Add a ListBuilder helper for constructing list values

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);

src/libexpr/eval.cc

@@ -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);
 }
 
 

src/libexpr/eval.hh

@@ -186,6 +186,11 @@ public:
      */
     Value vEmptyList;
 
+    /**
+     * Null constant.
+     */
+    Value vNull;
+
     /**
      * The accessor for the root filesystem.
      */
@@ -615,7 +620,11 @@ public:
         return BindingsBuilder(*this, allocBindings(capacity));
     }
 
-    void mkList(Value & v, size_t length);
+    ListBuilder buildList(size_t size)
+    {
+        return ListBuilder(*this, size);
+    }
+
     void mkThunk_(Value & v, Expr * expr);
     void mkPos(Value & v, PosIdx pos);
 
@@ -756,6 +765,7 @@ private:
     friend void prim_split(EvalState & state, const PosIdx pos, Value * * args, Value & v);
 
     friend struct Value;
+    friend class ListBuilder;
 };
 
 struct DebugTraceStacker {

src/libexpr/json-to-value.cc

@@ -57,11 +57,10 @@ class JSONSax : nlohmann::json_sax<json> {
         ValueVector values;
         std::unique_ptr<JSONState> resolve(EvalState & state) override
         {
-            Value & v = parent->value(state);
-            state.mkList(v, values.size());
-            for (size_t n = 0; n < values.size(); ++n) {
-                v.listElems()[n] = values[n];
-            }
+            auto list = state.buildList(values.size());
+            for (const auto & [n, v2] : enumerate(list))
+                v2 = values[n];
+            parent->value(state).mkList(list);
             return std::move(parent);
         }
         void add() override {