more explanation in the guide for startupitems

[dweber@…]

I found the guide for startupitems is too terse, without adequate background explanation of the tools that implement the process. Please include information on how to get further information, such as:

man launchd
man launchctl
daemondo --help

The latter is helpful, as {{{man daemondo}} doesn't exist (can we create it?).

In my case, I found it useful to read through the entire thread that was engaged during the creation of the guide for this section (esp. comments therein from James Berry), ie:

Ryan Schmidt, "daemondo defeats purpose of launchd?" at ​http://lists.macosforge.org/pipermail/macports-dev/2007-September/002669.html

If I had time and an understanding of the documentation dev process, I would contribute directly, but it may be easier for the current dev folks to update this section of the guide. I would really like to see a few paragraphs from James' comments adapted into the background for this section of the guide. His comments are succinct and illuminating (esp. in the context of that thread). For example,

daemondo will indeed quit when it detects that the launched process  
has quit, and thus will "keep alive" the process, since launchd will  
then restart daemondo. In this way, daemondo acts as a shim or  
adapter between the scripts supported by the startupitem command,
and the single process expected by launchd.

The reason that startupitem.executable is preferred is that this  
gives the best possible chance that daemondo will be able to detect  
the death of the launched process: since daemondo can launch the  
process, it can also detect when it quits, stop it, etc.

For those cases where startupitem.executable cannot be used, daemondo  
also supports the startupitem.pidfile commands that allow the  
process' pidfile to be monitored: daemondo will read the pidfile and  
watch for the death of that process.

So daemondo, and thus launchd, will be aware of the daemon process  
death (and be able to restart the daemon process) only under two  
circumstances:

(1) startupitem.executable was supplied (thus daemondo starts the process)
(2) startupitem.pidfile was supplied (thus daemondo reads the process id)

Under all other circumstances, daemondo will not know that the daemon  
process has died, and will not exit when the process does die, and  
thus launchd won't restart the process since it doesn't  know it  
died. Put another way, if daemondo can know the process has died,  
then launchd will know too, but not otherwise.

Note that for simplicity, startupitem.executable is handled by  
daemondo at present. This has two purposes:

	- It keeps the startupitem generating code a little simpler.
	- It allows the potential support for higher value services to be  
provided by daemondo. In particular, note daemondo's --restart- 
netchange option, which can be quite useful, but for which there is  
no current support by the startupitem keys.

Also, later in the thread, we have some comments that could be adapted for the guide:

...

I don't see  the incompatibility of those statements, but then I again  
I know what i meant, not necessarily what it means to others ;). The  
later sentence, btw, is missing a word on the end. It should read:  
"the pidfile keyword is likely useful only if the executable keyword  
is not specified." Does that help any?

> Looks to me like startupitem.pidfile must be set for a deamon to be
> tracked whether it is executable startupitem or not.

No, daemondo will track an "executable" in an case (and it doesn't  
need to know where their pidfile is, generally, since it launches the  
code and thus knows the pid). In the case of script code (non  
"executable") daemondo doesn't know the pid, since it doesn't know  
what the script code did. In this case, it has to rely on reading a  
pidfile to get the process id, or else simply not know.

> And the man page
> says startupitem.pidfile is "particularly useful" for
> startupitem.executable.  Can you explain this?

That was either garbage to begin with, or else got messed up in  
creation of the man page. Off the top of my head I can't see any  
particular reason to use a pidfile keyword in conjunction with the  
executable keyword, unless it's to specify that it should delete a  
pidfile created by the executable, and I'm not sure that even works  
for that case.

Hope that helps.

James

Best, Darren

PS, In the description of the startupitem.pidfile, can we remove the OR operator, ie:

Default: [none] | [${prefix}/var/run/${name}.pid]

It's easy to confuse this syntax as either:

Default: [none]

OR

Default: [${prefix}/var/run/${name}.pid]

However, we do need two arguments: [daemondoAction] [pidFile]

[(none)]

Milestone Website & Documentation deleted

[cooljeanius (Eric Gallager)]

Cc Me!

[mf2k (Frank Schima)]

markd has retired. See #44782.

[raimue (Rainer Müller)]

See also #22449.