Partially implement flush for RFC2217

serialx/platforms/serial_rfc2217/__init__.py

@@ -665,7 +665,12 @@ def _get_modem_pins(self) -> ModemPins:
         return self._engine.get_modem_pins()
 
     def _flush(self) -> None:
-        """Flush write buffers (no-op, TCP handles buffering)."""
+        """Wait for the server to acknowledge all preceding writes."""
+        if not self._engine.negotiated:
+            return
+
+        # RFC2217 has no flush. Instead, we "flush" the pipe with a req/rsp sequence.
+        self._send_and_wait(SetBaudrateCmd(baudrate=self._baudrate))
 
 
 class _RFC2217ProxyProtocol(asyncio.Protocol):
@@ -1007,7 +1012,13 @@ def abort(self) -> None:
             self._tcp_connection_lost(None)
 
     async def flush(self) -> None:
-        """Flush write buffers (no-op, TCP transport handles buffering)."""
+        """Wait for the server to acknowledge all preceding writes."""
+        assert self._serial is not None
+        if not self._serial._engine.negotiated:
+            return
+
+        # RFC2217 has no flush. Instead, we "flush" the pipe with a req/rsp sequence.
+        await self._send_and_wait(SetBaudrateCmd(baudrate=self._serial._baudrate))
 
     def get_write_buffer_size(self) -> int:
         """Get the number of bytes currently in the write buffer."""

tests/test_sync_transports.py

@@ -869,6 +869,11 @@ def test_fast_open_close(serial_pair: SerialPair) -> None:
     with Serial.from_url(serial_pair.left, baudrate=115200) as left:
         with Serial.from_url(serial_pair.right, baudrate=115200) as right:
             right.write(message)
+            right.flush()
+
+            # Some backends (notably complex chained RFC2217) lose data on close, making
+            # this test flaky without a tiny delay
+            time.sleep(0.01)
 
         assert left.readexactly(len(message)) == message