Add PROTOCOL_VERSION_LEGACY constant and configurable encoder version

Go defines VersionLegacy = 0x86104dd760433fe5 for pre-1.0 dqlite
servers and falls back to it when version 1 is rejected. Added the
legacy constant and made MessageEncoder accept a version parameter
so callers can implement the same fallback strategy.

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

Author: antoineleclair

src/dqlitewire/__init__.py

@@ -4,6 +4,7 @@
 from dqlitewire.codec import MessageDecoder, MessageEncoder
 from dqlitewire.constants import (
     PROTOCOL_VERSION,
+    PROTOCOL_VERSION_LEGACY,
     ROW_DONE_BYTE,
     ROW_DONE_MARKER,
     ROW_PART_BYTE,
@@ -18,6 +19,7 @@
     "MessageDecoder",
     "MessageEncoder",
     "PROTOCOL_VERSION",
+    "PROTOCOL_VERSION_LEGACY",
     "ProtocolError",
     "DecodeError",
     "EncodeError",

src/dqlitewire/codec.py

@@ -93,6 +93,16 @@
 class MessageEncoder:
     """Encodes messages to wire protocol format."""
 
+    def __init__(self, version: int = PROTOCOL_VERSION) -> None:
+        """Initialize encoder.
+
+        Args:
+            version: Protocol version to use in handshake. Defaults to
+                     PROTOCOL_VERSION (1). Use PROTOCOL_VERSION_LEGACY
+                     (0x86104dd760433fe5) for pre-1.0 dqlite servers.
+        """
+        self._version = version
+
     def encode(self, message: Message) -> bytes:
         """Encode a message to bytes."""
         return message.encode()
@@ -102,7 +112,7 @@ def encode_handshake(self) -> bytes:
 
         Must be sent before any other message.
         """
-        return PROTOCOL_VERSION.to_bytes(8, "little")
+        return self._version.to_bytes(8, "little")
 
 
 class MessageDecoder:

src/dqlitewire/constants.py

@@ -2,8 +2,9 @@
 
 from enum import IntEnum
 
-# Protocol version
+# Protocol versions
 PROTOCOL_VERSION = 1
+PROTOCOL_VERSION_LEGACY = 0x86104DD760433FE5  # Pre-1.0 dqlite servers
 
 # Word size in bytes (all messages are padded to 8-byte boundaries)
 WORD_SIZE = 8

tests/test_codec.py

@@ -8,7 +8,7 @@
     decode_message,
     encode_message,
 )
-from dqlitewire.constants import PROTOCOL_VERSION
+from dqlitewire.constants import PROTOCOL_VERSION, PROTOCOL_VERSION_LEGACY
 from dqlitewire.exceptions import DecodeError
 from dqlitewire.messages import (
     ClientRequest,
@@ -31,6 +31,17 @@ def test_encode_handshake(self) -> None:
         assert len(handshake) == 8
         assert int.from_bytes(handshake, "little") == PROTOCOL_VERSION
 
+    def test_encode_handshake_legacy(self) -> None:
+        """Encoder with legacy version should produce the legacy handshake word."""
+        encoder = MessageEncoder(version=PROTOCOL_VERSION_LEGACY)
+        handshake = encoder.encode_handshake()
+        assert len(handshake) == 8
+        assert int.from_bytes(handshake, "little") == PROTOCOL_VERSION_LEGACY
+
+    def test_legacy_version_constant_matches_go(self) -> None:
+        """PROTOCOL_VERSION_LEGACY must match Go's VersionLegacy = 0x86104dd760433fe5."""
+        assert PROTOCOL_VERSION_LEGACY == 0x86104DD760433FE5
+
     def test_encoder_has_no_buffer_attribute(self) -> None:
         """MessageEncoder should not have unused _buffer attribute."""
         encoder = MessageEncoder()