Fix defragment_symbol_data dropping user metadata (#9889436827)

cpp/arcticdb/version/version_core.cpp

@@ -2620,13 +2620,14 @@ VersionedItem defragment_symbol_data_impl(
     return util::variant_match(
             std::move(result),
             [&slices, &pre_defragmentation_info, &store](CompactionWrittenKeys&& written_keys) -> VersionedItem {
+                const auto& user_meta_ptr = pre_defragmentation_info.pipeline_context->user_meta_;
                 return collate_and_write(
                         store,
                         pre_defragmentation_info.pipeline_context,
                         slices,
                         std::move(written_keys),
                         pre_defragmentation_info.append_after.value(),
-                        std::nullopt
+                        user_meta_ptr ? std::make_optional(*user_meta_ptr) : std::nullopt
                 );
             },
             [](Error&& error) -> VersionedItem {

python/arcticdb/version_store/library.py

@@ -3243,7 +3243,9 @@ def defragment_symbol_data(
         Returns
         -------
         VersionedItem
-            Structure containing metadata and version number of the defragmented symbol in the store.
+            Structure containing version number of the defragmented symbol in the store. The ``metadata``
+            and ``data`` fields of the returned object will always be ``None``; call ``read_metadata`` or
+            ``read`` to retrieve the metadata or data associated with the defragmented version.
 
         Raises
         ------

python/tests/unit/arcticdb/version_store/test_append.py

@@ -14,7 +14,7 @@
 from arcticdb.version_store import NativeVersionStore
 from arcticdb_ext.exceptions import InternalException, NormalizationException, SortingException, SchemaException
 from arcticdb_ext import set_config_int
-from arcticdb.util.test import random_integers, assert_frame_equal
+from arcticdb.util.test import random_integers, assert_frame_equal, config_context
 from arcticdb.config import set_log_level
 from arcticdb.util.test_utils import generate_random_numpy_array, supported_types_list
 from arcticdb.util.logger import get_logger
@@ -697,6 +697,22 @@ def test_defragment_no_work_to_do(sym, lmdb_version_store):
         lmdb_version_store.defragment_symbol_data(sym)
 
 
+def test_defragment_preserves_metadata(sym, lmdb_version_store):
+    meta = {"key": "value", "number": 42}
+    df1 = pd.DataFrame({"a": [1, 2]}, index=pd.date_range("2020-01-01", periods=2))
+    df2 = pd.DataFrame({"a": [3, 4]}, index=pd.date_range("2020-01-03", periods=2))
+
+    lmdb_version_store.write(sym, df1, metadata=meta)
+    lmdb_version_store.append(sym, df2, metadata=meta)
+
+    with config_context("SymbolDataCompact.SegmentCount", 1):
+        assert lmdb_version_store.is_symbol_fragmented(sym)
+        versioned_item = lmdb_version_store.defragment_symbol_data(sym)
+
+    assert versioned_item.metadata is None
+    assert lmdb_version_store.read_metadata(sym).metadata == meta
+
+
 @pytest.mark.parametrize(
     "to_write, to_append",
     [