fix: handle oversized continuation frames by clearing flag and poisoning

decode_continuation() now catches DecodeError from read_message() for
oversized continuation frames, clears _continuation_expected, and
poisons the buffer. Previously the decoder got stuck in a deadlock
where decode(), skip_message(), and decode_continuation() were all
blocked, with reset() as the only escape.

Closes #123

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

Author: antoineleclair

src/dqlitewire/codec.py

@@ -276,7 +276,17 @@ def decode_continuation(self) -> RowsResponse | None:
                 "decode_continuation() called but no ROWS continuation "
                 "is in progress. Use decode() for the initial message."
             )
-        data = self._buffer.read_message()
+        try:
+            data = self._buffer.read_message()
+        except DecodeError:
+            # Oversized continuation frame — no skip_message() recovery
+            # available during continuation mode. Clear the flag and
+            # poison so the caller knows reset() is required.
+            self._continuation_expected = False
+            self._buffer.poison(
+                DecodeError("Oversized ROWS continuation frame; call reset() to recover")
+            )
+            raise
         if data is None:
             return None
 

tests/test_codec.py

@@ -1726,3 +1726,39 @@ def test_decode_continuation_rejects_unsupported_schema(self) -> None:
 
         with pytest.raises(DecodeError, match="Unsupported schema version"):
             decoder.decode_continuation()
+
+    def test_oversized_continuation_frame_clears_flag_and_poisons(self) -> None:
+        """123: oversized continuation frame must clear flag and poison."""
+        import struct
+
+        from dqlitewire.constants import ValueType
+        from dqlitewire.exceptions import ProtocolError
+        from dqlitewire.messages.responses import RowsResponse
+
+        initial = RowsResponse(
+            column_names=["id"],
+            column_types=[ValueType.INTEGER],
+            rows=[[1]],
+            has_more=True,
+        )
+
+        decoder = MessageDecoder(is_request=False)
+        decoder._buffer._max_message_size = 128
+
+        # Oversized continuation header: 200 words = 1600 bytes > 128
+        oversized_header = struct.pack("<IBBH", 200, 7, 0, 0)  # type 7 = ROWS
+
+        decoder.feed(initial.encode() + oversized_header)
+
+        msg = decoder.decode()
+        assert isinstance(msg, RowsResponse) and msg.has_more
+
+        with pytest.raises(DecodeError, match="exceeds maximum"):
+            decoder.decode_continuation()
+
+        # Flag must be cleared and buffer poisoned
+        assert not decoder._continuation_expected
+        assert decoder.is_poisoned
+
+        with pytest.raises(ProtocolError, match="poisoned"):
+            decoder.decode()