"Fossies" - the Fresh Open Source Software Archive 
"Fossies" - the Fresh Open Source Software Archive
lockmail − create mail lock files
|
lockmail [−r] [−t timeout] {lockfile} {program} [argument...] |
lockmail is a helper utility for working with mailbox files. Mailbox files must be locked to prevent other applications from modifying the mailbox at the same time. Different system use different locking conventions. lockmail uses two of the most common locking mechanisms in use, which should work reliably on most systems.
lockfile is the pathname to an existing mailbox file. By default, lockmail tries to lock the mailbox every five seconds (if the mailbox is already locked), and will give up after three minutes. After the mailbox is successfully locked, lockmail runs program as a child process, with any optional arguments. When program terminates, lockmail removes the mailbox lock, and terminates itself.
−r
If a regular lock fails, try a read−only lock. Use this option to lock mailbox files in a read−only directory.
−t timeout
If the lock attempt fails, try again for up to timeout seconds. The actual timeout is rounded up to the next five second interval (a lock attempt is tried every five seconds).
This section briefly describes the locking mechanism used by lockmail. lockmail uses three different locking conventions in order to maximize compatibility with other mail software: C−Client folder locks, dot−locks, and file locks.
C−Client
folder locks
Mail software based on the C−Client library creates
lock files named /tmp/.dddddd.iiiiii. Here,
dddddd and iiiiii are the device number and
the inode number of the mailbox file (the st_dev and
st_ino fields in the inode), in hexadecimal. If the
process ID saved in the C−Client folder lock file is
not valid, lockmail concludes that it's a stale lock
file, and will remove it.
Note
A race condition exists where a C−Client process is
killed after it creates a lock file, but before saving its
process ID in the lock file. The race window is very small,
but it exists. The C−Client library does not appear to
ever clear out the lock file.
lockmail attempts to resolve this race condition by deleting zero−length lock files that are at least five minutes old.
dot−locks
lockmail also creates, and honors dot−lock files.
Dot−lock files are first created as temporary files,
then linked to lockfile.lock. The link operation
fails if the dot−lock file already exists.
lockmail uses an enhanced method of
dot−locking, where its process ID, and the name of the
server where lockmail is running is also saved in its
dot−lock file. If the operation fails due to an
existing dot−lock file that was created by another
lockmail process on the same server, and the process
ID no longer exists, this stale dot−lock file is
removed immediately. In all other situations a
dot−lock file older than five minutes is considered
stale, and removed.
Note
A failure to create a dot−lock file is silently
ignored if the reason for the failure is because
lockmail does not have the write permission in the
dot−lock file's directory. The incoming mail spool
directory (usually /var/mail) typically does not have global
write permissions, so the attempt to create the
dot−lock file in the spool directory will fail, and
lockmail will be content with using
file−locking only.
File
locks
The final locking mechanism lockmail uses is the
operating system's file locking facility. If lockmail
fails to obtain all three locks, lockmail will sleep
for five seconds and try again. The only exception is a
failure to create a dot−lock because of no write
access to the dot−lock file's directory, which is
ignored. If lockmail still fails to obtain all
required locks in the amount of time specified by the
−t option (or its default value),
lockmail will terminate with the EX_TEMPFAIL exit
code.
lockmail runs program after obtaining the last file lock, waits until program terminates, and releases all locks. program must terminate before any of the locks obtained by lockmail expire, and are considered stale. lockmail will then terminate with the same exit code as program.
lockmail terminates with the same exit status as program lockmail terminates with the EX_TEMPFAIL exit status if it was unable to obtain a lock, or if program was killed by a signal.
maildrop(1) [1] , sendmail(8).
Sam Varshavchik
Author
|
1. |
maildrop(1) |
http://www.courier-mta.org/maildrop.html