sendmail, 4th Edition

Queue File Locking

When old versions of sendmail process a queued message (attempt to redeliver it) they create an empty lock file. That lock file was needed to signal other running sendmail processes that the mail message was busy so that they shouldn’t try to deliver the message too. Current versions simply flock(2) or fcntl(2) lock the qf file.

Current-style file locking

The method that sendmail uses to initially create an exclusive lock when first queueing a file is twofold. First it attempts to creat(2) the file with the argument:

O_CREAT|O_WRONLY|O_EXCL

If that succeeds, it then attempts to lock the file. If HASFLOCK (HAS... on page 114) is defined when sendmail is compiled, the file is locked with flock(2). Otherwise, it is locked with a fcntl(2) F_SETLK argument.

Locks shown when printing the queue

When mailq is run (or the -bp command-line switch is given to sendmail), the contents of the queue are listed. In that listing, an asterisk that appears to the right of an identifier indicates that a lock exists on the message:

/var/spool/mqueue/df (1 request)
 ----Q-ID---- --Size-- -----Q-Time----- ------------Sender/Recipient------------
 dB91UPA04168*       0 Wed Dec  8 17:30 <[e-mail]>
             ↑
                                             <[e-mail]
             note

Locks can get stuck

Occasionally, a file will become locked and remain that way for a long time. One indication of a stuck lock is a series of syslog messages about a given identifier:

Apr 12 00:33:38 ourhost sendmail[641]: dB91UPA04168: locked Apr 12 01:22:14 ourhost sendmail[976]: ...