PYTHON-3502 GridFSBucket.download_to_stream slow (#1108)

Author: blink1073

doc/changelog.rst

@@ -1,6 +1,14 @@
 Changelog
 =========
 
+Changes in Version 4.3.3
+------------------------
+
+- Fixed a performance regression in :meth:`~gridfs.GridOut.download_to_stream`
+  and :meth:`~gridfs.GridOut.download_to_stream_by_name` by reading in chunks
+  instead of line by line.
+
+
 Changes in Version 4.3 (4.3.2)
 ------------------------------
 

gridfs/__init__.py

@@ -796,7 +796,10 @@ def download_to_stream(
            Added ``session`` parameter.
         """
         with self.open_download_stream(file_id, session=session) as gout:
-            for chunk in gout:
+            while True:
+                chunk = gout.readchunk()
+                if not len(chunk):
+                    break
                 destination.write(chunk)
 
     @_csot.apply
@@ -977,7 +980,10 @@ def download_to_stream_by_name(
            Added ``session`` parameter.
         """
         with self.open_download_stream_by_name(filename, revision, session=session) as gout:
-            for chunk in gout:
+            while True:
+                chunk = gout.readchunk()
+                if not len(chunk):
+                    break
                 destination.write(chunk)
 
     def rename(