Raise DecodeError when zero-column RowsResponse has no end marker

Previously, a truncated zero-column result with no DONE/PART marker
was silently accepted as has_more=False. Now raises DecodeError,
consistent with the non-zero-column path.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: antoineleclair

src/dqlitewire/messages/responses.py

@@ -290,20 +290,21 @@ def decode_body(
         # Zero-column results cannot have row data (each row would be zero
         # bytes), so skip the row loop and consume the end marker directly.
         if column_count == 0:
-            from dqlitewire.constants import ROW_DONE_BYTE, ROW_PART_BYTE
-
-            has_more = False
-            if offset < len(data):
-                marker_byte = data[offset]
-                if marker_byte == ROW_DONE_BYTE:
-                    has_more = False
-                elif marker_byte == ROW_PART_BYTE:
-                    has_more = True
-                else:
-                    raise DecodeError(
-                        f"Expected DONE or PART marker for zero-column result, "
-                        f"got 0x{marker_byte:02x}"
-                    )
+            from dqlitewire.constants import ROW_DONE_BYTE, ROW_PART_BYTE, WORD_SIZE
+
+            if offset + WORD_SIZE > len(data):
+                raise DecodeError(
+                    "RowsResponse body exhausted without end marker (zero-column result)"
+                )
+            marker_byte = data[offset]
+            if marker_byte == ROW_DONE_BYTE:
+                has_more = False
+            elif marker_byte == ROW_PART_BYTE:
+                has_more = True
+            else:
+                raise DecodeError(
+                    f"Expected DONE or PART marker for zero-column result, got 0x{marker_byte:02x}"
+                )
             return cls(
                 column_names=[],
                 column_types=[],
@@ -383,20 +384,21 @@ def decode_rows_continuation(
         column_types: list[ValueType] = []
 
         if column_count == 0:
-            from dqlitewire.constants import ROW_DONE_BYTE, ROW_PART_BYTE
-
-            has_more = False
-            if offset < len(data):
-                marker_byte = data[offset]
-                if marker_byte == ROW_DONE_BYTE:
-                    has_more = False
-                elif marker_byte == ROW_PART_BYTE:
-                    has_more = True
-                else:
-                    raise DecodeError(
-                        f"Expected DONE or PART marker for zero-column result, "
-                        f"got 0x{marker_byte:02x}"
-                    )
+            from dqlitewire.constants import ROW_DONE_BYTE, ROW_PART_BYTE, WORD_SIZE
+
+            if offset + WORD_SIZE > len(data):
+                raise DecodeError(
+                    "RowsResponse continuation exhausted without end marker (zero-column result)"
+                )
+            marker_byte = data[offset]
+            if marker_byte == ROW_DONE_BYTE:
+                has_more = False
+            elif marker_byte == ROW_PART_BYTE:
+                has_more = True
+            else:
+                raise DecodeError(
+                    f"Expected DONE or PART marker for zero-column result, got 0x{marker_byte:02x}"
+                )
             return cls(
                 column_names=column_names,
                 column_types=[],

tests/test_messages_responses.py

@@ -302,6 +302,32 @@ def test_zero_columns_malformed_no_infinite_loop(self) -> None:
         with pytest.raises(DecodeError, match="Expected DONE or PART marker"):
             RowsResponse.decode_body(data)
 
+    def test_zero_columns_missing_marker_raises(self) -> None:
+        """Zero-column result with no end marker should raise DecodeError."""
+        import pytest
+
+        from dqlitewire.exceptions import DecodeError
+        from dqlitewire.types import encode_uint64
+
+        # column_count=0, but no marker follows
+        data = encode_uint64(0)
+        with pytest.raises(DecodeError, match="end marker"):
+            RowsResponse.decode_body(data)
+
+    def test_zero_columns_continuation_missing_marker_raises(self) -> None:
+        """Zero-column continuation with no end marker should raise DecodeError."""
+        import pytest
+
+        from dqlitewire.exceptions import DecodeError
+
+        # Empty data for a zero-column continuation
+        with pytest.raises(DecodeError, match="end marker"):
+            RowsResponse.decode_rows_continuation(
+                data=b"",
+                column_names=[],
+                column_count=0,
+            )
+
     def test_decode_rows_continuation(self) -> None:
         """Continuation messages (after PART marker) have rows but no column header."""
         from dqlitewire.tuples import encode_row_header, encode_row_values