fix: do not spam the log with checksum related INFO messages when downloading using transfer_manager (#1357)

* fix: do not spam the log with checksum related INFO messages when downloading using transfer_manager

`download_chunks_concurrently` function does not allow to set `checksum` field in `download_kwargs`. It also does not set it on its own so it takes the default value of `"md5"` (see `Blob._prep_and_do_download`). Because ranged downloads do not return checksums it results in a lot of INFO messages (tens/hundreds):
```
INFO google.resumable_media._helpers - No MD5 checksum was returned from the service while downloading ...
(which happens for composite objects), so client-side content integrity checking is not being performed.
```
To fix it set the `checksum` field to `None` which means no checksum checking for individual chunks. Note that `transfer_manager` has its own checksum checking logic (enabled by `crc32c_checksum` argument)

* fix tests

Author: rafalh

google/cloud/storage/transfer_manager.py

@@ -885,6 +885,8 @@ def download_chunks_concurrently(
             "'checksum' is in download_kwargs, but is not supported because sliced downloads have a different checksum mechanism from regular downloads. Use the 'crc32c_checksum' argument on download_chunks_concurrently instead."
         )
 
+    download_kwargs = download_kwargs.copy()
+    download_kwargs["checksum"] = None
     download_kwargs["command"] = "tm.download_sharded"
 
     # We must know the size and the generation of the blob.

tests/unit/test_transfer_manager.py

@@ -606,6 +606,7 @@ def test_download_chunks_concurrently():
 
     expected_download_kwargs = EXPECTED_DOWNLOAD_KWARGS.copy()
     expected_download_kwargs["command"] = "tm.download_sharded"
+    expected_download_kwargs["checksum"] = None
 
     with mock.patch("google.cloud.storage.transfer_manager.open", mock.mock_open()):
         result = transfer_manager.download_chunks_concurrently(
@@ -636,9 +637,6 @@ def test_download_chunks_concurrently_with_crc32c():
     blob_mock.size = len(BLOB_CONTENTS)
     blob_mock.crc32c = "eOVVVw=="
 
-    expected_download_kwargs = EXPECTED_DOWNLOAD_KWARGS.copy()
-    expected_download_kwargs["command"] = "tm.download_sharded"
-
     def write_to_file(f, *args, **kwargs):
         f.write(BLOB_CHUNK)
 
@@ -664,9 +662,6 @@ def test_download_chunks_concurrently_with_crc32c_failure():
     blob_mock.size = len(BLOB_CONTENTS)
     blob_mock.crc32c = "invalid"
 
-    expected_download_kwargs = EXPECTED_DOWNLOAD_KWARGS.copy()
-    expected_download_kwargs["command"] = "tm.download_sharded"
-
     def write_to_file(f, *args, **kwargs):
         f.write(BLOB_CHUNK)