Provided by: aegis_4.24.3-3_amd64 bug

NAME

        aegis locks - how locking works, and which commands use them

DESCRIPTION

        Aegis maintains a database of information about the projects in its care, and the various
        changes, both completed and in progress.  In order to ensure the integrity of this
        database, and also your project repository, it uses locks.

        From time to time, these locks are visible to the users, because they will be told that a
        command is waiting for a particular lock.  For some transactions, this can be a long
        wait.

   Dining Philosophers
        While UNIX supplies locks in various flavors, if you need an entire set of locks
        simultaneously, there is no elegant “all or nothing ” interface available.  This is
        unsurprising, as this is one of the classic computer science problems, known as the
        Dining Philosophers problem.

        The master lock is used to solve the Dining Philosophers problem, and is meant to be very
        transient.  It is only held while the other locks which are required (frequently two or
        more, hence the problem) are requested - non-blocking.  Once they are all obtained (or
        not, and any partials given back) the master lock is released.  It is usually held for
        much less than a second.  If you notice the master lock being held, it is almost always a
        symptom of the NFS lock daemon misbehaving.

        If the lock(s) could not be obtained, the blocking lock is waited on (without the
        master).  This is when the "waiting for" message is issued.  When obtained, it is
        released and the whole cycle starts again.  This is why you occasionally see a series of
        "waiting for" messages.  (This could maybe be optimized some, but it is still possible to
        block on yet another lock, and they you have to release all and wait again.  As yet, I'm
        not convinced the extra code complexity is required.)

   Listing Locks
        There is a command available to list the current Aegis locks.
                aegis -list locks
        Note that the project names are change numbers are guesses as the locks are hashed over a
        16-bit range, and range overlaps are possible.  Collisions are also possible, but
        fortunately rarer.

   Known Problems
        There is a known problem with the HP/UX NFS clients.  If you see persistent "no locks
        available" error messages when  is NFS mounted, try making the /lockfile file world
        writable.  chmod 666 /lockfile There is the possibility of a denial of service attack
        (which is why the default is 0600) but since you are presently denied service anyway,
        it's academic.

COMMANDS

        The following table shows the locks taken by the various commands.  Note that
        theoretically some of the commands take too few locks, but this has yet to prove to be a
        problem in practice.  Also, "project state file" and "change state file" are the same
        thing for branches, it just depends which way you are looking at them at the time.

        Command      Global      Project    Project     Ancestor     Change       User
                     State       State      Baseline    Baselines    State        State
                     File        File                                File         File
        ──────────────────────────────────────────────────────────────────────────────────
        aeb (dev)       .           .        shared      shared     exclusive       .
        aeb (int)       .           .           .        shared     exclusive       .
        aeca            .           .           .           .       exclusive       .
        aechown         .           .           .           .       exclusive   exclusive
        aeclean         .           .           .           .       exclusive       .

        aeclone         .       exclusive       .           .       exclusive   exclusive
        aecp            .           ,           ,           ,       exclusive       .
        aecpu           .           .           .           .       exclusive       .
        aed             .           ,           ,           ,       exclusive       .
        aedb            .           .           .           .       exclusive   exclusive
        aedbu           .           .           .           .       exclusive   exclusive
        aede            .           ,           .           .       exclusive   exclusive
        aedeu           .           .           .           .       exclusive   exclusive
        aedn            .       exclusive       .           .           .           .
        aeib            .       exclusive       .           .       exclusive   exclusive
        aeibu           .       exclusive       .           .       exclusive   exclusive
        aeifail         .       exclusive       .           .       exclusive   exclusive
        aeipass         .       exclusive   exclusive       .       exclusive   exclusive
        aemv            .           ,           ,           ,       exclusive       .
        aena            .       exclusive       .           .           .           .
        aenbr           .       exclusive       .           .           .           .
        aenbru      exclusive   exclusive       .           .           .           .
        aenc            .       exclusive       .           .           .           .
        aencu           .       exclusive       .           .           .           .
        aend            .       exclusive       .           .           .           .
        aenf            .           .           .           .       exclusive       .
        aenfu           .           .           .           .       exclusive       .
        aeni            .       exclusive       .           .           .           .
        aenpa       exclusive       .           .           .           .           .
        aenpr       exclusive       .           .           .           .           .
        aenrv           .       exclusive       .           .           .           .
        aent            .       exclusive       .           .       exclusive       .
        aentu           .           .           .           .       exclusive       .
        aepa            .       exclusive       .           .           .           .
        aera            .       exclusive       .           .           .           .
        aerd            .       exclusive       .           .           .           .
        aerfail         .           .           .           .       exclusive   exclusive
        aeri            .       exclusive       .           .           .           .
        aerm            .           ,           .           .       exclusive       .
        aermu           .           .           .           .       exclusive       .
        aermpr      exclusive       .           .           .           .           .
        aerpa       exclusive       .           .           .           .           .
        aerpass         .           .           .           .       exclusive       .
        aerpu           .           .           .           .       exclusive       .
        aerrv           .       exclusive       .           .           .           .
        aet             .           .           ,           ,       exclusive       .

SEE ALSO

        You may wish to see the manual pages of all of the above commands.  Many have
        descriptions of the locking interactions.

COPYRIGHT

        aegis version 4.24.3.D001
        Copyright (C) 1991, 1992, 1993, 1994, 1995, 1996, 1997, 1998, 1999, 2000, 2001, 2002,
        2003, 2004, 2005, 2006, 2007, 2008, 2009, 2010 Peter Miller

        The aegis program comes with ABSOLUTELY NO WARRANTY; for details use the 'aegis -VERSion
        License' command.  This is free software and you are welcome to redistribute it under
        certain conditions; for details use the 'aegis -VERSion License' command.

AUTHOR

        Peter Miller   E-Mail:   millerp@canb.auug.org.au
        /\/\*             WWW:   http://www.canb.auug.org.au/~millerp/