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Re: Summary: File locking discussion


I noticed that during this discussion two issues that are not intrinsically 
related keep on getting tied together:

 1) Reliable file locking, including over NFS

 2) Reliable mail delivery to users' inboxes

I cannot claim to be an expert on NFS locking, but I have a fairly strong 
feeling that 1) is almost impossible.  Of course, if it is possible then I 
think we should do the Unix community a great service by providing an easy to 
use library, as has been suggested.

Regardless of whether 1) is attainable, 2) is _not_ possible by use of locks 
alone.  The reasons for this are covered in a fair amount of detail in the 
qmail documentation, but boil down to the fact that it is possible to corrupt 
a conventional mbox by writing a partial message to the end.

If we are serious about changing our mail delivery policy, then I think we 
should seriously consider moving to Maildir delivery as our default, with 
dot-locking as a required alternative (for backwards compatibility).

Maildir delivery does not need any locking to be performed, and allows 
multiple processes to create/modify/delete mail messages simultaneously 
without the possibility of inbox corruption.  No deadlocks or stale locks can 
occur because no locking is required.

I think we need to write two libraries:

  1)  libnfslock        (or whatever)

  2)  libmailaccess     (or whatever)

libmailaccess should be something like PAM for mail delivery, providing access 
to a user's mailbox by use of either Maildir, or dot-locking (via libnfslock 
say), or whatever other method --- as selected by the user.

Cheers, Phil.

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