Ensure paho.mqtt.client is imported in the executor (#118412)

fixes #118405

homeassistant/components/mqtt/client.py

@@ -39,9 +39,11 @@ from homeassistant.core import (
 )
 from homeassistant.exceptions import HomeAssistantError
 from homeassistant.helpers.dispatcher import async_dispatcher_send
+from homeassistant.helpers.importlib import async_import_module
 from homeassistant.helpers.start import async_at_started
 from homeassistant.helpers.typing import ConfigType
 from homeassistant.loader import bind_hass
+from homeassistant.setup import SetupPhases, async_pause_setup
 from homeassistant.util.async_ import create_eager_task
 from homeassistant.util.collection import chunked_or_all
 from homeassistant.util.logging import catch_log_exception, log_exception
@@ -491,13 +493,13 @@ class MQTT:
         """Handle HA stop."""
         await self.async_disconnect()
 
-    def start(
+    async def async_start(
         self,
         mqtt_data: MqttData,
     ) -> None:
         """Start Home Assistant MQTT client."""
         self._mqtt_data = mqtt_data
-        self.init_client()
+        await self.async_init_client()
 
     @property
     def subscriptions(self) -> list[Subscription]:
@@ -528,8 +530,11 @@ class MQTT:
             mqttc.on_socket_open = self._async_on_socket_open
             mqttc.on_socket_register_write = self._async_on_socket_register_write
 
-    def init_client(self) -> None:
+    async def async_init_client(self) -> None:
         """Initialize paho client."""
+        with async_pause_setup(self.hass, SetupPhases.WAIT_IMPORT_PACKAGES):
+            await async_import_module(self.hass, "paho.mqtt.client")
+
         mqttc = MqttClientSetup(self.conf).client
         # on_socket_unregister_write and _async_on_socket_close
         # are only ever called in the event loop