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Add a ListBuilder helper for constructing list values

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Previously, `state.mkList()` would set the type of the value to tList
and allocate the list vector, but it would not initialize the values
in the list. This has two problems:

* If an exception occurs, the list is left in an undefined state.

* More importantly, for multithreaded evaluation, if a value
  transitions from thunk to non-thunk, it should be final (i.e. other
  threads should be able to access the value safely).

To address this, there now is a `ListBuilder` class (analogous to
`BindingsBuilder`) to build the list vector prior to the call to
`Value::mkList()`. Typical usage:

   auto list = state.buildList(size);
   for (auto & v : list)
       v = ... set value ...;
   vRes.mkList(list);
This commit is contained in:
Eelco Dolstra 2024-03-14 19:10:31 +01:00
parent bff5c94184
commit fecff520d7
10 changed files with 228 additions and 157 deletions
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24
src/libexpr/eval.cc
View file

@ -435,7 +435,8 @@ EvalState::EvalState(
static_assert(sizeof(Env) <= 16, "environment must be <= 16 bytes");
vEmptyList.mkList(0);
vEmptyList.mkList(buildList(0));
vNull.mkNull();
/* Initialise the Nix expression search path. */
if (!evalSettings.pureEval) {
@ -923,12 +924,11 @@ inline Value * EvalState::lookupVar(Env * env, const ExprVar & var, bool noEval)
}
}
void EvalState::mkList(Value & v, size_t size)
ListBuilder::ListBuilder(EvalState & state, size_t size)
: size(size)
, elems(size <= 2 ? inlineElems : (Value * *) allocBytes(size * sizeof(Value *)))
{
v.mkList(size);
if (size > 2)
v.bigList.elems = (Value * *) allocBytes(size * sizeof(Value *));
nrListElems += size;
state.nrListElems += size;
}
@ -1353,9 +1353,10 @@ void ExprLet::eval(EvalState & state, Env & env, Value & v)
void ExprList::eval(EvalState & state, Env & env, Value & v)
{
state.mkList(v, elems.size());
for (auto [n, v2] : enumerate(v.listItems()))
const_cast<Value * &>(v2) = elems[n]->maybeThunk(state, env);
auto list = state.buildList(elems.size());
for (const auto & [n, v2] : enumerate(list))
v2 = elems[n]->maybeThunk(state, env);
v.mkList(list);
}
@ -1963,14 +1964,15 @@ void EvalState::concatLists(Value & v, size_t nrLists, Value * * lists, const Po
return;
}
mkList(v, len);
auto out = v.listElems();
auto list = buildList(len);
auto out = list.elems;
for (size_t n = 0, pos = 0; n < nrLists; ++n) {
auto l = lists[n]->listSize();
if (l)
memcpy(out + pos, lists[n]->listElems(), l * sizeof(Value *));
pos += l;
}
v.mkList(list);
}