From 1b9813e4e60836ddb1467efd50c572e7579ac945 Mon Sep 17 00:00:00 2001
From: Robert Hensing <robert@roberthensing.nl>
Date: Thu, 16 Nov 2023 13:04:27 +0100
Subject: [PATCH] primops: Name stack reservation limits

---
 src/libexpr/eval.cc    |  3 ++-
 src/libexpr/primops.cc |  6 +++---
 src/libexpr/primops.hh | 16 ++++++++++++++++
 3 files changed, 21 insertions(+), 4 deletions(-)

diff --git a/src/libexpr/eval.cc b/src/libexpr/eval.cc
index 2fcbf3311..8b0ada517 100644
--- a/src/libexpr/eval.cc
+++ b/src/libexpr/eval.cc
@@ -1,6 +1,7 @@
 #include "eval.hh"
 #include "eval-settings.hh"
 #include "hash.hh"
+#include "primops.hh"
 #include "types.hh"
 #include "util.hh"
 #include "store-api.hh"
@@ -2015,7 +2016,7 @@ void ExprConcatStrings::eval(EvalState & state, Env & env, Value & v)
         return result;
     };
 
-    boost::container::small_vector<Value, 16> values(es->size());
+    boost::container::small_vector<Value, conservativeStackReservation> values(es->size());
     Value * vTmpP = values.data();
 
     for (auto & [i_pos, i] : *es) {
diff --git a/src/libexpr/primops.cc b/src/libexpr/primops.cc
index 7aa212281..adce95bed 100644
--- a/src/libexpr/primops.cc
+++ b/src/libexpr/primops.cc
@@ -2728,7 +2728,7 @@ 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 *, 256> res(args[1]->listSize());
+    boost::container::small_vector<Value *, nonRecursiveStackReservation> res(args[1]->listSize());
     size_t found = 0;
 
     for (auto v2 : args[1]->listItems()) {
@@ -3063,7 +3063,7 @@ 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 *, 256> vs(args[1]->listSize());
+    boost::container::small_vector<Value *, nonRecursiveStackReservation> vs(args[1]->listSize());
     size_t k = 0;
 
     bool same = true;
@@ -3452,7 +3452,7 @@ 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, 16> lists(nrLists);
+    boost::container::small_vector<Value, conservativeStackReservation> lists(nrLists);
     size_t len = 0;
 
     for (unsigned int n = 0; n < nrLists; ++n) {
diff --git a/src/libexpr/primops.hh b/src/libexpr/primops.hh
index 930e7f32a..1d5d5710d 100644
--- a/src/libexpr/primops.hh
+++ b/src/libexpr/primops.hh
@@ -8,6 +8,22 @@
 
 namespace nix {
 
+/**
+ * For functions where we do not expect deep recursion, we can use a sizable
+ * part of the stack a free allocation space.
+ *
+ * Note: this is expected to be multiplied by sizeof(Value), or about 24 bytes.
+ */
+constexpr size_t nonRecursiveStackReservation = 256;
+
+/**
+ * Functions that maybe applied to self-similar inputs, such as concatMap on a
+ * tree, should reserve a smaller part of the stack for allocation.
+ *
+ * Note: this is expected to be multiplied by sizeof(Value), or about 24 bytes.
+ */
+constexpr size_t conservativeStackReservation = 16;
+
 struct RegisterPrimOp
 {
     typedef std::vector<PrimOp> PrimOps;