fix: handle random length bytes before version bytes

roborock/protocol.py

@@ -341,9 +341,32 @@ class PrefixedStruct(Struct):
     def _parse(self, stream, context, path):
         subcon1 = Peek(Optional(Bytes(3)))
         peek_version = subcon1.parse_stream(stream, **context)
-        if peek_version not in (b"1.0", b"A01", b"B01", b"L01"):
-            subcon2 = Bytes(4)
-            subcon2.parse_stream(stream, **context)
+
+        valid_versions = (b"1.0", b"A01", b"B01", b"L01")
+        if peek_version not in valid_versions:
+            # Current stream position does not start with a valid version.
+            # Scan forward to find one.
+            current_pos = stream_tell(stream, path)
+            # Read remaining data to find a valid header
+            data = stream.read()
+
+            start_index = -1
+            # Find the earliest occurrence of any valid version
+            candidates = [idx for v in valid_versions if (idx := data.find(v)) != -1]
+            if candidates:
+                start_index = min(candidates)
+
+            if start_index != -1:
+                # Found a valid version header at `start_index`.
+                # Seek to that position (original_pos + index).
+                if start_index != 4:
+                    # 4 is the typical/expected amount we prune off,
+                    # therefore, we only want a debug if we have a different length.
+                    _LOGGER.debug("Stripping %d bytes of invalid data from stream", start_index)
+                stream_seek(stream, current_pos + start_index, 0, path)
+            else:
+                _LOGGER.debug("No valid version header found in stream, continuing anyways...")
+
         return super()._parse(stream, context, path)
 
     def _build(self, obj, stream, context, path):

tests/devices/test_local_decoder_padding.py

@@ -0,0 +1,89 @@
+from roborock.protocol import create_local_decoder, create_local_encoder
+from roborock.roborock_message import RoborockMessage, RoborockMessageProtocol
+
+TEST_LOCAL_KEY = "local_key"
+
+
+def test_decoder_clean_message():
+    encoder = create_local_encoder(TEST_LOCAL_KEY)
+    decoder = create_local_decoder(TEST_LOCAL_KEY)
+
+    msg = RoborockMessage(
+        protocol=RoborockMessageProtocol.RPC_REQUEST,
+        payload=b"test_payload",
+        version=b"1.0",
+        seq=1,
+        random=123,
+    )
+    encoded = encoder(msg)
+
+    decoded = decoder(encoded)
+    assert len(decoded) == 1
+    assert decoded[0].payload == b"test_payload"
+
+
+def test_decoder_4byte_padding():
+    """Test existing behavior: 4 byte padding should be skipped."""
+    encoder = create_local_encoder(TEST_LOCAL_KEY)
+    decoder = create_local_decoder(TEST_LOCAL_KEY)
+
+    msg = RoborockMessage(
+        protocol=RoborockMessageProtocol.RPC_REQUEST,
+        payload=b"test_payload",
+        version=b"1.0",
+    )
+    encoded = encoder(msg)
+
+    # Prepend 4 bytes of garbage
+    garbage = b"\x00\x00\x05\xa1"
+    data = garbage + encoded
+
+    decoded = decoder(data)
+    assert len(decoded) == 1
+    assert decoded[0].payload == b"test_payload"
+
+
+def test_decoder_variable_padding():
+    """Test proposed fix: variable length padding should be skipped."""
+    encoder = create_local_encoder(TEST_LOCAL_KEY, connect_nonce=123, ack_nonce=456)
+    decoder = create_local_decoder(TEST_LOCAL_KEY, connect_nonce=123, ack_nonce=456)
+
+    msg = RoborockMessage(
+        protocol=RoborockMessageProtocol.RPC_REQUEST,
+        payload=b"test_payload",
+        version=b"L01",
+    )
+    encoded = encoder(msg)
+
+    # Prepend 6 bytes of garbage
+    garbage = b"\x00\x00\x05\xa1\xff\xff"
+    data = garbage + encoded
+
+    decoded = decoder(data)
+    assert len(decoded) == 1
+    assert decoded[0].payload == b"test_payload"
+
+
+def test_decoder_split_padding_variable():
+    """Test variable padding split across chunks."""
+    encoder = create_local_encoder(TEST_LOCAL_KEY, connect_nonce=123, ack_nonce=456)
+    decoder = create_local_decoder(TEST_LOCAL_KEY, connect_nonce=123, ack_nonce=456)
+
+    msg = RoborockMessage(
+        protocol=RoborockMessageProtocol.RPC_REQUEST,
+        payload=b"test_payload",
+        version=b"L01",
+    )
+    encoded = encoder(msg)
+
+    garbage = b"\x00\x00\x05\xa1\xff\xff"  # 6 bytes
+
+    # Send garbage
+    decoded1 = decoder(garbage)
+    assert len(decoded1) == 0
+
+    # Send message
+    decoded2 = decoder(encoded)
+
+    assert len(decoded2) == 1
+    assert decoded2[0].payload == b"test_payload"