refactor(tvix): json-to-value: use unique_ptr instead of raw pointers

Backported from upstream at a350d0beb0e13d0f58698510bd6a96d894cd06fd

Change-Id: Ib486bc1b36ef65041fab1a4634a0a82e13036fd4
Reviewed-on: https://cl.tvl.fyi/c/depot/+/2139
Tested-by: BuildkiteCI
Reviewed-by: andi <andi@notmuch.email>

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

@@ -15,14 +15,15 @@ namespace nix {
 class JSONSax : nlohmann::json_sax<json> {
   class JSONState {
    protected:
-    JSONState* parent;
+    std::unique_ptr<JSONState> parent;
     Value* v;
 
    public:
-    virtual JSONState* resolve(EvalState&) {
+    virtual std::unique_ptr<JSONState> resolve(EvalState&) {
       throw std::logic_error("tried to close toplevel json parser state");
     };
-    explicit JSONState(JSONState* p) : parent(p), v(nullptr){};
+    explicit JSONState(std::unique_ptr<JSONState>&& p)
+        : parent(std::move(p)), v(nullptr){};
     explicit JSONState(Value* v) : v(v){};
     JSONState(JSONState& p) = delete;
     Value& value(EvalState& state) {
@@ -36,13 +37,13 @@ class JSONSax : nlohmann::json_sax<json> {
   class JSONObjectState : public JSONState {
     using JSONState::JSONState;
     ValueMap attrs = ValueMap();
-    virtual JSONState* resolve(EvalState& state) override {
+    std::unique_ptr<JSONState> resolve(EvalState& state) override {
       Value& v = parent->value(state);
       state.mkAttrs(v, attrs.size());
       for (auto& i : attrs) v.attrs->push_back(Attr(i.first, i.second));
-      return parent;
+      return std::move(parent);
     }
-    virtual void add() override { v = nullptr; };
+    void add() override { v = nullptr; };
 
    public:
     void key(string_t& name, EvalState& state) {
@@ -51,29 +52,30 @@ class JSONSax : nlohmann::json_sax<json> {
   };
 
   class JSONListState : public JSONState {
+    using JSONState::JSONState;
     std::vector<Value*> values;
-    virtual JSONState* resolve(EvalState& state) override {
+    std::unique_ptr<JSONState> resolve(EvalState& state) override {
       Value& v = parent->value(state);
       state.mkList(v, values.size());
       for (size_t n = 0; n < values.size(); ++n) {
         (*v.list)[n] = values[n];
       }
-      return parent;
+      return std::move(parent);
     }
-
     void add() override {
       values.push_back(v);
       v = nullptr;
     };
 
    public:
-    JSONListState(JSONState* p, std::size_t reserve) : JSONState(p) {
+    JSONListState(std::unique_ptr<JSONState>&& p, std::size_t reserve)
+        : JSONState(std::move(p)) {
       values.reserve(reserve);
     }
   };
 
   EvalState& state;
-  JSONState* rs;
+  std::unique_ptr<JSONState> rs;
 
   template <typename T, typename... Args>
   inline bool handle_value(T f, Args... args) {
@@ -84,7 +86,6 @@ class JSONSax : nlohmann::json_sax<json> {
 
  public:
   JSONSax(EvalState& state, Value& v) : state(state), rs(new JSONState(&v)){};
-  ~JSONSax() { delete rs; };
 
   bool null() { return handle_value(mkNull); }
 
@@ -105,20 +106,17 @@ class JSONSax : nlohmann::json_sax<json> {
   }
 
   bool start_object(std::size_t len) {
-    JSONState* old = rs;
+    rs = std::make_unique<JSONObjectState>(std::move(rs));
-    rs = new JSONObjectState(old);
     return true;
   }
 
   bool key(string_t& name) {
-    dynamic_cast<JSONObjectState*>(rs)->key(name, state);
+    dynamic_cast<JSONObjectState*>(rs.get())->key(name, state);
     return true;
   }
 
   bool end_object() {
-    JSONState* old = rs;
+    rs = rs->resolve(state);
-    rs = old->resolve(state);
-    delete old;
     rs->add();
     return true;
   }
@@ -126,9 +124,8 @@ class JSONSax : nlohmann::json_sax<json> {
   bool end_array() { return end_object(); }
 
   bool start_array(size_t len) {
-    JSONState* old = rs;
+    rs = std::make_unique<JSONListState>(
-    rs = new JSONListState(
+        std::move(rs), len != std::numeric_limits<size_t>::max() ? len : 128);
-        old, len != std::numeric_limits<size_t>::max() ? len : 128);
     return true;
   }