VM is behaving badly with read-only folders.
If I visit a folder read-only, I don't expect it to be changed and
written back to disk. However, this happens:
(a) If I visit a folder created by my MTA, VM says "upgrading
attributes", and then the folder is modified, and VM writes it
back to disk when I quit. This of course causes corruption if the
MTA is writing new mail at the same time, since they use different
locking mechanisms.
(b) If I visit a virtual read-only folder, and then I change the
attributes of a message (by, say, filing it), the underlying
folder has its attributes changed and gets written when I quit the
virtual folder. I don't think this is right either.
My understanding of a read-only folder is that nothing I do to it
should cause any modification of the disk file.
