Tolerate STOMP notification with empty body (#2088)

Details:

* Fixed a JSONDecodeError when a STOMP notification from the HMC contained the
  empty string as the message body, by tolerating that.

Signed-off-by: Andreas Maier <maiera@de.ibm.com>

Author: andy-maier

changes/2087.fix.rst

@@ -0,0 +1,2 @@
+Fixed a JSONDecodeError when a STOMP notification from the HMC contained the
+empty string as the message body, by tolerating that.

zhmcclient/_notification.py

@@ -627,9 +627,9 @@ def notifications(self):
             "Message format" in chapter 4. "Asynchronous notification" in the
             :term:`HMC API` book.
 
-          * message (:term:`JSON object`): Body of the HMC notification,
-            converted into a JSON object. `None` for notifications that
-            have no content in their response body.
+          * message (:term:`JSON object` or `None`): Body of the HMC
+            notification, converted into a JSON object. `None` for notifications
+            that have no content in their response body.
 
             The properties of the JSON object vary by notification type.
 
@@ -688,13 +688,16 @@ def notifications(self):
 
             # Now we have an item from the listener
             if item.msgtype == 'message':
-                try:
-                    msg_obj = json.loads(item.message)
-                except Exception as exc:
-                    raise NotificationParseError(
-                        "Cannot convert JMS message body to JSON: "
-                        f"{exc.__class__.__name__}: {exc}",
-                        item.message)
+                if item.message is None:
+                    msg_obj = None
+                else:
+                    try:
+                        msg_obj = json.loads(item.message)
+                    except Exception as exc:
+                        raise NotificationParseError(
+                            "Cannot convert JMS message body to JSON: "
+                            f"{exc.__class__.__name__}: {exc}",
+                            item.message)
             elif item.msgtype == 'error':
                 if 'message' in item.headers:
                     # Not sure that is always the case, but it was the case

zhmcclient/_utils.py

@@ -842,35 +842,39 @@ def get_headers_message(frame_args):
     Transform STOMP event method parameters to a tuple(headers, message),
     dependent on the stomp-py version that is used.
 
+    A message that is the empty string is returned as `None`.
+
     Parameters:
 
       frame_args: The STOMP frame, represented depending on the stomp-py
         package version as follows:
 
-          * if stomp-py uses Frames, a single stomp.Frame object:
+          * On stomp.py >= 7.0.0, a stomp.Frame object with STOMP message
+            headers and message body in attributes 'headers' and 'body'.
 
-            frame (stomp.Frame): Object with STOMP message headers and
-              message body.
+          * On stomp.py < 7.0.0, a tuple(headers, body).
 
-          * else, a tuple(headers, message):
+        In both cases, the STOMP headers and body are represented as follows:
 
-            headers (dict): STOMP message headers.
-              The headers are described in the `headers` tuple item
-              returned by the
-              :meth:`~zhmcclient.NotificationReceiver.notifications`
-              method.
+          headers (dict): STOMP message headers.
+            The headers are described in the `headers` tuple item
+            returned by the
+            :meth:`~zhmcclient.NotificationReceiver.notifications`
+            method.
 
-            message (string): STOMP message body as a string, which
-              contains a serialized JSON object.
-              The JSON object is described in the `message` tuple item
-              returned by the
-              :meth:`~zhmcclient.NotificationReceiver.notifications`
-              method.
+          message (str): STOMP message body as a Unicode string, which
+            contains a serialized JSON object or the empty string.
+            The JSON object is described in the `message` tuple item
+            returned by the
+            :meth:`~zhmcclient.NotificationReceiver.notifications`
+            method.
     """
     if len(frame_args) == 1:
         frame = frame_args[0]
         headers = frame.headers
         message = frame.body
     else:
         headers, message = frame_args
+    if message == '':
+        message = None
     return headers, message