fix: bail out when ROWS continuation makes no progress

A continuation frame with zero rows and has_more=True would otherwise
spin the client forever. This can happen on the server when the column
header alone exceeds the per-batch page buffer (query__batch emits a
partial frame with no encoded rows). Detect and raise ProtocolError
rather than livelock.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: antoineleclair

src/dqliteclient/protocol.py

@@ -201,6 +201,14 @@ async def query_sql(
         all_rows = list(response.rows)
         while response.has_more:
             next_response = await self._read_continuation()
+            if not next_response.rows and next_response.has_more:
+                # Server claimed "more coming" but delivered zero rows in a
+                # continuation frame. That would spin forever (known
+                # pathological case: column header larger than the server's
+                # page buffer). Bail out instead of livelocking.
+                raise ProtocolError(
+                    "ROWS continuation made no progress: frame had 0 rows and has_more=True"
+                )
             all_rows.extend(next_response.rows)
             response = next_response
 

tests/test_protocol.py

@@ -215,6 +215,36 @@ async def test_query_sql_reads_single_response(
         # Stray response must remain in the buffer, not silently consumed.
         assert protocol._decoder.has_message()
 
+    async def test_query_sql_raises_if_continuation_has_no_progress(
+        self,
+        protocol: DqliteProtocol,
+        mock_reader: AsyncMock,
+    ) -> None:
+        """A ROWS continuation that sets has_more=True but delivers 0 rows
+        must raise rather than loop forever. Known pathological server case:
+        column header alone exceeds the page buffer, so query__batch sends a
+        frame with no rows encoded but still marks it as partial.
+        """
+        from dqlitewire.constants import ValueType
+        from dqlitewire.messages import RowsResponse
+
+        first = RowsResponse(
+            column_names=["x"],
+            column_types=[ValueType.INTEGER],
+            rows=[],
+            has_more=True,
+        )
+        stuck = RowsResponse(
+            column_names=["x"],
+            column_types=[ValueType.INTEGER],
+            rows=[],
+            has_more=True,
+        )
+        mock_reader.read.return_value = first.encode() + stuck.encode()
+
+        with pytest.raises(ProtocolError, match="no progress|no rows"):
+            await protocol.query_sql(1, "SELECT x FROM wide_table")
+
     async def test_query_sql(
         self,
         protocol: DqliteProtocol,