[EH] Set AMQP Client Link Max Message Size to 0 (#43462)

* no restriction on max message size in amqp attach

* eh constant changed correctly, sb change reverted

* log addition

Author: PIRO-V

sdk/eventhub/azure-eventhub/CHANGELOG.md

@@ -21,6 +21,7 @@
 
 - Fixed a bug where service errors were incorrectly required and expected to have info/description fields.
 - Fixed a bug so that the BufferedProducer ThreadPoolExecutor uses one worker per partition. ([#38961](https://github.com/Azure/azure-sdk-for-python/issues/38961))
+- Fixed a bug in the EventHub client where the maximum message size negotiation during the AMQP attach frame was handled incorrectly. The client now correctly sends a max message size of 0 (unlimited), allowing the server to apply its internal limit (20 MB) as intended.
 
 ### Other Changes
 

sdk/eventhub/azure-eventhub/azure/eventhub/_pyamqp/client.py

@@ -46,6 +46,7 @@
     OUTGOING_WINDOW,
     DEFAULT_AUTH_TIMEOUT,
     MESSAGE_DELIVERY_DONE_STATES,
+    LINK_MAX_MESSAGE_SIZE
 )
 
 from .management_operation import ManagementOperation
@@ -576,7 +577,7 @@ class SendClient(AMQPClient):
     def __init__(self, hostname, target, **kwargs):
         self.target = target
         # Sender and Link settings
-        self._max_message_size = kwargs.pop("max_message_size", MAX_FRAME_SIZE_BYTES)
+        self._max_message_size = kwargs.pop("max_message_size", LINK_MAX_MESSAGE_SIZE)
         self._link_properties = kwargs.pop("link_properties", None)
         self._link_credit = kwargs.pop("link_credit", None)
         super(SendClient, self).__init__(hostname, **kwargs)

sdk/eventhub/azure-eventhub/azure/eventhub/_pyamqp/constants.py

@@ -59,6 +59,7 @@
 #: size until they have agreed a definitive maximum frame size for that Connection.
 MIN_MAX_FRAME_SIZE = 512
 MAX_FRAME_SIZE_BYTES = 1024 * 1024
+LINK_MAX_MESSAGE_SIZE = 0
 MAX_CHANNELS = 65535
 INCOMING_WINDOW = 64 * 1024
 OUTGOING_WINDOW = 64 * 1024