libexpr: fix stack overflow in deepSeq on deeply nested structures

builtins.deepSeq on deeply nested structures (e.g., a linked list with
100,000 elements) caused an uncontrolled OS-level stack overflow with
no Nix stack trace.

Fix by adding call depth tracking to forceValueDeep, integrating with
Nix's existing max-call-depth mechanism. Now produces a controlled
"stack overflow; max-call-depth exceeded" error with a proper stack
trace.

Closes: https://github.com/NixOS/nix/issues/7816

src/libexpr/eval.cc

@@ -2188,6 +2188,8 @@ void EvalState::forceValueDeep(Value & v)
     std::set<const Value *> seen;
 
     [&, &state(*this)](this const auto & recurse, Value & v) {
+        auto _level = state.addCallDepth(v.determinePos(noPos));
+
         if (!seen.insert(&v).second)
             return;
 

tests/functional/lang/eval-fail-deepseq-stack-overflow.err.exp

@@ -0,0 +1,30 @@
+error:
+       … while calling the 'deepSeq' builtin
+         at /pwd/lang/eval-fail-deepseq-stack-overflow.nix:8:1:
+            7| in
+            8| builtins.deepSeq reverseLinkedList (
+             | ^
+            9|   throw "unexpected success; expected a controlled stack overflow instead"
+
+       … while evaluating the attribute 'tail'
+         at /pwd/lang/eval-fail-deepseq-stack-overflow.nix:6:67:
+            5|   long = builtins.genList (x: x) 100000;
+            6|   reverseLinkedList = builtins.foldl' (tail: head: { inherit head tail; }) null long;
+             |                                                                   ^
+            7| in
+
+       (9997 duplicate frames omitted)
+
+       … while evaluating the attribute 'head'
+         at /pwd/lang/eval-fail-deepseq-stack-overflow.nix:6:62:
+            5|   long = builtins.genList (x: x) 100000;
+            6|   reverseLinkedList = builtins.foldl' (tail: head: { inherit head tail; }) null long;
+             |                                                              ^
+            7| in
+
+       error: stack overflow; max-call-depth exceeded
+       at /pwd/lang/eval-fail-deepseq-stack-overflow.nix:5:28:
+            4| let
+            5|   long = builtins.genList (x: x) 100000;
+             |                            ^
+            6|   reverseLinkedList = builtins.foldl' (tail: head: { inherit head tail; }) null long;

tests/functional/lang/eval-fail-deepseq-stack-overflow.nix

@@ -0,0 +1,10 @@
+# Test that deepSeq on a deeply nested structure produces a controlled
+# stack overflow error rather than a segfault.
+
+let
+  long = builtins.genList (x: x) 100000;
+  reverseLinkedList = builtins.foldl' (tail: head: { inherit head tail; }) null long;
+in
+builtins.deepSeq reverseLinkedList (
+  throw "unexpected success; expected a controlled stack overflow instead"
+)