//! 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)
}
}
}
