sendmail, 4th Edition

Prevent Simultaneous Rebuilds

The alias database files can be rebuilt in two ways: automatically, by the daemon or by users sending mail (and thereby indirectly running sendmail),^[207] or explicitly, by users rebuilding the database with newaliases (or the -bi command-line switch). To prevent one rebuild from compromising and corrupting another, sendmail uses file locking.

The sendmail program uses flock(2) or fcntl(2) with F_SETLK to lock the aliases file (depending on how it was compiled). If the aliases file is already locked (because the database is currently being rebuilt), sendmail prints the following message:

Alias file name is already being rebuilt

If sendmail is attempting to rebuild because it was run as newaliases or with the -bi command-line switch, the previous message is printed, and the program exits. Otherwise, the previous message is printed, and sendmail waits for the aliases file to become unlocked.

Once the aliases file is locked, sendmail next looks to see whether the key @ appears in the database. If that key is missing, sendmail knows the database is still being rebuilt. If the AliasWait option (AliasWait on page 973) has a value, sendmail waits that amount of time for the other rebuild to finish. If the AliasWait option is missing or has a zero value, sendmail plows ahead, trusting the previous lock to prevent simultaneous rebuilds.

The sendmail program waits the number of seconds specified by the AliasWait option for an @ key to appear in the database. If that ...