Assign description atomically with fetched rows in async adapter

AsyncAdaptedCursor.execute and .executemany previously set
self.description from the underlying cursor before awaiting
fetchall. If fetchall raised (CancelledError from an outer
timeout, mid-stream server fault, etc.), the adapter was left
with description populated and _rows empty — a state
SQLAlchemy's Result layer treats as an empty result set,
indistinguishable from "execute succeeded but fetched no rows".
Fetch first into a local, then assign both together so a raise
at fetch time leaves description at None.

Author: antoineleclair

src/sqlalchemydqlite/aio.py

@@ -79,8 +79,16 @@ def execute(self, operation: str, parameters: Any = None) -> None:
                 await_only(cursor.execute(operation))
 
             if cursor.description:
+                # Fetch first, assign atomically. If ``fetchall`` raises
+                # (CancelledError from an outer timeout, server fault
+                # mid-stream, etc.), ``self.description`` must not be left
+                # set while ``self._rows`` is still empty — SQLAlchemy's
+                # Result layer treats (description, empty rows) as an
+                # empty result set, indistinguishable from "execute
+                # succeeded but fetched no rows".
+                fetched = deque(await_only(cursor.fetchall()))
                 self.description = cursor.description
-                self._rows = deque(await_only(cursor.fetchall()))
+                self._rows = fetched
             else:
                 self.lastrowid = cursor.lastrowid
                 self.rowcount = cursor.rowcount
@@ -105,8 +113,12 @@ def executemany(self, operation: str, seq_of_parameters: Iterable[Sequence[Any]]
             # when SQLAlchemy's insertmanyvalues + RETURNING path is
             # driven through the async engine.
             if cursor.description:
+                # Same fetch-first-then-assign pattern as ``execute``:
+                # a raise from ``fetchall`` must not leave description
+                # populated with empty rows.
+                fetched = deque(await_only(cursor.fetchall()))
                 self.description = cursor.description
-                self._rows = deque(await_only(cursor.fetchall()))
+                self._rows = fetched
             else:
                 self.lastrowid = cursor.lastrowid
                 self.rowcount = cursor.rowcount

tests/test_async_adapter.py

@@ -463,6 +463,41 @@ def test_cursor_returns_async_adapted_cursor_wrapping_inner(self) -> None:
         assert isinstance(cursor, AsyncAdaptedCursor)
 
 
+class TestAsyncAdaptedCursorDescriptionConsistency:
+    """If ``fetchall`` raises mid-call, ``description`` must not be
+    left set with an empty ``_rows`` buffer — SQLAlchemy's Result
+    layer treats that pair as an empty result set, indistinguishable
+    from "execute succeeded but fetched no rows"."""
+
+    def test_fetchall_raise_leaves_description_none(self) -> None:
+        import pytest
+
+        cursor = _make_cursor()
+
+        mock_inner = MagicMock()
+        mock_inner.description = (("id", 1, None, None, None, None, None),)
+        mock_inner.execute.return_value = None
+        mock_inner.close.return_value = None
+        # ``fetchall`` raises before returning — the adapter must not
+        # commit ``description`` nor leave ``_rows`` in a half-assigned
+        # state.
+        mock_inner.fetchall.side_effect = RuntimeError("synthetic fetchall failure")
+        cursor._connection.cursor.return_value = mock_inner
+
+        with (
+            patch("sqlalchemydqlite.aio.await_only", side_effect=_run_sync),
+            pytest.raises(RuntimeError, match="synthetic fetchall failure"),
+        ):
+            cursor.execute("SELECT id FROM t")
+
+        assert cursor.description is None, (
+            "description must roll back to None when fetchall raises, "
+            "so SQLAlchemy's Result layer cannot misread the cursor as "
+            "holding an empty result set"
+        )
+        assert len(cursor._rows) == 0
+
+
 class TestAsyncAdaptedCursorDescriptionType:
     """``cursor.description`` is a PEP 249 sequence of sequences. The
     adapter must accept (and pass through) any sequence the underlying