From 99d9f6b50afa65b442120169ddf1ef71b5f521fe Mon Sep 17 00:00:00 2001
From: puddly <32534428+puddly@users.noreply.github.com>
Date: Sun, 3 May 2026 23:50:54 -0400
Subject: [PATCH 1/2] RFC2217 flush

---
 serialx/platforms/serial_rfc2217/__init__.py | 16 ++++++++++++++--
 tests/test_sync_transports.py                |  5 +++++
 2 files changed, 19 insertions(+), 2 deletions(-)

diff --git a/serialx/platforms/serial_rfc2217/__init__.py b/serialx/platforms/serial_rfc2217/__init__.py
index ea07e16..ffe08a3 100644
--- a/serialx/platforms/serial_rfc2217/__init__.py
+++ b/serialx/platforms/serial_rfc2217/__init__.py
@@ -52,6 +52,7 @@
     SetModemstateMaskCmd,
     SetParityCmd,
     SetStopsizeCmd,
+    SignatureCmd,
     TelnetCmdId,
     TelnetCommand,
     TelnetOption,
@@ -665,7 +666,12 @@ 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(SignatureCmd())
 
 
 class _RFC2217ProxyProtocol(asyncio.Protocol):
@@ -1007,7 +1013,13 @@ 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(SignatureCmd())
 
     def get_write_buffer_size(self) -> int:
         """Get the number of bytes currently in the write buffer."""
diff --git a/tests/test_sync_transports.py b/tests/test_sync_transports.py
index 641c7c8..27e04ad 100644
--- a/tests/test_sync_transports.py
+++ b/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
 

From ee9ef582c1de71904df301f49dce32805e5ce57e Mon Sep 17 00:00:00 2001
From: puddly <32534428+puddly@users.noreply.github.com>
Date: Mon, 4 May 2026 01:08:37 -0400
Subject: [PATCH 2/2] Try a different command

---
 serialx/platforms/serial_rfc2217/__init__.py | 5 ++---
 1 file changed, 2 insertions(+), 3 deletions(-)

diff --git a/serialx/platforms/serial_rfc2217/__init__.py b/serialx/platforms/serial_rfc2217/__init__.py
index ffe08a3..d9f7eb9 100644
--- a/serialx/platforms/serial_rfc2217/__init__.py
+++ b/serialx/platforms/serial_rfc2217/__init__.py
@@ -52,7 +52,6 @@
     SetModemstateMaskCmd,
     SetParityCmd,
     SetStopsizeCmd,
-    SignatureCmd,
     TelnetCmdId,
     TelnetCommand,
     TelnetOption,
@@ -671,7 +670,7 @@ def _flush(self) -> None:
             return
 
         # RFC2217 has no flush. Instead, we "flush" the pipe with a req/rsp sequence.
-        self._send_and_wait(SignatureCmd())
+        self._send_and_wait(SetBaudrateCmd(baudrate=self._baudrate))
 
 
 class _RFC2217ProxyProtocol(asyncio.Protocol):
@@ -1019,7 +1018,7 @@ async def flush(self) -> None:
             return
 
         # RFC2217 has no flush. Instead, we "flush" the pipe with a req/rsp sequence.
-        await self._send_and_wait(SignatureCmd())
+        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."""