refactor(tvix): JSONSax: Use a RootValue

More #3377.

Backported from upstream at 9f46f54de4e55267df492456fc0393f74616366

Change-Id: I11bfca4ec56bd4e45291ce3f98a60f198dff0196
Reviewed-on: https://cl.tvl.fyi/c/depot/+/2140
Tested-by: BuildkiteCI
Reviewed-by: andi <andi@notmuch.email>

third_party/nix/src/libexpr/json-to-value.cc

@@ -16,22 +16,21 @@ class JSONSax : nlohmann::json_sax<json> {
   class JSONState {
    protected:
     std::unique_ptr<JSONState> parent;
-    Value* v;
+    std::shared_ptr<Value*> v;
 
    public:
     virtual std::unique_ptr<JSONState> resolve(EvalState&) {
       throw std::logic_error("tried to close toplevel json parser state");
-    };
-    explicit JSONState(std::unique_ptr<JSONState>&& p)
-        : parent(std::move(p)), v(nullptr){};
-    explicit JSONState(Value* v) : v(v){};
+    }
+    explicit JSONState(std::unique_ptr<JSONState>&& p) : parent(std::move(p)) {}
+    explicit JSONState(Value* v) : v(allocRootValue(v)) {}
     JSONState(JSONState& p) = delete;
     Value& value(EvalState& state) {
-      if (v == nullptr) v = state.allocValue();
-      return *v;
-    };
-    virtual ~JSONState(){};
-    virtual void add(){};
+      if (!v) v = allocRootValue(state.allocValue());
+      return **v;
+    }
+    virtual ~JSONState() {}
+    virtual void add() {}
   };
 
   class JSONObjectState : public JSONState {
@@ -63,7 +62,7 @@ class JSONSax : nlohmann::json_sax<json> {
       return std::move(parent);
     }
     void add() override {
-      values.push_back(v);
+      values.push_back(*v);
       v = nullptr;
     };
 
@@ -87,50 +86,52 @@ class JSONSax : nlohmann::json_sax<json> {
  public:
   JSONSax(EvalState& state, Value& v) : state(state), rs(new JSONState(&v)){};
 
-  bool null() { return handle_value(mkNull); }
+  bool null() override { return handle_value(mkNull); }
 
-  bool boolean(bool val) { return handle_value(mkBool, val); }
+  bool boolean(bool val) override { return handle_value(mkBool, val); }
 
-  bool number_integer(number_integer_t val) { return handle_value(mkInt, val); }
-
-  bool number_unsigned(number_unsigned_t val) {
+  bool number_integer(number_integer_t val) override {
     return handle_value(mkInt, val);
   }
 
-  bool number_float(number_float_t val, const string_t& s) {
+  bool number_unsigned(number_unsigned_t val) override {
+    return handle_value(mkInt, val);
+  }
+
+  bool number_float(number_float_t val, const string_t&) override {
     return handle_value(mkFloat, val);
   }
 
-  bool string(string_t& val) {
+  bool string(string_t& val) override {
     return handle_value<void(Value&, const char*)>(mkString, val.c_str());
   }
 
-  bool start_object(std::size_t len) {
+  bool start_object(std::size_t) override {
     rs = std::make_unique<JSONObjectState>(std::move(rs));
     return true;
   }
 
-  bool key(string_t& name) {
+  bool key(string_t& name) override {
     dynamic_cast<JSONObjectState*>(rs.get())->key(name, state);
     return true;
   }
 
-  bool end_object() {
+  bool end_object() override {
     rs = rs->resolve(state);
     rs->add();
     return true;
   }
 
-  bool end_array() { return end_object(); }
+  bool end_array() override { return end_object(); }
 
-  bool start_array(size_t len) {
+  bool start_array(size_t len) override {
     rs = std::make_unique<JSONListState>(
         std::move(rs), len != std::numeric_limits<size_t>::max() ? len : 128);
     return true;
   }
 
   bool parse_error(std::size_t, const std::string&,
-                   const nlohmann::detail::exception& ex) {
+                   const nlohmann::detail::exception& ex) override {
     throw JSONParseError(ex.what());
   }
 };