fix(async): synchronize connect() promise with connection_made callback

pymodbus.log

@@ -0,0 +1,81 @@
+2026-09-10 01:22:16,121 DEBUG test_logging:69 test 1
+2026-09-10 01:22:16,122 DEBUG test_logging:70 Repeating....
+2026-09-10 01:22:16,122 ERROR test_logging:72 get frames
+2026-09-10 01:22:16,123 CRITICAL test_logging:73 Repeating....
+2026-09-10 01:22:16,123 WARNING test_logging:77 test 2
+2026-09-10 01:22:16,124 WARNING test_logging:78 Repeating....
+2026-09-10 01:22:16,125 CRITICAL test_logging:81 test 3no
+>>>>> send: 0x61 0x62 0x63
+>>>>> Repeating....
+>>>>> None
+2026-09-10 01:22:16,125 CRITICAL test_logging:83 test 3
+2026-09-10 01:22:16,125 CRITICAL test_logging:84 Repeating....
+2026-09-10 01:22:16,126 ERROR test_logging:89 test 4
+2026-09-10 01:22:16,126 ERROR test_logging:90 Repeating....
+2026-09-10 01:22:16,127 INFO  test_logging:95 test 5
+2026-09-10 01:22:16,127 INFO  test_logging:96 Repeating....
+2026-09-10 01:22:16,594 DEBUG test_logging:136 send: 0x31 0x32 0x33
+2026-09-10 01:22:16,859 WARNING context:65 ModbusDeviceContext, ModbusSequentialDataBlock, ModbusSparseDataBlock are deprecated and will be removed in v4.
+Please convert to SimData/SimDevice.
+Please read https://pymodbus.readthedocs.io/en/dev/source/upgrade_40.html#convert-to-simdata-simdevice
+2026-09-10 01:22:16,860 WARNING context:120 ModbusServerContext is deprecated and will be removed in v4.
+Please convert to SimData/SimDevice.
+Please read https://pymodbus.readthedocs.io/en/dev/source/upgrade_40.html#convert-to-simdata-simdevice
+2026-09-10 01:22:17,343 WARNING context:65 ModbusDeviceContext, ModbusSequentialDataBlock, ModbusSparseDataBlock are deprecated and will be removed in v4.
+Please convert to SimData/SimDevice.
+Please read https://pymodbus.readthedocs.io/en/dev/source/upgrade_40.html#convert-to-simdata-simdevice
+2026-09-10 01:22:17,344 WARNING context:120 ModbusServerContext is deprecated and will be removed in v4.
+Please convert to SimData/SimDevice.
+Please read https://pymodbus.readthedocs.io/en/dev/source/upgrade_40.html#convert-to-simdata-simdevice
+2026-09-10 01:22:17,466 WARNING context:65 ModbusDeviceContext, ModbusSequentialDataBlock, ModbusSparseDataBlock are deprecated and will be removed in v4.
+Please convert to SimData/SimDevice.
+Please read https://pymodbus.readthedocs.io/en/dev/source/upgrade_40.html#convert-to-simdata-simdevice
+2026-09-10 01:22:17,468 WARNING context:120 ModbusServerContext is deprecated and will be removed in v4.
+Please convert to SimData/SimDevice.
+Please read https://pymodbus.readthedocs.io/en/dev/source/upgrade_40.html#convert-to-simdata-simdevice
+2026-09-10 01:22:17,472 INFO  base:92 Server listening.
+2026-09-10 01:22:17,472 INFO  base:92 Repeating....
+2026-09-10 01:22:17,473 INFO  base:96 Server graceful shutdown.
+2026-09-10 01:22:17,591 WARNING context:65 ModbusDeviceContext, ModbusSequentialDataBlock, ModbusSparseDataBlock are deprecated and will be removed in v4.
+Please convert to SimData/SimDevice.
+Please read https://pymodbus.readthedocs.io/en/dev/source/upgrade_40.html#convert-to-simdata-simdevice
+2026-09-10 01:22:17,592 WARNING context:120 ModbusServerContext is deprecated and will be removed in v4.
+Please convert to SimData/SimDevice.
+Please read https://pymodbus.readthedocs.io/en/dev/source/upgrade_40.html#convert-to-simdata-simdevice
+2026-09-10 01:22:17,710 WARNING context:65 ModbusDeviceContext, ModbusSequentialDataBlock, ModbusSparseDataBlock are deprecated and will be removed in v4.
+Please convert to SimData/SimDevice.
+Please read https://pymodbus.readthedocs.io/en/dev/source/upgrade_40.html#convert-to-simdata-simdevice
+2026-09-10 01:22:17,711 WARNING context:120 ModbusServerContext is deprecated and will be removed in v4.
+Please convert to SimData/SimDevice.
+Please read https://pymodbus.readthedocs.io/en/dev/source/upgrade_40.html#convert-to-simdata-simdevice
+2026-09-10 01:22:17,831 WARNING context:65 ModbusDeviceContext, ModbusSequentialDataBlock, ModbusSparseDataBlock are deprecated and will be removed in v4.
+Please convert to SimData/SimDevice.
+Please read https://pymodbus.readthedocs.io/en/dev/source/upgrade_40.html#convert-to-simdata-simdevice
+2026-09-10 01:22:17,832 WARNING context:120 ModbusServerContext is deprecated and will be removed in v4.
+Please convert to SimData/SimDevice.
+Please read https://pymodbus.readthedocs.io/en/dev/source/upgrade_40.html#convert-to-simdata-simdevice
+2026-09-10 01:22:17,954 WARNING context:65 ModbusDeviceContext, ModbusSequentialDataBlock, ModbusSparseDataBlock are deprecated and will be removed in v4.
+Please convert to SimData/SimDevice.
+Please read https://pymodbus.readthedocs.io/en/dev/source/upgrade_40.html#convert-to-simdata-simdevice
+2026-09-10 01:22:17,955 WARNING context:120 ModbusServerContext is deprecated and will be removed in v4.
+Please convert to SimData/SimDevice.
+Please read https://pymodbus.readthedocs.io/en/dev/source/upgrade_40.html#convert-to-simdata-simdevice
+2026-09-10 01:22:18,811 DEBUG base:75 Processing: 0x0 0x1 0x0 0x0 0x0 0x5 0x1 0x4 0x2 0x0 0x3
+2026-09-10 01:22:18,812 DEBUG decoders:86 decoded PDU function_code(4 sub -1) -> ReadInputRegistersResponse(dev_id=0, transaction_id=0, address=0, count=0, bits=[], registers=[3], status=1, retries=0) 
+2026-09-10 01:22:18,813 DEBUG base:75 Processing: 0x0 0x1 0x0 0x0 0x0 0x5 0x1 0x4 0x2 0x0 0x3
+2026-09-10 01:22:18,814 ERROR base:86 ERROR: request ask for transaction_id=2 but got id=1, Skipping.
+2026-09-10 01:22:18,815 ERROR transaction:180 No response received after 3 retries, continue with next request
+2026-09-10 01:24:38,019 DEBUG test_logging:69 test 1
+2026-09-10 01:24:38,021 DEBUG test_logging:70 Repeating....
+2026-09-10 01:24:38,021 ERROR test_logging:72 get frames
+2026-09-10 01:24:38,027 CRITICAL test_logging:73 Repeating....
+2026-09-10 01:24:38,027 WARNING test_logging:77 test 2
+2026-09-10 01:24:38,028 WARNING test_logging:78 Repeating....
+2026-09-10 01:24:38,028 CRITICAL test_logging:81 test 3no
+2026-09-10 01:24:38,028 CRITICAL test_logging:83 test 3
+2026-09-10 01:24:38,029 CRITICAL test_logging:84 Repeating....
+2026-09-10 01:24:38,030 ERROR test_logging:89 test 4
+2026-09-10 01:24:38,030 ERROR test_logging:90 Repeating....
+2026-09-10 01:24:38,030 INFO  test_logging:95 test 5
+2026-09-10 01:24:38,031 INFO  test_logging:96 Repeating....
+2026-09-10 01:24:38,494 DEBUG test_logging:136 send: 0x31 0x32 0x33

pymodbus/transport/transport.py

@@ -136,6 +136,7 @@ def __init__(
         if is_sync:
             return
         self.loop = asyncio.get_running_loop()
+        self.connected_evt = asyncio.Event()
         if self.is_server:
             if self.comm_params.source_address is not None:
                 host = self.comm_params.source_address[0]
@@ -218,11 +219,17 @@ async def connect(self) -> bool:
         """Handle generic connect and call on to specific transport connect."""
         Log.debug("Connecting {}", self.comm_params.comm_name)
         self.is_closing = False
+        self.connected_evt.clear()
         try:
             self.transport, _protocol = await asyncio.wait_for(
                 self.call_create(),
                 timeout=self.comm_params.timeout_connect,
             )
+            if self.transport:
+                await asyncio.wait_for(
+                    self.connected_evt.wait(),
+                    timeout=self.comm_params.timeout_connect,
+                )
         except (asyncio.TimeoutError, OSError) as exc:  # pylint: disable=overlapping-except
             Log.warning("Failed to connect {}", exc)
             return False
@@ -256,12 +263,16 @@ def connection_made(self, transport: asyncio.BaseTransport) -> None:
         self.transport = transport
         self.reset_delay()
         self.callback_connected()
+        self.connected_evt.set()
 
     def connection_lost(self, exc: Exception | None) -> None:
         """Call from asyncio, when the connection is lost or closed.
 
         :param exc: None or an exception object
         """
+        if not getattr(self, "is_sync", False) and hasattr(self, "connected_evt"):
+            self.connected_evt.clear()
+
         if not self.transport or self.is_closing:
             return
         Log.debug("Connection lost {} due to {}", self.comm_params.comm_name, exc)

test/client/test_client.py

@@ -1,5 +1,6 @@
 """Test client sync."""
 
+import asyncio
 import socket
 import ssl
 from typing import cast
@@ -610,6 +611,62 @@ async def test_client_connection_made(self):
         client.close()
         assert rc
 
+    async def test_async_connect_state_machine(self):
+        """Test async client state-machine hydration after connect."""
+        client = lib_client.AsyncModbusTcpClient("127.0.0.1")
+
+        # Mock create_connection to return transport and protocol but also simulate calling connection_made
+        transport_mock = mock.AsyncMock()
+        transport_mock.close = lambda: ()
+
+        async def mock_create_connection():
+            client.ctx.connection_made(transport_mock)
+            return transport_mock, client.ctx
+
+        client.ctx.call_create = mock_create_connection
+
+        # Override the normal client connect behavior to ensure we use our mocked create_connection
+        connected = await client.connect()
+        assert connected
+
+        # This is the crux of the fix: client.connected MUST be true right after await client.connect()
+        assert client.connected
+        assert client.ctx.transport is not None
+        client.close()
+
+    async def test_async_connect_race_condition(self):
+        """Test that client.connect() correctly waits for connection_made callback even if delayed.
+
+        This demonstrates why relying solely on `create_connection`'s return (the old behavior)
+        results in `client.connected == False` if the callback is delayed by the event loop.
+        """
+        client = lib_client.AsyncModbusTcpClient("127.0.0.1")
+
+        transport_mock = mock.AsyncMock()
+        transport_mock.close = lambda: ()
+
+        async def mock_create_connection():
+            # Artificially delay the firing of protocol.connection_made(transport)
+            async def delayed_connection_made():
+                await asyncio.sleep(0.01)
+                client.ctx.connection_made(transport_mock)
+
+            _task = asyncio.create_task(delayed_connection_made())  # noqa: RUF006
+            return transport_mock, client.ctx
+
+        client.ctx.call_create = mock_create_connection
+
+        # connect() should await until connection_made fires, rather than just returning immediately
+        connected = await client.connect()
+        assert connected
+
+        # With the new synchronization logic, await client.connect() successfully waits for the delayed
+        # connection_made, resulting in client.connected == True.
+        assert client.connected
+        assert client.ctx.transport is not None
+
+        client.close()
+
     async def test_client_base_async(self):
         """Test modbus base client class."""
         async with ModbusBaseClient(

test/transport/test_protocol.py

@@ -61,7 +61,13 @@ async def test_init_source_addr_none(self, use_clc, dummy_protocol):
 
     async def test_loop_connect(self, client, dummy_protocol):
         """Test properties."""
-        client.call_create = mock.AsyncMock(return_value=(dummy_protocol(), None))
+
+        async def mock_call_create():
+            prot = dummy_protocol()
+            client.connection_made(prot)
+            return (prot, None)
+
+        client.call_create = mock.AsyncMock(side_effect=mock_call_create)
         assert await client.connect()
 
     async def test_loop_listen(self, server, dummy_protocol):
@@ -73,7 +79,13 @@ async def test_loop_listen(self, server, dummy_protocol):
 
     async def test_connect_ok(self, client, dummy_protocol):
         """Test properties."""
-        client.call_create = mock.AsyncMock(return_value=(dummy_protocol(), None))
+
+        async def mock_call_create():
+            prot = dummy_protocol()
+            client.connection_made(prot)
+            return (prot, None)
+
+        client.call_create = mock.AsyncMock(side_effect=mock_call_create)
         assert await client.connect()
 
     async def test_connect_not_ok(self, client, dummy_protocol):

test/transport/test_reconnect.py

@@ -64,9 +64,14 @@ async def test_multi_reconnect_call(self, client):
     async def test_reconnect_call_ok(self, client):
         """Test connection_lost()."""
         client.loop = asyncio.get_running_loop()
-        client.call_create = mock.AsyncMock(return_value=(mock.Mock(), mock.Mock()))
+
+        async def mock_call_create():
+            transport = mock.Mock()
+            client.connection_made(transport)
+            return (transport, mock.Mock())
+
+        client.call_create = mock.AsyncMock(side_effect=mock_call_create)
         await client.connect()
-        client.connection_made(mock.Mock())
         client.connection_lost(RuntimeError("Connection lost"))
         await asyncio.sleep(client.reconnect_delay_current * 1.8)
         assert client.call_create.call_count == 2