Create infrastructure for "checkpoint" characterization tests

These do a read/write test in the middles of some computation. They are
an imperative way to test intermediate values rather than functionally
testing end outputs.

src/libutil-test-support/include/nix/util/tests/characterization.hh

@@ -31,16 +31,14 @@ static inline bool testAccept()
 /**
  * Mixin class for writing characterization tests
  */
-class CharacterizationTest : public virtual ::testing::Test
+struct CharacterizationTest : virtual ::testing::Test
 {
-protected:
     /**
      * While the "golden master" for this characterization test is
      * located. It should not be shared with any other test.
      */
     virtual std::filesystem::path goldenMaster(PathView testStem) const = 0;
 
-public:
     /**
      * Golden test for reading
      *

src/libutil-test-support/include/nix/util/tests/json-characterization.hh

@@ -39,6 +39,30 @@ void writeJsonTest(CharacterizationTest & test, PathView testStem, const T & val
         [](const auto & file, const auto & got) { return writeFile(file, got.dump(2) + "\n"); });
 }
 
+/**
+ * Golden test in the middle of something
+ */
+template<typename T>
+void checkpointJson(CharacterizationTest & test, PathView testStem, const T & got)
+{
+    using namespace nlohmann;
+
+    auto file = test.goldenMaster(Path{testStem} + ".json");
+
+    json gotJson = static_cast<json>(got);
+
+    if (testAccept()) {
+        std::filesystem::create_directories(file.parent_path());
+        writeFile(file, gotJson.dump(2) + "\n");
+        ADD_FAILURE() << "Updating golden master " << file;
+    } else {
+        json expectedJson = json::parse(readFile(file));
+        ASSERT_EQ(gotJson, expectedJson);
+        T expected = adl_serializer<T>::from_json(expectedJson);
+        ASSERT_EQ(got, expected);
+    }
+}
+
 /**
  * Mixin class for writing characterization tests for `nlohmann::json`
  * conversions for a given type.
@@ -67,6 +91,11 @@ struct JsonCharacterizationTest : virtual CharacterizationTest
     {
         nix::writeJsonTest(*this, testStem, value);
     }
+
+    void checkpointJson(PathView testStem, const T & value)
+    {
+        nix::checkpointJson(*this, testStem, value);
+    }
 };
 
 } // namespace nix