Concurrent FTP access

How does FTP servers handle concurrent access to the same file, i.e. if one user is updating a file while another is reading the same file? Should I be worried about getting corrupt data? Is it dependent on the FTP server or even the operating system?

I think the FTP Server itself does not handle this.
The underlying file-system will manage the accesses involved here.

If a read starts before a write, the read would typically get the older version.

You should get the answer to this question based on
the server file-system handling for a file being overwritten.

I think FTP implementations just don't deal with this, and the OS will vary on how it does, windows might lock the file, linux will give you partial data.

Yes you should be worried, specially under high usage. The solutions I found on the past were sketchy at best, including different folders for upload/download and a monitor process to copy from upload to download when the file is fully uploaded.

This gets worse as files are longer and/or people upload from slow connections.