Fix bug causing KafkaProducer to double-compress message batches on retry

kafka/producer/buffer.py

@@ -89,6 +89,12 @@ class MessageSetBuffer(object):
         return self._buffer.tell() >= self._batch_size
 
     def close(self):
+        # This method may be called multiple times on the same batch
+        # i.e., on retries
+        # we need to make sure we only close it out once
+        # otherwise compressed messages may be double-compressed
+        # see Issue 718
+        if not self._closed:
             if self._compressor:
                 # TODO: avoid copies with bytearray / memoryview
                 self._buffer.seek(4)
@@ -105,6 +111,7 @@ class MessageSetBuffer(object):
             size = self._buffer.tell() - 4
             self._buffer.seek(0)
             self._buffer.write(Int32.encode(size))
+
         self._buffer.seek(0)
         self._closed = True