Uploading a file via sftp is silently corrupted if the transfer is interrupted and later resumed

Large files uploaded via sftp from a users compuer to a network file server will sometimes become corrupted. WinSCP reports the transfer as complete so it's difficult for end-users to notice the corruption immediately.
It's only later discovered by other users who download the file and can't unzip the archive.

We performed several tests but we can't get a reliable method to replicate the issue.

Due to WFH, most users are uploading these archives from their home network. Some users have very slow connections so the upload of a file takes several hours.

Our best attempts at replicating this occurs when:
- for some of the users, the network connection drops several times during the transfer but WinSCP recovers and continues the transfer from where it left.
- some users forcefully close WinSCP and restart the application to continue the transfer due to the transfer hanging. This is most likely a good indicator that a file will become corrupted.

Version of WinSCP - 5.17.7
Version of Microsoft Windows - Windows 10 Enterprise 10.0.17763
Transfer protocol - SFTP.
Using the GUI (Commander).

Due to the randomness of this issue, it's difficult to say when the corruption occurs and if it is captured in the logs. We have a log file that's over 1.5GB. If there are specific sections from the log that you are interested in, let me know and I'll extract them.

Thank you.

I attached a few sections from the log that contains some errors that occur during the file transfer.

A file server engineer where we save these .zip files shared the following information (attached) with the time stamp in the log that I previously attached.

We also attempted to replicate this issue using CuteFTP but the files transferred with CuteFTP from the same machine are always intact, with no corruptions.

Please try to turn off "Optimize connection buffer size":
https://winscp.net/eng/docs/ui_login_connection

Thanks Martin, we ran a few more tests and while we are still experiencing several disconnections during the upload so far turning that option off seems to have worked.

We are still seeing several "out of memory" issues just like in the previous logs, what could be the cause of that? Both the laptop and the server have plenty of space.

I have sent you an email with a debug version of WinSCP to the address you have used to register on this forum.

Hello Martin,
Today we managed to get another corrupted archive while the "Optimize connection buffer size" was turned off.

Haven't had a chance to attempt it while using the debug version yet.

HappyCamper wrote:

Haven't had a chance to attempt it while using the debug version yet.

So please try that.