Unit test the "common protocol" too

Copy the relevant tests to ensure the new interfaces added in the last
commit are tested.

Perhaps I should try to deduplicat these tests some more. However its
not clear how to do that outside of a big ugly C++ macro.
https://github.com/google/googletest/blob/main/docs/advanced.md has some
stuff but it is cumbersome and I didn't figure it out yet.

This is done in a separate commit in order to be sure that the first
commit really didn't change any behavior; if we changed the
implementation and the tests at once, it would be harder to tell whether
or not some behavioral changes slipped in what is supposed to be a "pure
refactor".

Co-Authored-By: Valentin Gagarin <valentin.gagarin@tweag.io>

src/libstore/tests/protocol.hh

@@ -0,0 +1,88 @@
+#include <nlohmann/json.hpp>
+#include <gtest/gtest.h>
+
+#include "tests/libstore.hh"
+#include "tests/characterization.hh"
+
+namespace nix {
+
+template<class Proto, const char * protocolDir>
+class ProtoTest : public LibStoreTest
+{
+    /**
+     * Read this as simply `using S = Inner::Serialise;`.
+     *
+     * See `LengthPrefixedProtoHelper::S` for the same trick, and its
+     * rationale.
+     */
+    template<typename U> using S = typename Proto::template Serialise<U>;
+
+public:
+    Path unitTestData = getUnitTestData() + "/libstore/" + protocolDir;
+
+    Path goldenMaster(std::string_view testStem) {
+        return unitTestData + "/" + testStem + ".bin";
+    }
+
+    /**
+     * Golden test for `T` reading
+     */
+    template<typename T>
+    void readTest(PathView testStem, T value)
+    {
+        if (testAccept())
+        {
+            GTEST_SKIP() << "Cannot read golden master because another test is also updating it";
+        }
+        else
+        {
+            auto expected = readFile(goldenMaster(testStem));
+
+            T got = ({
+                StringSource from { expected };
+                S<T>::read(
+                    *store,
+                    typename Proto::ReadConn { .from = from });
+            });
+
+            ASSERT_EQ(got, value);
+        }
+    }
+
+    /**
+     * Golden test for `T` write
+     */
+    template<typename T>
+    void writeTest(PathView testStem, const T & value)
+    {
+        auto file = goldenMaster(testStem);
+
+        StringSink to;
+        Proto::write(
+            *store,
+            typename Proto::WriteConn { .to = to },
+            value);
+
+        if (testAccept())
+        {
+            createDirs(dirOf(file));
+            writeFile(file, to.s);
+            GTEST_SKIP() << "Updating golden master";
+        }
+        else
+        {
+            auto expected = readFile(file);
+            ASSERT_EQ(to.s, expected);
+        }
+    }
+};
+
+#define CHARACTERIZATION_TEST(FIXTURE, NAME, STEM, VALUE) \
+    TEST_F(FIXTURE, NAME ## _read) { \
+        readTest(STEM, VALUE); \
+    } \
+    TEST_F(FIXTURE, NAME ## _write) { \
+        writeTest(STEM, VALUE); \
+    }
+
+}