thorny (for me) permissions problem

[John Almberg]

The following permissions problem has me stumped:

1. User A uploads a file (using ftp) to the server, into a directory  
called 'data' owned by user B. Permissions on directory set to allow  
this, like this:
	drwxrwxr-x  2 user_b user_b   512 Oct  7 08:40 data

2. A cron job, run by user B, then processes the file

3. When the processing is complete, the cron job needs to delete the  
file from the server

4. however, after upload, the file has the ownership A:B (i.e, owned  
by A, group B) with permissions -rw-r--r--. So B does not have  
permission to delete the file.
	-rw-r--r-- 1 user_a user_b 154879 Oct 7 08:40 data_file.csv

The ftp user can manually change the permissions on the file to -rw- 
rw-r--, but I do not want to depend on the user remembering to change  
permissions. If he forgets, the cronjob will process the file over  
and over again. I need the server to handle this, so it gets done  
correctly 100% of the time.

B does not have sufficient permissions to delete the file or change  
it's permissions. The only thing I can think of is to have ANOTHER  
cron job, run by A, run every few minutes to check for the existence  
of a file, and change the permissions so B can delete it. But this  
smells like a kludge to me.

Is there a correct way to handle this? For instance, is there  
something I can set in A's profile, so when he uploads a file, the  
group permission is set to rw? That would be a nice clean way to do  
it, but I can't find anything like that.

Any help, much appreciated.

-- John