From 67769e6fc146d9b1b2db777c56e2bba9f3ddf698 Mon Sep 17 00:00:00 2001
From: Eelco Dolstra <edolstra@gmail.com>
Date: Tue, 15 Jul 2025 20:16:39 +0200
Subject: [PATCH] Make EvalState::callDepth thread-local

This is needed to make it thread-safe.
---
 src/libexpr/eval.cc                  |  2 ++
 src/libexpr/include/nix/expr/eval.hh | 20 +++++++++++---------
 2 files changed, 13 insertions(+), 9 deletions(-)

diff --git a/src/libexpr/eval.cc b/src/libexpr/eval.cc
index 47cc35daa..e93ca5593 100644
--- a/src/libexpr/eval.cc
+++ b/src/libexpr/eval.cc
@@ -1533,6 +1533,8 @@ void ExprLambda::eval(EvalState & state, Env & env, Value & v)
     v.mkLambda(&env, this);
 }
 
+thread_local size_t EvalState::callDepth = 0;
+
 void EvalState::callFunction(Value & fun, std::span<Value *> args, Value & vRes, const PosIdx pos)
 {
     auto _level = addCallDepth(pos);
diff --git a/src/libexpr/include/nix/expr/eval.hh b/src/libexpr/include/nix/expr/eval.hh
index 27294d114..5d9847182 100644
--- a/src/libexpr/include/nix/expr/eval.hh
+++ b/src/libexpr/include/nix/expr/eval.hh
@@ -52,15 +52,15 @@ namespace eval_cache {
  * Increments a count on construction and decrements on destruction.
  */
 class CallDepth {
-  size_t & count;
+    size_t & count;
 
 public:
-  CallDepth(size_t & count) : count(count) {
-    ++count;
-  }
-  ~CallDepth() {
-    --count;
-  }
+    CallDepth(size_t & count) : count(count) {
+        ++count;
+    }
+    ~CallDepth() {
+        --count;
+    }
 };
 
 /**
@@ -710,9 +710,11 @@ private:
         std::shared_ptr<StaticEnv> & staticEnv);
 
     /**
-     * Current Nix call stack depth, used with `max-call-depth` setting to throw stack overflow hopefully before we run out of system stack.
+     * Current Nix call stack depth, used with `max-call-depth`
+     * setting to throw stack overflow hopefully before we run out of
+     * system stack.
      */
-    size_t callDepth = 0;
+    thread_local static size_t callDepth;
 
 public: