AVRO-4351: [C++] Preserve primitive custom attributes

lang/c++/impl/Compiler.cc

@@ -552,6 +552,13 @@ static NodePtr makeNode(const Entity &e, const Object &m,
         result = makeMapNode(e, m, st, ns);
     } else {
         result = makePrimitive(type);
+        if (result) {
+            CustomAttributes customAttributes;
+            getCustomAttributes(m, customAttributes);
+            if (!customAttributes.attributes().empty()) {
+                result->addCustomAttributesForField(customAttributes);
+            }
+        }
     }
 
     if (result) {

lang/c++/impl/NodeImpl.cc

@@ -220,8 +220,10 @@ NodeSymbolic::resolve(const Node &reader) const {
 
 void NodePrimitive::printJson(std::ostream &os, size_t depth) const {
     bool hasLogicalType = logicalType().type() != LogicalType::NONE;
+    bool hasCustomAttributes = customAttributes_.size() != 0;
+    bool printAsObject = hasLogicalType || hasCustomAttributes;
 
-    if (hasLogicalType) {
+    if (printAsObject) {
         os << "{\n"
            << indent(depth) << "\"type\": ";
     }
@@ -232,6 +234,11 @@ void NodePrimitive::printJson(std::ostream &os, size_t depth) const {
         os << ",\n"
            << indent(depth);
         logicalType().printJson(os);
+    }
+    for (size_t i = 0; i != customAttributes_.size(); ++i) {
+        printCustomAttributes(customAttributes_.get(i), depth, os);
+    }
+    if (printAsObject) {
         os << "\n}";
     }
     if (!getDoc().empty()) {

lang/c++/test/SchemaTests.cc

@@ -890,6 +890,30 @@ static void testCustomAttributesJson2Schema2Json() {
     BOOST_CHECK_EQUAL(removeWhitespaceFromSchema(json), removeWhitespaceFromSchema(schema));
 }
 
+static void testPrimitiveCustomAttributesJsonRoundTrip() {
+    const std::vector<std::string> schemas = {
+        R"({
+            "type": "long",
+            "logicalType": "timestamp-micros",
+            "adjust-to-utc": true
+        })",
+        R"({
+            "type": "long",
+            "custom-property": "value"
+        })",
+    };
+
+    // Iceberg relies on adjust-to-utc to distinguish timestamp from timestamptz.
+    // Primitive schema properties must also round-trip without a logical type.
+    for (const auto &schema : schemas) {
+        ValidSchema compiledSchema = compileJsonSchemaFromString(schema);
+        BOOST_REQUIRE_EQUAL(compiledSchema.root()->customAttributes(), 1);
+
+        std::string json = compiledSchema.toJson();
+        BOOST_CHECK_EQUAL(removeWhitespaceFromSchema(json), removeWhitespaceFromSchema(schema));
+    }
+}
+
 static void testCustomAttributesSchema2Json2Schema() {
     const std::string expected = R"({
         "type": "record",
@@ -951,6 +975,7 @@ init_unit_test_suite(int /*argc*/, char * /*argv*/[]) {
     ts->add(BOOST_TEST_CASE(&avro::schema::testParseCustomAttributes));
     ts->add(BOOST_TEST_CASE(&avro::schema::testAddCustomAttributes));
     ts->add(BOOST_TEST_CASE(&avro::schema::testCustomAttributesJson2Schema2Json));
+    ts->add(BOOST_TEST_CASE(&avro::schema::testPrimitiveCustomAttributesJsonRoundTrip));
     ts->add(BOOST_TEST_CASE(&avro::schema::testCustomAttributesSchema2Json2Schema));
     return ts;
 }