Make EvalState::callDepth thread-local

This is needed to make it thread-safe.

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);

src/libexpr/include/nix/expr/eval.hh

@@ -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: