dotlock - execute a command with a lock on a mailbox
dotlock [-LPW] mbox-file command [arg ...]
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.
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.)
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.
With this option, dotlock simply exits non-zero and does not run
command if it cannot immediately acquire the lock.
avenger(1), deliver(1), avenger.local(8)
The Mail Avenger home page: <http://www.mailavenger.org/>.
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).