Re: ITS#1679 gentle SIGHUP handling

["Pierangelo Masarati" <masarati@aero.polimi.it>]

Hallvard, 

I didn't apply your patch since discussion is still going on
on this subject.  I had very little time to consider your ideas
about making slapd restart after a (gentle SIG)HUP; however,
before anybody goes ahead, maybe we'd better focus what's to be
done. 

Quoting a previous message of yours, 

> 1. tell slapd to do a "gentle stop".
> 2. slapd enters read-only mode (i.e. rejects future update operations).
> 3. slapd waits for currently active update operations to finish.
> 4. slapd flushes the database cache, or better, closes the database
>    and reopens it in readonly mode (can that be done fairly simply?)
> 5. slapd tells the user that the database now can be backed up. (e.g.
>    #1 could be an ldapmodify on "cn=Control" setting "readonly=on",
>    and that operation would wait to finish until after step #4.)
> 6. you back up the database and start a new slapd with the backup.
> 7. you tell old slapd that it may now stop.  (kill -HUP, maybe.)
> 8. old slapd waits for current clients to finish, and stops when they
>    do or after a timeout.

- we need to check whether each backend can safely turn into readonly
mode (maybe we need a dedicated API call);
- we need a means to tell slapd that another slapd is going to take
its listener(s, plus more), which might well be the SIGHUP itself;
- we need a way for the old slapd to notify that readonly mode has
been reached by the database(s); 

I note that one intriguing means to do this is (as you envisaged)
to start another slapd in "overtake" mode, which takes care
of driving the old slapd in a safe status, inherits most of the
stuff the old slapd was using (I'm referring to listener(s) and
file handles in general). 

Pierangelo. 

Dr. Pierangelo Masarati               | voice: +39 02 2399 8309
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