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Re: Summary: File locking discussion

On Tue, 24 Jun 1997, Philip Hands wrote:

> Hi,
> 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.

It would be good if a "real NFS export" could comment on this. I always
thought that there is a single safe way of file locking over NFS, that is
the one Miquel implemented in his new lib (and the one that is mentioned
in open(2)).

> 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.

Of course, there are other ways to do "mail folder locking". Simpliest
(IMHO) would be to have a daemon run on the server where "/var/spool/mail"
is located. Each mail "client" (MTA, MUA, etc.) will have to contact that
server over a socket and ask for permission to modify the file. 

This has another advantage: When the client dies, the socket goes down and
the server could unlock the file automatically.

Of course, this would require another daemon, and a little more code to
the client's lib.

> 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).

I don't know the "Maildir" locking technique yet. Could you please
describe it with a little more details?

Thanks for your input,


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