VMRequest::ReadDir(path)).await {
VMResponse::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_(VMRequest::Span).await {
VMResponse::Span(span) => span,
msg => panic!(
"Tvix bug: VM responded with incorrect generator message: {}",
msg
),
}
}
pub(crate) async fn request_to_json(co: &GenCo, value: Value) -> serde_json::Value {
match co.yield_(VMRequest::ToJson(value)).await {
VMResponse::Value(Value::Json(json)) => json,
msg => panic!(
"Tvix bug: VM responded with incorrect generator message: {}",
msg
),
}
}
/// 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)
}
}
}
