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.
If a regular lock fails, try a read-only lock. Use this option to lock mailbox files in a read-only directory.
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.
Mail software based on the C-Client library creates
lock files named
/tmp/..
Here, dddddd.iiiiiidddddd 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.
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.
lockmail
also creates, and honors dot-lock files.
Dot-lock files are first created as temporary files, then linked to
lockfile.lock
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.
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.