Remove consumer assignment for backups

When instantiating the consumer we already a subscribe to the topic that
is to be backed up (ie. __schemas). Using assign on top of that causes
problems - once the assignment actually takes effect, the consumer
starts to consume from the beginning, ultimately resulting in an
`InconsistentOffset` error.

The default subscribe is enough at the moment, as we do not support
backing up a multi-partition topic.

karapace/backup/api.py

@@ -530,7 +530,6 @@ def create_backup(
         with _consumer(config, topic_name) as consumer:
             (partition,) = consumer.partitions_for_topic(topic_name)
             topic_partition = TopicPartition(topic_name, partition)
-            consumer.assign([topic_partition])
 
             try:
                 data_file = _write_partition(

tests/integration/kafka/test_consumer.py

@@ -12,6 +12,7 @@
 from tests.integration.utils.kafka_server import KafkaServers
 
 import pytest
+import time
 
 
 class TestPartitionsForTopic:
@@ -77,3 +78,46 @@ def test_get_watermark_offsets_topic_with_one_message(
 
         assert beginning == 0
         assert end == 1
+
+
+def test_assign_after_subscribe_consumes_from_beginning(
+    producer: KafkaProducer,
+    kafka_servers: KafkaServers,
+    new_topic: NewTopic,
+) -> None:
+    """This test is an example of how **not** to use a consumer.
+
+    Instantiating the consumer subscribes to the given topic. However, having
+    assign called afterwards with a `TopicPartition` with the default offset
+    will make the consumer (eventually) reset and start consuming from the
+    beginning.
+    """
+    number_of_messages = 1000
+    for i in range(number_of_messages):
+        producer.send(
+            new_topic.topic,
+            key=f"message-key-{i}",
+            value=f"message-value-{i}-" + 1000 * "X",
+        )
+    producer.flush()
+
+    consumer = KafkaConsumer(
+        bootstrap_servers=kafka_servers.bootstrap_servers,
+        topic=new_topic.topic,
+        enable_auto_commit=False,
+        auto_offset_reset="earliest",
+    )
+    consumer.assign([TopicPartition(new_topic.topic, partition=0)])
+
+    out_of_order_message_seen = False
+    last_offset = -1
+
+    for i in range(number_of_messages):
+        time.sleep(0.01)  # To simulate processing of message
+        message = consumer.poll(60)
+        out_of_order_message_seen = message.offset() <= last_offset
+        if out_of_order_message_seen:
+            break
+        last_offset = message.offset()
+
+    assert out_of_order_message_seen