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locking mechanism
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dotlock(1) Mail Avenger 0.8.3 dotlock(1) 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 Mail Avenger 0.8.3 2012-04-05 dotlock(1)
