make checksum problems

[RW]

There seems to be a change in "make checksum" where it's deleting
preexisting files that don't match their checksums. 

This is having two effects:

1. downloads don't resume any more because the partial file is deleted

2. if a make checksum runs while another make process is downloading a
   distfile, the file is deleted and a new file is created. The first
   process is then writing to a deleted file, so when it completes it
   checks the new incomplete file and deletes that. This can cycle until
   they both reach their FETCH_REGET limit. Usually one ends up with a
   checksum error. 


I suspect that 2  may be affecting portmaster, which runs make checksum
in the background. I have a script that does something similar, and I've
been seeing occasional failed builds like this with portupgrade.

 
Is this a known problem? I don't like to file a PR without doing a
search, and Bugzilla searches are all hanging with "Please stand
by ...".