From 1d6abec993a371091459d5e23f985c6d69621ce7 Mon Sep 17 00:00:00 2001
From: Eelco Dolstra <edolstra@gmail.com>
Date: Mon, 20 Nov 2023 12:35:35 +0100
Subject: [PATCH] Revert use of boost::container::small_vector in the evaluator

It caused random crashes (https://hydra.nixos.org/build/241514506,
https://hydra.nixos.org/build/241443330) because the heap allocation
done by small_vector in the not-small case is not scanned for GC
roots.
---
 src/libexpr/eval.cc    | 11 +++++------
 src/libexpr/primops.cc |  7 ++++---
 2 files changed, 9 insertions(+), 9 deletions(-)

diff --git a/src/libexpr/eval.cc b/src/libexpr/eval.cc
index e9b8cacfd..46a49c891 100644
--- a/src/libexpr/eval.cc
+++ b/src/libexpr/eval.cc
@@ -31,7 +31,6 @@
 
 #include <sys/resource.h>
 #include <nlohmann/json.hpp>
-#include <boost/container/small_vector.hpp>
 
 #if HAVE_BOEHMGC
 
@@ -1710,7 +1709,7 @@ void EvalState::callFunction(Value & fun, size_t nrArgs, Value * * args, Value &
                 /* We have all the arguments, so call the primop with
                    the previous and new arguments. */
 
-                Value * vArgs[maxPrimOpArity];
+                Value * vArgs[arity];
                 auto n = argsDone;
                 for (Value * arg = &vCur; arg->isPrimOpApp(); arg = arg->primOpApp.left)
                     vArgs[--n] = arg->primOpApp.right;
@@ -1773,11 +1772,11 @@ void ExprCall::eval(EvalState & state, Env & env, Value & v)
     // 4: about 60
     // 5: under 10
     // This excluded attrset lambdas (`{...}:`). Contributions of mixed lambdas appears insignificant at ~150 total.
-    boost::container::small_vector<Value *, 4> vArgs(args.size());
+    Value * vArgs[args.size()];
     for (size_t i = 0; i < args.size(); ++i)
         vArgs[i] = args[i]->maybeThunk(state, env);
 
-    state.callFunction(vFun, args.size(), vArgs.data(), v, pos);
+    state.callFunction(vFun, args.size(), vArgs, v, pos);
 }
 
 
@@ -2016,8 +2015,8 @@ void ExprConcatStrings::eval(EvalState & state, Env & env, Value & v)
         return result;
     };
 
-    boost::container::small_vector<Value, conservativeStackReservation> values(es->size());
-    Value * vTmpP = values.data();
+    Value values[es->size()];
+    Value * vTmpP = values;
 
     for (auto & [i_pos, i] : *es) {
         Value & vTmp = *vTmpP++;
diff --git a/src/libexpr/primops.cc b/src/libexpr/primops.cc
index 27f502830..a8d44d8b7 100644
--- a/src/libexpr/primops.cc
+++ b/src/libexpr/primops.cc
@@ -2729,7 +2729,7 @@ static void prim_catAttrs(EvalState & state, const PosIdx pos, Value * * args, V
     auto attrName = state.symbols.create(state.forceStringNoCtx(*args[0], pos, "while evaluating the first argument passed to builtins.catAttrs"));
     state.forceList(*args[1], pos, "while evaluating the second argument passed to builtins.catAttrs");
 
-    boost::container::small_vector<Value *, nonRecursiveStackReservation> res(args[1]->listSize());
+    Value * res[args[1]->listSize()];
     size_t found = 0;
 
     for (auto v2 : args[1]->listItems()) {
@@ -3064,7 +3064,8 @@ static void prim_filter(EvalState & state, const PosIdx pos, Value * * args, Val
 
     state.forceFunction(*args[0], pos, "while evaluating the first argument passed to builtins.filter");
 
-    boost::container::small_vector<Value *, nonRecursiveStackReservation> vs(args[1]->listSize());
+    // FIXME: putting this on the stack is risky.
+    Value * vs[args[1]->listSize()];
     size_t k = 0;
 
     bool same = true;
@@ -3453,7 +3454,7 @@ static void prim_concatMap(EvalState & state, const PosIdx pos, Value * * args,
     state.forceList(*args[1], pos, "while evaluating the second argument passed to builtins.concatMap");
     auto nrLists = args[1]->listSize();
 
-    boost::container::small_vector<Value, conservativeStackReservation> lists(nrLists);
+    Value lists[nrLists];
     size_t len = 0;
 
     for (unsigned int n = 0; n < nrLists; ++n) {