pulse_worker: don't prefetch more than one message (bug 1981900)

If the first message fails and needs requeuing, the Consumer will
immediately be given the next message in the batch. This leads to those
message being processed out-of-order.

By only fetching one message at a time, the Consumer will retrieve the
failed message again on the next fetch. It will continue looping on this
message forever until it succeeds, or someone intervenes.

This is the desired behaviour. This ensures that no message goes
silently unprocessed.

Author: shtrom

git_hg_sync/pulse_worker.py

@@ -52,7 +52,10 @@ def get_consumers(
         _channel: Any,
     ) -> list[kombu.Consumer]:
         consumer = consumer_class(
-            self.task_queue, auto_declare=False, callbacks=[self.on_task]
+            self.task_queue,
+            auto_declare=False,
+            callbacks=[self.on_task],
+            prefetch_count=1,
         )
         logger.debug(f"Using consumer {consumer=}")
         return [consumer]