[BUG] - SSHClient.open_sftp().open(...) cause file downloaded being modified about paramiko HOT 2 OPEN

@lizaw : Consistently, despite the comment in the code (see sftp_client.SFTPClient.open()'s docstring/ documentation), you will find that people have better adding two arguments to the sftp.open

Try the following:

with sftp.open(sftp_root_dir+output_file_path, 'wb', 32768) as f:
  s3client.download_fileobj(s3_bucket, s3_key, f)

I have found that setting the buffer limit to 32kb and adding the binary flag does at times help with S3 connections, as mentioned on a few SO posts.

I know SSH by default transmits all data as binary, but I can't help feeling some implementations on hosts don't behave as expected.

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@prokopto-dev : I did try to put max buffer size in open(...) method and got same error. Per the known issue with download_fileobj:

download_fileobj with a file object in append mode can mess up file content #1446 - boto/boto3#1446

I do not know if the paramiko SFTP open file is conflicted with download_fileobj setting

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