Use the frame start once we've found it

tests/unit/test_serial_connection.py

@@ -0,0 +1,31 @@
+import asyncio
+
+import pytest
+
+from meshcore.serial_cx import SerialConnection
+
+
+class RecordingReader:
+    def __init__(self):
+        self.frames = []
+
+    async def handle_rx(self, data):
+        self.frames.append(bytes(data))
+
+
+@pytest.mark.asyncio
+async def test_handle_rx_discards_leading_junk_before_frame_start():
+    conn = SerialConnection("/dev/null", 115200)
+    reader = RecordingReader()
+    conn.set_reader(reader)
+
+    payload = b"\x00\x01\x02\x53"
+    frame = b"\x3e" + len(payload).to_bytes(2, "little") + payload
+
+    conn.handle_rx(b"junk bytes\r\n" + frame)
+    await asyncio.sleep(0)
+
+    assert reader.frames == [payload]
+    assert conn.header == b""
+    assert conn.inframe == b""
+    assert conn.frame_expected_size == 0