feat: add max_message_size parameter to MessageDecoder

MessageDecoder now accepts a max_message_size parameter (default 64 MiB)
that is forwarded to the internal ReadBuffer. Previously the only way
to customize the limit was to access the private _buffer attribute.

Closes #138

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

Author: antoineleclair

src/dqlitewire/codec.py

@@ -164,7 +164,12 @@ class MessageDecoder:
     layer that owns the socket.
     """
 
-    def __init__(self, is_request: bool = False, version: int = PROTOCOL_VERSION) -> None:
+    def __init__(
+        self,
+        is_request: bool = False,
+        version: int = PROTOCOL_VERSION,
+        max_message_size: int = ReadBuffer.DEFAULT_MAX_MESSAGE_SIZE,
+    ) -> None:
         """Initialize decoder.
 
         Args:
@@ -174,13 +179,16 @@ def __init__(self, is_request: bool = False, version: int = PROTOCOL_VERSION) ->
                     Defaults to PROTOCOL_VERSION (1). Use PROTOCOL_VERSION_LEGACY
                     for pre-1.0 dqlite servers (affects LeaderResponse format).
                     Ignored for request decoders (version comes from handshake).
+            max_message_size: Maximum allowed message size in bytes.
+                    Defaults to 64 MiB. Messages exceeding this limit are
+                    rejected with DecodeError.
         """
         if not is_request and version not in _SUPPORTED_VERSIONS:
             raise ProtocolError(
                 f"Unsupported protocol version: {version:#x}. "
                 f"Supported: {', '.join(f'{v:#x}' for v in sorted(_SUPPORTED_VERSIONS))}"
             )
-        self._buffer = ReadBuffer()
+        self._buffer = ReadBuffer(max_message_size=max_message_size)
         self._is_request = is_request
         self._type_map = REQUEST_TYPES if is_request else RESPONSE_TYPES
         # For client-side decoders, version is set from the constructor parameter.

tests/test_codec.py

@@ -95,6 +95,26 @@ def test_decoder_request_ignores_version(self) -> None:
         decoder = MessageDecoder(is_request=True, version=0xDEADBEEF)
         assert decoder.version is None  # not set until handshake
 
+    def test_decoder_custom_max_message_size(self) -> None:
+        """138: max_message_size should be configurable via constructor."""
+        import struct
+
+        from dqlitewire.exceptions import DecodeError
+
+        decoder = MessageDecoder(max_message_size=64)
+        # Feed a message larger than 64 bytes
+        oversized = struct.pack("<IBBH", 100, 0, 0, 0)  # 808-byte body
+        decoder.feed(oversized)
+        with pytest.raises(DecodeError, match="exceeds maximum"):
+            decoder.decode()
+
+    def test_decoder_default_max_message_size(self) -> None:
+        """138: default max_message_size should match ReadBuffer default."""
+        from dqlitewire.buffer import ReadBuffer
+
+        decoder = MessageDecoder()
+        assert decoder._buffer._max_message_size == ReadBuffer.DEFAULT_MAX_MESSAGE_SIZE
+
     def test_decode_handshake(self) -> None:
         decoder = MessageDecoder(is_request=True)
         decoder.feed(PROTOCOL_VERSION.to_bytes(8, "little"))