When uploading/downloading, make sure to truncate the remote/local file when opened.

[jlesage]

This fixes issue when replacing an existing remote/local file. If the file is not first truncated, the resulting file will be incorrect when new file is smaller than the existing one.

[helloyi]

Sorry for the late reply.

Implicitly truncating files is usually not safe, and this decision is usually left to the developer.
There are two options to fix this problem:

1. Explicitly truncate the file using the Truncate API

// upload
sftp.Truncate(remotePath, 0 /*size*/) or sftp.Remove(remotePath)
sftp.Upload(localPath, remotePath)

// download
os.Truncate(localPath, 0 /*size*/) or os.Remove(localPath)
sftp.Download(remotePath, localPath)

2. Add a truncated version of the upload/download API in go-sshclient (explicitly indicating that it will overwrite existing files)

// force upload and overwrite existing remote files
sftp.ForceUpload(localPath, remotePath)

// force download and overwrite existing local files
sftp.ForceDownload(remotePath, localPath)

If you are interested, please add ForceUpload and ForceDownload APIs.
looking forward to your work!