I would recommend renaming both files before you start. If the ftp server is actively writing data to the files, they will continue writing to the files. If the ftp client starts sending a new file after the rename(), it won't overwrite the renamed files.

You can then loop for some reasonable amount of time, and if the mtime doesn't change, you can guess that the ftp server isn't writing to them anymore.

Something like:

if ( -f $control_file && -f $data_file ) {
    rename($data_file,$data_file. $$);
    rename($control_file,$control_file . $$);
} else {
    exit;
}
while (1) {
    $mtime=(stat($data_file . $$))[9];
    # drop out of the loop if the file hasn't
    # changed in 5 minutes (perhaps longer
    # for a wan connection ?)
    last if (time > $mtime + 300);
    sleep(30);
}
#process the files
[download]

This still leaves a small race condition between the two rename(s) in which the control file might have been overwritten. (a real small timeslice, but a timeslice nonetheless...)

You mentioned you didn't have control over the process, but if you ever do get control, here's two ideas:

• Have the ftp client send the files as file.tmp, then use the "rename file.tmp file" ftp command when the file is sent.
• Lacking that, install an ftp server that does something similar. proftpd has a configuration parameter called HiddenStor that does this.

Update: yep..I missed a sleep in the lower loop, which would burn lots of resources...fixed

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