Bug#43651: ACCEPTED] mailbox locking

[Julian Gilbey <J.D.Gilbey@qmw.ac.uk>]

I am about to include this amendment in policy.  However, I am stuck
with the wording, as you say the following.

Questions: What version number should be used in footnote 2?
           What do we do with the reference implementation?

> So I think we should change the above paragraph to something which
> explicitly says, how locking has to be implemented:
> 
>      All Debian MUAs, MTAs, MDAs and other mailbox accessing programs
>      (like IMAP daemons) have to lock the mailbox in a NFS-safe way.
>      This means that fcntl() locking has to be combined with dot
>      locking.  To avoid dead locks, a program has to use fcntl() first
>      and dot locking after this or alternatively implement the two
>      locking methods in a non blocking way[1].  Using the functions
>      `maillock' and `mailunlock' provided by the `liblockfile*'[2]
>      packages is the recommended way to realize this.
> 
>      Footnotes: 
>      [1] If it is not possible to establish both locks, the system
>          shouldn't wait for the second lock to be established, but
>          remove the first lock, wait a (random) time, and start over
>          locking again.
>      [2] liblockfile version >= .... (fill in a version number here,
>          which implements the above noted non blocking mechanism
>          without blocking).
> 
> 
> Thomas Roessler <roessler@guug.de> posted a skeleton of maillock() to
> debian-devel, maybe we should place this somewhere in the policy or in
> /usr/[share/]doc/debian-policy/ or take this as a basis for a new
> version of liblockfile:
> 
> int do_lock (const char *path, int fd, int retries)
> {
>         int i;
>         int rv;
>         
>         for (i = 0; i < retries; i++)
>         {
>                 if ((rv = do_fcntl_lock (fd)) == -1 && errno != EAGAIN)
>                         return -1;
>                 else if (rv == 0)
>                 {
>                         if (do_dotlock (path) == 0)
>                                 return 0;
>                         do_fcntl_unlock (fd);
>                 }
>                 sleep (rand() % 10);
>         }
>         return -1;
> }


   Julian

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  Julian Gilbey, Dept of Maths, QMW, Univ. of London. J.D.Gilbey@qmw.ac.uk
        Debian GNU/Linux Developer,  see http://www.debian.org/~jdg