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Re: [VM] VM writes r/o folders

From: Uday Reddy
Subject: Re: [VM] VM writes r/o folders
Date: Tue, 14 Feb 2012 13:33:29 +0000

Julian Bradfield writes:

> 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.

I don't think that that kind of semantics has ever been promised for
`vm-folder-read-only'.  Actually, I don't find it documented in the manual
at all, which means to me that no promises were ever made.

I haven't taken out any read-only checks that were present in VM.  The
behaviour should be the same as it always was.  (Of course, some bugs here
and there are always possible, but there was no deliberate attempt to change
the behaviour here.)

If you do not want a folder changed, your best bet is to use
`vm-quit-no-change' when you are done.

We can discuss assigning a strong semantics to read-only, but I suspect that
it may be difficult to implement.  "Upgrading attributes", for example,
needs to be done when an old folder is read because the new code base will
only work with the upgraded version of data.  We can perhaps refrain from
writing the folder back to the disk at the end if the read-only flag is set.

Personally, I don't think you should ever be using VM on folders that an MTA
is writing to.  Failsafe concurrency is only possible if you use a proper
mail server.  Concurrency is the main reason I myself switched to IMAP.  I
regard what I used to do in the old days as "stone age"!

> (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.

Indeed, this wishlist item has been recorded.

Please feel free to contribute a patch if you are able to.


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