NAME¶
dotlock - execute a command with a lock on a mailbox
SYNOPSIS¶
dotlock [-LPW]
mbox-file command [
arg ...]
DESCRIPTION¶
dotlock acquires a lock on the mailbox file
mbox-file using both
flock and a lock file, then executes
command with any arguments
specified. When
command exits, dotlock releases the lock.
dotlock attempts to clean up stale lockfiles. If it succeeds in locking an
mbox-file with
flock, and roughly 30 seconds elapse without
there being any changes to
mbox-file or the lockfile, then dotlock will
delete the lockfile and try again.
While it holds a lock, lockfile will keep updating the modification time of the
lockfile every 15 seconds, to prevent the lock from getting cleaned up in the
event that
command is slow.
OPTION¶
- --noflock (-L)
- Ordinarily, dotlock uses both flock and dotfile locking. (It uses flock
first, but releases that lock in the even that dotfile locking fails, so
as to avoid deadlocking with applications that proceed in the reverse
order.) The -L option disables flock locking, so that dotlock only
uses dotfile locking.
This is primarily useful as a wrapper around an application that already
does flock locking, but to which you want to add dotfile locking. (Even if
your mail delivery system doesn't use flock, flock actually improves the
efficiency of dotlock, so there is no reason to disable it.)
- --fcntl (-P)
- This option enables fcntl (a.k.a. POSIX) file locking of mail spools, in
addition to flock and dotfile locking. The advantage of fcntl locking is
that it may do the right thing over NFS. However, if either the NFS client
or server does not properly support fcntl locking, or if the file system
is not mounted with the appropriate options, fcntl locking can fail in one
of several ways. It can allow different processes to lock the same file
concurrently--even on the same machine. It can simply hang when trying to
acquire a lock, even if no other process holds a lock on the file. Also,
on some OSes it can interact badly with flock locking, because those OSes
actually implement flock in terms of fcntl.
- --nowait (-W)
- With this option, dotlock simply exits non-zero and does not run
command if it cannot immediately acquire the lock.
SEE ALSO¶
avenger(1),
deliver(1),
avenger.local(8)
The Mail Avenger home page: <
http://www.mailavenger.org/>.
BUGS¶
dotlock does not perform
fcntl/
lockf-style locking by default.
Thus, if your mail reader exclusively uses
fcntl for locking, there
will be race conditions unless you specify the
--fcntl option.
flock does not work over network file systems. Thus, because of dotlock's
mechanism for cleaning stale lock files, there is a possibility that a network
outage could lead to a race condition where the lockfile is cleared before
command finishes executing. If lockfile detects that the lock has been
stolen, it prints a message to standard error, but does not do anything else
(like try to kill
command).
AUTHOR¶
David Mazieres