upstart as cron replacement

upstart as cron replacement

Posted Sep 12, 2006 12:02 UTC (Tue) by robbe (subscriber, #16131)
In reply to: upstart as cron replacement by smurf
Parent article: What's happening at Ubuntu: from X.org updates to upstart

> init has the unique property that it gets SIGCHLD for all processes which
> happen to die after their parent. Thus it is able to keep track of exactly
> when a job is finished, even if that job daemonizes or forks itself. (For
> the most part.)

Let's say init starts a daemon (e.g. apache as pid 100) directly and it daemonizes itself (i.e. forks to pid 105, and pid 100 terminates). The init will then get a SIGCHLD from pid 100 and supposes the daemon job as "finished". It doesn't know that pid 100 and pid 105 are connected. If the daemon exits or crashes later init will notice that pid 105 has terminated, but does not connect this fact with the apache job it started sometime in the past. (One can usually suppress daemonization via a commandline argument, though.)

Right now most daemons are started via scripts that setup the environment etc. If init runs this script it knows its pid and when it finishes. Again, it does not know the pid of the real worker process that the script forks off.

Some of your other comments to this article rely on part on this assumption which I argue to be wrong...

---

(Log in to post comments)