Half-close TCP connections so the last write is not lost

close() went straight to shutdown(SHUT_RDWR) + close(). Doing that while either
side still has unread data makes the stack send RST rather than FIN, and the
device's FromRadio stream means there is essentially always unread data.

Winsock discards data that was received but not yet read when an RST arrives, so
an admin message written moments earlier is thrown away before the device reads
it. Linux delivers the buffered bytes before reporting ECONNRESET, which is why
this only surfaced on Windows: every one-shot TCP write there (--set, --seturl,
--sendtext) reported success and did nothing.

writeConfig() does not wait for an ack, so close() runs microseconds after
sendall(). The device cannot compensate; it polls every 5ms and the RST beats its
next read.

Send FIN first with shutdown(SHUT_WR), which lets the device consume what we
wrote, wait briefly for the reader thread to drain and exit, then tear down
exactly as before. The full shutdown is kept so a reader still blocked in recv()
is unblocked for the join in StreamInterface.close(). The wait is bounded: the
device is not obliged to close just because we half-closed.

Fixes #957

meshtastic/tcp_interface.py

@@ -11,6 +11,11 @@ from typing import Optional
 from meshtastic.stream_interface import StreamInterface
 
 DEFAULT_TCP_PORT = 4403
+
+# How long close() gives the device to consume what we last wrote before forcing
+# the connection down. See close() for why this exists.
+GRACEFUL_CLOSE_TIMEOUT = 0.25
+
 logger = logging.getLogger(__name__)
 
 
@@ -80,6 +85,18 @@ class TCPInterface(StreamInterface):
         server_address = (self.hostname, self.portNumber)
         self.socket = socket.create_connection(server_address)
 
+    def _wait_for_reader_exit(self, timeout: float) -> None:
+        """Wait briefly for the reader thread to drain and exit after a half-close.
+
+        Returns as soon as it exits, or after timeout: the device is not obliged
+        to close just because we did.
+        """
+        rx = getattr(self, "_rxThread", None)
+        if rx is None or rx is threading.current_thread():
+            return
+        with contextlib.suppress(Exception):
+            rx.join(timeout)
+
     def close(self) -> None:
         """Close a connection to the device."""
         logger.debug("Closing TCP stream")
@@ -87,6 +104,20 @@ class TCPInterface(StreamInterface):
         # Therefore force a shutdown first to unblock reader thread reads.
         self._wantExit = True
         if self.socket is not None:
+            # Half-close first. shutdown(SHUT_WR) sends FIN, which tells the
+            # device we are done writing and lets it consume what we last wrote
+            # before the connection goes away -- typically the admin message a
+            # one-shot command such as `--set` just sent.
+            #
+            # Going straight to shutdown(SHUT_RDWR) + close() while either side
+            # still has unread data makes the stack send RST instead. Winsock
+            # then discards data the peer had already received but not yet read,
+            # so the write is lost; Linux delivers it before reporting
+            # ECONNRESET, which is why this only bites on Windows.
+            with contextlib.suppress(Exception):
+                self.socket.shutdown(socket.SHUT_WR)
+            self._wait_for_reader_exit(GRACEFUL_CLOSE_TIMEOUT)
+
             with contextlib.suppress(
                 Exception
             ):  # Ignore errors in shutdown, because we might have a race with the server

meshtastic/tests/test_tcp_interface.py

@@ -1,6 +1,7 @@
 """Meshtastic unit tests for tcp_interface.py"""
 
 import re
+import socket
 from unittest.mock import MagicMock, patch
 
 import pytest
@@ -78,6 +79,56 @@ def test_TCPInterface_close_shutdowns_socket_before_super_close():
     assert iface.socket is None
 
 
+@pytest.mark.unit
+def test_TCPInterface_close_half_closes_before_shutdown():
+    """Close should send FIN and let the device drain before forcing the link down.
+
+    Going straight to shutdown(SHUT_RDWR)/close() with data still unread makes the
+    stack send RST, and Winsock then discards data the device had received but not
+    yet read, silently losing writes such as `--set`.
+    """
+    iface = TCPInterface(hostname="localhost", noProto=True, connectNow=False)
+    sock = MagicMock()
+    iface.socket = sock
+    call_order = []
+
+    with patch.object(TCPInterface, "_socket_shutdown", autospec=True) as mock_shutdown:
+        with patch.object(
+            TCPInterface, "_wait_for_reader_exit", autospec=True
+        ) as mock_wait:
+            with patch(
+                "meshtastic.stream_interface.StreamInterface.close", autospec=True
+            ) as mock_super_close:
+                sock.shutdown.side_effect = lambda how: call_order.append(f"shutdown_{how}")
+                mock_wait.side_effect = lambda _self, _t: call_order.append("wait")
+                mock_shutdown.side_effect = lambda _self: call_order.append("full_shutdown")
+                mock_super_close.side_effect = lambda _self: call_order.append("super_close")
+
+                iface.close()
+
+    assert call_order == [
+        f"shutdown_{socket.SHUT_WR}",
+        "wait",
+        "full_shutdown",
+        "super_close",
+    ]
+
+
+@pytest.mark.unit
+def test_TCPInterface_close_survives_half_close_failure():
+    """A peer that already vanished must not break close()."""
+    iface = TCPInterface(hostname="localhost", noProto=True, connectNow=False)
+    sock = MagicMock()
+    sock.shutdown.side_effect = OSError("already gone")
+    iface.socket = sock
+
+    with patch("meshtastic.stream_interface.StreamInterface.close", autospec=True):
+        iface.close()  # must not raise
+
+    sock.close.assert_called_once()
+    assert iface.socket is None
+
+
 @pytest.mark.unit
 def test_TCPInterface_reconnect():
     """Test that _reconnect correctly reconnects"""