Ensure backward compatibility for meta field in DerivationOptions JSON

Make the meta field optional in DerivationOptions JSON deserialization to maintain backward compatibility with JSON that predates the derivation-meta feature.

Changes:
- Use optionalValueAt instead of valueAt for meta field deserialization
- Mark meta as optional in derivation-options-v1 schema
- Add unit test for backward compatibility

Author: roberth

doc/manual/source/protocols/json/schema/derivation-options-v1.yaml

@@ -26,7 +26,8 @@ required:
   - requiredSystemFeatures
   - preferLocalBuild
   - allowSubstitutes
-  - meta
+  # Note: `meta` is not in the required list to maintain backward compatibility
+  # with existing JSON that predates the derivation-meta feature
 properties:
   outputChecks:
     type: object

src/libstore-tests/derivation-advanced-attrs.cc

@@ -523,4 +523,17 @@ TEST_JSON_OPTIONS(CaDerivationAdvancedAttrsTest, structuredAttrs_all_set, struct
 
 #undef TEST_JSON_OPTIONS
 
+// Test backward compatibility: JSON without 'meta' field should be ingestible
+TEST_F(DerivationAdvancedAttrsTest, DerivationOptions_backward_compat_no_meta)
+{
+    // Read existing JSON and remove the 'meta' field to simulate old format
+    this->readTest("defaults.json", [&](auto encoded) {
+        auto j = json::parse(encoded);
+        j.erase("meta"); // Remove meta field to simulate old JSON
+        auto got = j.template get<DerivationOptions<SingleDerivedPath>>();
+        // Should successfully deserialize with meta = std::nullopt
+        EXPECT_EQ(got.meta, std::nullopt);
+    });
+}
+
 } // namespace nix

src/libstore/derivation-options.cc

@@ -628,7 +628,11 @@ DerivationOptions<SingleDerivedPath> adl_serializer<DerivationOptions<SingleDeri
         .requiredSystemFeatures = getStringSet(valueAt(json, "requiredSystemFeatures")),
         .preferLocalBuild = getBoolean(valueAt(json, "preferLocalBuild")),
         .allowSubstitutes = getBoolean(valueAt(json, "allowSubstitutes")),
-        .meta = valueAt(json, "meta").get<std::optional<nlohmann::json::object_t>>(),
+        .meta = [&]() -> std::optional<nlohmann::json::object_t> {
+            if (auto * metaPtr = optionalValueAt(json, "meta"))
+                return metaPtr->get<std::optional<nlohmann::json::object_t>>();
+            return std::nullopt;
+        }(),
     };
 }