libexpr: fix stack overflow in toJSON on deeply nested structures

Similar to the deepSeq fix, toJSON on deeply nested structures caused
an uncontrolled OS-level stack overflow.

Fix by adding call depth tracking to printValueAsJSON.

src/libexpr/value-to-json.cc

@@ -16,6 +16,8 @@ json printValueAsJSON(
 {
     checkInterrupt();
 
+    auto _level = state.addCallDepth(pos);
+
     if (strict)
         state.forceValue(v, pos);
 

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

@@ -0,0 +1,54 @@
+error:
+       … while evaluating the attribute 'outPath'
+         at «nix-internal»/derivation-internal.nix:50:7:
+           49|     value = commonAttrs // {
+           50|       outPath = builtins.getAttr outputName strict;
+             |       ^
+           51|       drvPath = strict.drvPath;
+
+       … while calling the 'getAttr' builtin
+         at «nix-internal»/derivation-internal.nix:50:17:
+           49|     value = commonAttrs // {
+           50|       outPath = builtins.getAttr outputName strict;
+             |                 ^
+           51|       drvPath = strict.drvPath;
+
+       … while calling the 'derivationStrict' builtin
+         at «nix-internal»/derivation-internal.nix:37:12:
+           36|
+           37|   strict = derivationStrict drvAttrs;
+             |            ^
+           38|
+
+       … while evaluating derivation 'test'
+         whose name attribute is located at /pwd/lang/eval-fail-derivation-structuredAttrs-stack-overflow.nix:5:3
+
+       … while evaluating attribute 'nested' of derivation 'test'
+         at /pwd/lang/eval-fail-derivation-structuredAttrs-stack-overflow.nix:9:3:
+            8|   __structuredAttrs = true;
+            9|   nested =
+             |   ^
+           10|     let
+
+       … while evaluating attribute 'tail'
+         at /pwd/lang/eval-fail-derivation-structuredAttrs-stack-overflow.nix:12:71:
+           11|       long = builtins.genList (x: x) 100000;
+           12|       reverseLinkedList = builtins.foldl' (tail: head: { inherit head tail; }) null long;
+             |                                                                       ^
+           13|     in
+
+       (9994 duplicate frames omitted)
+
+       … while evaluating attribute 'head'
+         at /pwd/lang/eval-fail-derivation-structuredAttrs-stack-overflow.nix:12:66:
+           11|       long = builtins.genList (x: x) 100000;
+           12|       reverseLinkedList = builtins.foldl' (tail: head: { inherit head tail; }) null long;
+             |                                                                  ^
+           13|     in
+
+       error: stack overflow; max-call-depth exceeded
+       at /pwd/lang/eval-fail-derivation-structuredAttrs-stack-overflow.nix:12:66:
+           11|       long = builtins.genList (x: x) 100000;
+           12|       reverseLinkedList = builtins.foldl' (tail: head: { inherit head tail; }) null long;
+             |                                                                  ^
+           13|     in

tests/functional/lang/eval-fail-derivation-structuredAttrs-stack-overflow.nix

@@ -0,0 +1,15 @@
+# Test that derivations with __structuredAttrs and deeply nested structures
+# produce a controlled stack overflow error rather than a segfault.
+
+derivation {
+  name = "test";
+  system = "x86_64-linux";
+  builder = "/bin/sh";
+  __structuredAttrs = true;
+  nested =
+    let
+      long = builtins.genList (x: x) 100000;
+      reverseLinkedList = builtins.foldl' (tail: head: { inherit head tail; }) null long;
+    in
+    reverseLinkedList;
+}

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

@@ -0,0 +1,30 @@
+error:
+       … while calling the 'toJSON' builtin
+         at /pwd/lang/eval-fail-toJSON-stack-overflow.nix:8:1:
+            7| in
+            8| builtins.toJSON reverseLinkedList
+             | ^
+            9|
+
+       … while evaluating attribute 'tail'
+         at /pwd/lang/eval-fail-toJSON-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 attribute 'head'
+         at /pwd/lang/eval-fail-toJSON-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-toJSON-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

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

@@ -0,0 +1,8 @@
+# Test that toJSON 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.toJSON reverseLinkedList