Drop the socket when a send fails, so the client can reconnect

pymodbus/client/tcp.py

@@ -219,7 +219,13 @@ def send(self, request, addr: tuple | None = None):
         if not self.socket:
             raise ConnectionException(str(self))
         if request:
-            return self.socket.send(request)
+            try:
+                return self.socket.send(request)
+            except (BlockingIOError, InterruptedError):
+                raise
+            except OSError:
+                self.close()
+                raise ConnectionException(str(self)) from None
         return 0
 
     def recv(self, size: int | None) -> bytes:

test/client/test_client_sync.py

@@ -150,6 +150,30 @@ def test_tcp_client_send(self):
         assert not client.send(b"")
         assert client.send(b"1234") == 4
 
+    def test_tcp_client_send_drops_socket_on_os_error(self):
+        """Test that a socket the OS tore down is not left in place as connected."""
+        client = ModbusTcpClient("127.0.0.1")
+        mock_socket = mock.MagicMock()
+        mock_socket.send.side_effect = BrokenPipeError(32, "Broken pipe")
+        client.socket = mock_socket
+        with pytest.raises(ConnectionException):
+            client.send(b"1234")
+        assert not client.connected
+        assert client.socket is None
+
+    def test_tcp_client_send_keeps_socket_on_transient_error(self):
+        """Test that a transient write error leaves a healthy socket in place."""
+        client = ModbusTcpClient("127.0.0.1")
+        mock_socket = mock.MagicMock()
+        mock_socket.send.side_effect = BlockingIOError(
+            11, "Resource temporarily unavailable"
+        )
+        client.socket = mock_socket
+        with pytest.raises(BlockingIOError):
+            client.send(b"1234")
+        assert client.connected
+        assert client.socket is mock_socket
+
     @mock.patch("pymodbus.client.tcp.time")
     @mock.patch("pymodbus.client.tcp.select")
     def test_tcp_client_recv(self, mock_select, mock_time):