Add on_pre_connect() callback

This is called immediately before a connection attempt is made.

ChangeLog.txt

@@ -1,3 +1,10 @@
+v1.7.0 - 2023-xx-xx
+===================
+
+- Add on_pre_connect() callback, which is called immediately before a
+  connection attempt is made.
+
+
 v1.6.1 - 2021-10-21
 ===================
 

README.rst

@@ -468,6 +468,10 @@ bind_address
 Callback
 ........
 
+Immediately prior to the connection attempt, the `on_pre_connect()` callback
+will be called. This is a useful callback to set information that must be done
+before the connection starts, e.g., calling `will_set()`.
+
 When the client receives a CONNACK message from the broker in response to the
 connect it generates an ``on_connect()`` callback.
 

src/paho/mqtt/client.py

@@ -1041,6 +1041,18 @@ def reconnect(self):
         # Put messages in progress in a valid state.
         self._messages_reconnect_reset()
 
+        with self._callback_mutex:
+            on_pre_connect = self.on_pre_connect
+
+        if on_pre_connect:
+            try:
+                on_pre_connect(self, self._userdata)
+            except Exception as err:
+                self._easy_log(
+                    MQTT_LOG_ERR, 'Caught exception in on_pre_connect: %s', err)
+                if not self.suppress_exceptions:
+                    raise
+
         sock = self._create_socket_connection()
 
         if self._ssl:
@@ -1844,6 +1856,35 @@ def decorator(func):
             return func
         return decorator
 
+    @property
+    def on_pre_connect(self):
+        """If implemented, called immediately prior to the connection is made
+        request."""
+        return self._on_pre_connect
+
+    @on_pre_connect.setter
+    def on_pre_connect(self, func):
+        """ Define the pre_connect callback implementation.
+
+        Expected signature:
+            connect_callback(client, userdata)
+
+        client:     the client instance for this callback
+        userdata:   the private user data as set in Client() or userdata_set()
+
+        Decorator: @client.pre_connect_callback() (```client``` is the name of the
+            instance which this callback is being attached to)
+
+        """
+        with self._callback_mutex:
+            self._on_pre_connect = func
+
+    def pre_connect_callback(self):
+        def decorator(func):
+            self.on_pre_connect = func
+            return func
+        return decorator
+
     @property
     def on_connect(self):
         """If implemented, called when the broker responds to our connection