Log warning if message set appears double-compressed in KafkaConsumer

kafka/consumer/fetcher.py

@@ -361,6 +361,14 @@ class Fetcher(six.Iterator):
                     # If relative offset is used, we need to decompress the entire message first to compute
                     # the absolute offset.
                     inner_mset = msg.decompress()
+
+                    # There should only ever be a single layer of compression
+                    if inner_mset[0][-1].is_compressed():
+                        log.warning('MessageSet at %s offset %d appears '
+                                    ' double-compressed. This should not'
+                                    ' happen -- check your producers!',
+                                    tp, offset)
+
                     if msg.magic > 0:
                         last_offset, _, _ = inner_mset[-1]
                         absolute_base_offset = offset - last_offset