fix(tvix): don't call function eagerly in genList, map & mapAttrs

mapAttrs, map and genList call Nix functions provided by the caller and
store the result of applying them in a Nix data structure that does not
force all of its contents when forced itself. This means that when such
a builtin application is forced, the Nix function calls performed by the
builtin should not be forced: They may be forced later, but it is also
possible that they will never be forced, e.g. in

    builtins.length (builtins.map (builtins.add 2) [ 1 2 3 ])

it is not necessary to compute a single application of builtins.add.

Since request_call_with immediately performs the function call
requested, Tvix would compute function applications unnecessarily before
this change. Because this was not followed by a request_force, the
impact of this was relatively low in Nix code (most functions return a
new thunk after being applied), but it was enough to cause a lot of
bogus builtins.trace applications when evaluating anything from
`lib.modules`. The newly added test includes many cases where Tvix
previously incorrectly applied a builtin, breaking a working expression.

To fix this we add a new helper to construct a Thunk performing a
function application at runtime from a function and argument given as
`Value`s. This mimics the compiler's compile_apply(), but does itself
not require a compiler, since the necessary Lambda can be constructed
independently.

I also looked into other builtins that call a Nix function to verify
that they don't exhibit such a problem:

- Many builtins immediately use the resulting value in a way that makes
  it necessary to compute all the function calls they do as soon as
  the outer builtin application is forced:

  * all
  * any
  * filter
  * groupBy
  * partition

- concatMap needs to (shallowly) force the returned list for
  concatenation.

- foldl' is strict in the application of `op` (I added a comment that
  makes this explicit).

- genericClosure needs to (shallowly) force the resulting list and some
  keys of the attribute sets inside.

Resolves b/272.

Change-Id: I1fa53f744bcffc035da84c1f97ed25d146830446
Reviewed-on: https://cl.tvl.fyi/c/depot/+/8651
Autosubmit: sterni <sternenseemann@systemli.org>
Tested-by: BuildkiteCI
Reviewed-by: tazjin <tazjin@tvl.su>
This commit is contained in:
sterni 2023-05-26 20:38:24 +02:00 committed by clbot
parent a4d0a80466
commit 3b33c19a9c
4 changed files with 88 additions and 5 deletions

View file

@ -17,7 +17,7 @@ use crate::vm::generators::{self, GenCo};
use crate::warnings::WarningKind; use crate::warnings::WarningKind;
use crate::{ use crate::{
errors::ErrorKind, errors::ErrorKind,
value::{CoercionKind, NixAttrs, NixList, NixString, SharedThunkSet, Value}, value::{CoercionKind, NixAttrs, NixList, NixString, SharedThunkSet, Thunk, Value},
}; };
use self::versions::{VersionPart, VersionPartsIter}; use self::versions::{VersionPart, VersionPartsIter};
@ -307,6 +307,9 @@ mod pure_builtins {
let mut nul = nul; let mut nul = nul;
let list = list.to_list()?; let list = list.to_list()?;
for val in list { for val in list {
// Every call of `op` is forced immediately, but `nul` is not, see
// https://github.com/NixOS/nix/blob/940e9eb8/src/libexpr/primops.cc#L3069-L3070C36
// and our tests for foldl'.
nul = generators::request_call_with(&co, op.clone(), [nul, val]).await; nul = generators::request_call_with(&co, op.clone(), [nul, val]).await;
nul = generators::request_force(&co, nul).await; nul = generators::request_force(&co, nul).await;
} }
@ -397,9 +400,15 @@ mod pure_builtins {
) -> Result<Value, ErrorKind> { ) -> Result<Value, ErrorKind> {
let mut out = imbl::Vector::<Value>::new(); let mut out = imbl::Vector::<Value>::new();
let len = length.as_int()?; let len = length.as_int()?;
// the best span we can get…
let span = generators::request_span(&co).await;
for i in 0..len { for i in 0..len {
let val = generators::request_call_with(&co, generator.clone(), [i.into()]).await; let val = Value::Thunk(Thunk::new_suspended_call(
generator.clone(),
i.into(),
span.clone(),
));
out.push_back(val); out.push_back(val);
} }
@ -551,8 +560,11 @@ mod pure_builtins {
async fn builtin_map(co: GenCo, f: Value, list: Value) -> Result<Value, ErrorKind> { async fn builtin_map(co: GenCo, f: Value, list: Value) -> Result<Value, ErrorKind> {
let mut out = imbl::Vector::<Value>::new(); let mut out = imbl::Vector::<Value>::new();
// the best span we can get…
let span = generators::request_span(&co).await;
for val in list.to_list()? { for val in list.to_list()? {
let result = generators::request_call_with(&co, f.clone(), [val]).await; let result = Value::Thunk(Thunk::new_suspended_call(f.clone(), val, span.clone()));
out.push_back(result) out.push_back(result)
} }
@ -564,9 +576,17 @@ mod pure_builtins {
let attrs = attrs.to_attrs()?; let attrs = attrs.to_attrs()?;
let mut out = imbl::OrdMap::new(); let mut out = imbl::OrdMap::new();
// the best span we can get…
let span = generators::request_span(&co).await;
for (key, value) in attrs.into_iter() { for (key, value) in attrs.into_iter() {
let result = let result = Value::Thunk(Thunk::new_suspended_call(
generators::request_call_with(&co, f.clone(), [key.clone().into(), value]).await; f.clone(),
key.clone().into(),
span.clone(),
));
let result = Value::Thunk(Thunk::new_suspended_call(result, value, span.clone()));
out.insert(key, result); out.insert(key, result);
} }

View file

@ -0,0 +1 @@
[ 2 [ "Hans" "James" "Joachim" ] 2 [ "Clawdia" "Mynheer" ] 981 3 2 2 ]

View file

@ -0,0 +1,31 @@
[
# This is independent of builtins
(builtins.length [ (builtins.throw "Ferge") (builtins.throw "Wehsal") ])
(builtins.attrNames {
Hans = throw "Castorp";
Joachim = throw "Ziemßen";
James = "Tienappel";
})
(builtins.length (builtins.map builtins.throw [ "Settembrini" "Naphta" ]))
(builtins.attrNames (builtins.mapAttrs builtins.throw {
Clawdia = "Chauchat";
Mynheer = "Peeperkorn";
}))
(builtins.length (builtins.genList (builtins.add "Marusja") 981))
(builtins.length (builtins.genList builtins.throw 3))
# These are hard to get wrong since the outer layer needs to be forced anyways
(builtins.length (builtins.genericClosure {
startSet = [
{ key = 1; initial = true; }
];
operator = { key, initial, ... }:
if initial
then [ { key = key - 1; initial = false; value = throw "lol"; } ]
else [ ];
}))
(builtins.length (builtins.concatMap (m: [ m (builtins.throw m) ]) [ "Marusja" ]))
]

View file

@ -27,6 +27,7 @@ use std::{
use crate::{ use crate::{
errors::ErrorKind, errors::ErrorKind,
opcode::OpCode,
spans::LightSpan, spans::LightSpan,
upvalues::Upvalues, upvalues::Upvalues,
value::Closure, value::Closure,
@ -124,6 +125,36 @@ impl Thunk {
))))) )))))
} }
/// Helper function to create a [`Thunk`] that calls a function given as the
/// [`Value`] `callee` with the argument `arg` when it is forced. This is
/// particularly useful in builtin implementations if the result of calling
/// a function does not need to be forced immediately, because e.g. it is
/// stored in an attribute set.
pub fn new_suspended_call(callee: Value, arg: Value, light_span: LightSpan) -> Self {
let mut lambda = Lambda::default();
let span = light_span.span();
let arg_idx = lambda.chunk().push_constant(arg);
let f_idx = lambda.chunk().push_constant(callee);
// This is basically a recreation of compile_apply():
// We need to push the argument onto the stack and then the function.
// The function (not the argument) needs to be forced before calling.
lambda.chunk.push_op(OpCode::OpConstant(arg_idx), span);
lambda.chunk().push_op(OpCode::OpConstant(f_idx), span);
lambda.chunk.push_op(OpCode::OpForce, span);
lambda.chunk.push_op(OpCode::OpCall, span);
// Inform the VM that the chunk has ended
lambda.chunk.push_op(OpCode::OpReturn, span);
Thunk(Rc::new(RefCell::new(ThunkRepr::Suspended {
upvalues: Rc::new(Upvalues::with_capacity(0)),
lambda: Rc::new(lambda),
light_span,
})))
}
fn prepare_blackhole(&self, forced_at: LightSpan) -> ThunkRepr { fn prepare_blackhole(&self, forced_at: LightSpan) -> ThunkRepr {
match &*self.0.borrow() { match &*self.0.borrow() {
ThunkRepr::Suspended { ThunkRepr::Suspended {