fix(python): close synchronous database connections (#4424)

Calling `close()` on a synchronous remote connection returned an
unawaited coroutine and left the connection usable. Local synchronous
connections had no close or context-manager API.

Add synchronous `close()` and `is_open()` to the connection interface,
delegate them to the existing connection handle for local, remote, and
namespace connections, and provide shared context-manager cleanup.
Closing is idempotent, and subsequent database operations use the
existing `Connection is closed` error. Document the lifecycle with
examples.

Validation:

- Reproduced the reported coroutine warning and missing local lifecycle
API on the original code; all 15 new regression cases failed before the
fix.
- All 168 tests in `test_db.py`, `test_remote_db.py`, and
`test_namespace.py` passed through the uv-managed environment, including
local, in-memory, namespace, native-derived, and mocked remote lifecycle
coverage.
- All 6 synchronous connection doctests passed.
- Repository-wide Ruff 0.9.9 lint and formatting passed; `git diff
--check` passed.

Fixes #4382

<!-- lance-gatekeeper-fix:v1 agent=a743139af98198f1d4c8934c9cc1d0c9
generation=1 -->

Co-authored-by: Gatefixer <313497061+lancedb-gatefixer[bot]@users.noreply.github.com>

python/python/lancedb/db.py

@@ -122,7 +122,48 @@ def _view_description(
 
 
 class DBConnection(EnforceOverrides):
-    """An active LanceDB connection interface."""
+    """An active LanceDB connection interface.
+
+    Use [close][lancedb.db.DBConnection.close] to release the connection's
+    underlying resources, or use the connection as a context manager to close it
+    automatically when leaving the block, including when an exception is raised.
+
+    Examples
+    --------
+    >>> import lancedb
+    >>> with lancedb.connect("memory://") as db:
+    ...     assert db.is_open()
+    >>> db.is_open()
+    False
+    """
+
+    def __enter__(self) -> DBConnection:
+        return self
+
+    def __exit__(self, *_) -> None:
+        self.close()
+
+    @abstractmethod
+    def is_open(self) -> bool:
+        """Return True if the connection is open."""
+        pass
+
+    @abstractmethod
+    def close(self) -> None:
+        """Close the connection, releasing any underlying resources.
+
+        It is safe to call this method multiple times. Database operations on a
+        closed connection raise ``RuntimeError: Connection is closed``.
+
+        Examples
+        --------
+        >>> import lancedb
+        >>> db = lancedb.connect("memory://")
+        >>> db.close()
+        >>> db.is_open()
+        False
+        """
+        pass
 
     def list_namespaces(
         self,
@@ -1275,6 +1316,14 @@ class LanceDBConnection(DBConnection):
     def __repr__(self) -> str:
         return f"{self.__class__.__name__}(uri={self._conn.uri!r})"
 
+    @override
+    def is_open(self) -> bool:
+        return self._conn.is_open()
+
+    @override
+    def close(self) -> None:
+        self._conn.close()
+
     @override
     def serialize(self) -> str:
         import json

python/python/lancedb/namespace.py

@@ -489,6 +489,14 @@ class LanceNamespaceDBConnection(DBConnection):
             )
         self._uri = self._inner.uri
 
+    @override
+    def is_open(self) -> bool:
+        return self._inner.is_open()
+
+    @override
+    def close(self) -> None:
+        self._inner.close()
+
     @override
     def serialize(self) -> str:
         import json

python/python/lancedb/remote/db.py

@@ -1054,6 +1054,10 @@ class RemoteDBConnection(DBConnection):
         """
         return LOOP.run(self._conn.namespace_client())
 
-    async def close(self):
-        """Close the connection to the database."""
+    @override
+    def is_open(self) -> bool:
+        return self._conn.is_open()
+
+    @override
+    def close(self) -> None:
         self._conn.close()

python/python/tests/test_db.py

@@ -2,6 +2,7 @@
 # SPDX-FileCopyrightText: Copyright The LanceDB Authors
 
 
+import contextlib
 import inspect
 import re
 import sys
@@ -405,6 +406,49 @@ async def test_connect(tmp_path):
     assert str(db) == f"ListingDatabase(uri={tmp_path}, read_consistency_interval=5s)"
 
 
+@pytest.fixture(params=["memory", "local", "namespace", "from_inner"])
+def sync_db(request, tmp_path):
+    if request.param == "memory":
+        return lancedb.connect("memory://")
+    if request.param == "namespace":
+        return lancedb.connect_namespace("dir", {"root": str(tmp_path)})
+    db = lancedb.connect(tmp_path)
+    if request.param == "from_inner":
+        return lancedb.db.LanceDBConnection.from_inner(db._inner, None)
+    return db
+
+
+def test_sync_close(sync_db):
+    assert sync_db.is_open()
+    assert sync_db.close() is None
+    assert not sync_db.is_open()
+    assert sync_db.close() is None
+
+    with pytest.warns(DeprecationWarning, match="table_names"):
+        with pytest.raises(RuntimeError, match="Connection is closed"):
+            sync_db.table_names()
+    with pytest.raises(RuntimeError, match="Connection is closed"):
+        sync_db.list_tables()
+    with pytest.raises(RuntimeError, match="Connection is closed"):
+        sync_db.open_table("test")
+
+
+@pytest.mark.parametrize("raise_error", [False, True])
+def test_sync_context_manager(sync_db, raise_error):
+    with contextlib.ExitStack() as stack:
+        if raise_error:
+            stack.enter_context(pytest.raises(ValueError, match="test error"))
+        with sync_db as db:
+            assert db is sync_db
+            assert db.is_open()
+            if raise_error:
+                raise ValueError("test error")
+
+    assert not sync_db.is_open()
+    with pytest.raises(RuntimeError, match="Connection is closed"):
+        sync_db.list_tables()
+
+
 @pytest.mark.asyncio
 async def test_close(mem_db_async: lancedb.AsyncConnection):
     assert mem_db_async.is_open()

python/python/tests/test_remote_db.py

@@ -2145,15 +2145,46 @@ async def test_header_provider_overrides_static_headers():
 
 
 def test_close():
-    """Test that close() works without AttributeError."""
-    import asyncio
-
     def handler(req):
         req.send_response(200)
         req.end_headers()
 
     with mock_lancedb_connection(handler) as db:
-        asyncio.run(db.close())
+        assert db.close() is None
+        assert not db.is_open()
+        assert db.close() is None
+
+        with pytest.warns(DeprecationWarning, match="table_names"):
+            with pytest.raises(RuntimeError, match="Connection is closed"):
+                db.table_names()
+        with pytest.raises(RuntimeError, match="Connection is closed"):
+            db.list_tables()
+        with pytest.raises(RuntimeError, match="Connection is closed"):
+            db.open_table("test")
+
+
+@pytest.mark.parametrize("raise_error", [False, True])
+def test_sync_context_manager(raise_error):
+    def handler(req):
+        req.send_response(200)
+        req.send_header("Content-Type", "application/json")
+        req.end_headers()
+        req.wfile.write(b'{"tables": []}')
+
+    with mock_lancedb_connection(handler) as db:
+        with contextlib.ExitStack() as stack:
+            if raise_error:
+                stack.enter_context(pytest.raises(ValueError, match="test error"))
+            with db as conn:
+                assert conn is db
+                assert conn.is_open()
+                assert conn.list_tables().tables == []
+                if raise_error:
+                    raise ValueError("test error")
+
+        assert not db.is_open()
+        with pytest.raises(RuntimeError, match="Connection is closed"):
+            db.list_tables()
 
 
 @pytest.mark.parametrize("exception", [KeyboardInterrupt, SystemExit, GeneratorExit])