fix: validate handshake version in MessageEncoder constructor

MessageEncoder now rejects unsupported protocol versions at construction
time rather than silently producing invalid handshake bytes that cause
the server to drop the connection with no useful error.

Closes #102

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

Author: antoineleclair

src/dqlitewire/codec.py

@@ -119,6 +119,11 @@ def __init__(self, version: int = PROTOCOL_VERSION) -> None:
                      PROTOCOL_VERSION (1). Use PROTOCOL_VERSION_LEGACY
                      (0x86104dd760433fe5) for pre-1.0 dqlite servers.
         """
+        if 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._version = version
 
     def encode(self, message: Message) -> bytes:

tests/test_codec.py

@@ -9,7 +9,7 @@
     encode_message,
 )
 from dqlitewire.constants import PROTOCOL_VERSION, PROTOCOL_VERSION_LEGACY
-from dqlitewire.exceptions import DecodeError
+from dqlitewire.exceptions import DecodeError, ProtocolError
 from dqlitewire.messages import (
     ClientRequest,
     DbResponse,
@@ -50,6 +50,21 @@ def test_encoder_has_no_buffer_attribute(self) -> None:
         encoder = MessageEncoder()
         assert not hasattr(encoder, "_buffer")
 
+    def test_encoder_rejects_invalid_version(self) -> None:
+        """102: invalid handshake version should raise at construction time."""
+        with pytest.raises(ProtocolError, match="Unsupported protocol version"):
+            MessageEncoder(version=0xDEADBEEF)
+
+    def test_encoder_accepts_legacy_version(self) -> None:
+        """102: PROTOCOL_VERSION_LEGACY must be accepted."""
+        encoder = MessageEncoder(version=PROTOCOL_VERSION_LEGACY)
+        assert encoder.encode_handshake() is not None
+
+    def test_encoder_accepts_default_version(self) -> None:
+        """102: default PROTOCOL_VERSION must be accepted."""
+        encoder = MessageEncoder(version=PROTOCOL_VERSION)
+        assert encoder.encode_handshake() is not None
+
     def test_encode_message(self) -> None:
         from dqlitewire.constants import RequestType
         from dqlitewire.messages.base import Header