[launchd-dev] early boot hook in launchd/launchctl?

[Damien Sorresso]

On Jun 23, 2009, at 6:20 PM, Greg Shenaut wrote:
> Until recently, it was possible to put arbitrary code into /etc/ 
> rc.server to perform scripted maintenance activities in the early  
> boot context (i.e., system disk still write-only, limited driver  
> access, etc). In the version distributed with 10.5.7, rc.server  
> shows up later in the sequence (i.e., system disk mounted read- 
> write). In any case, adding code to rc.server always was a kludge.

What exactly are you doing that requires a read-only filesystem (I  
assume that's what you meant). What do you mean by "limited driver  
access"? Single-user mode doesn't stop the kernel from loading kexts  
as needed.

> Now, the cleanest way to do it seems to be to put code into /.env  
> and use "nvram boot-args=-s ; reboot" to run it. This is actually a  
> much cooler kludge, because /.env (the value of ENV in single-user  
> mode) is normally run only in single-user mode, because otherwise  
> the HOME variable will be set, and for root that will cause ENV  
> normally to be defined as /var/root/.env. Because /.env only gets  
> run in single-user mode, you can put in fairly elaborate checks to  
> make sure you're in in the right place without negatively impacting  
> startup time for normal root shell scripts.
>
> But I'm concerned that this method, like rc.server, will someday be  
> seen by the launchd developers as a bug (that is, HOME not being  
> defined in single-user mode); in this case, it too will suddenly  
> disappear and some other kludge will have to be found.

.env is a feature of the shell, not launchd. Though it's not mentioned  
in bash(1). Perhaps you're referring to /etc/profile? If that's the  
case, I suppose we could someday invoke the shell with --noprofile or  
something in the future, but I don't see a compelling reason to do  
that. Also, we don't set ENV when invoking the shell for single-user  
mode.

Also, the existence of rc.server is considered to be more of a  
necessary evil than a bug. :)

> So what I'm wondering is whether it might not be a good idea to  
> provide a conventional hook for early-boot maintenance. It could  
> simply be a shell script in a standard location such as /etc/ 
> rc.earlyboot which gets executed by launchd if present, owned by  
> root, and not writable except by root. This would keep people who  
> insist on doing this kind of thing from having to keep finding new  
> and better kludges.
>
> Or, of course, the /.env thing and the HOME-less single-user shell  
> could become a feature of launchd that is reserved for this purpose.


This might come in the future, but not in the form you're expecting.
-- 
Damien Sorresso
BSD Engineering
Apple Inc.

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