Re: Executing a file only on later passes

[Chip Seraphine]

Mark.Burgess@iu.hio.no wrote:

> > OK, I now finally actually get the whole darned AddInstallable thing.  I 
> > just ran a couple quick tests, but I think this solves the problem.  The 
> > hell of it is that I've seen this explained before, but I had never 
> > quite *got* it.  Thanks.
>
> Perhaps you could explain it in your own words. That might help
> someone else. I could make your words into a FAQ. The Gospel
> according to Chip, Mark Luke and John...and Steve and Alva and...

I am seriously considering legally changing my name to Matt just for the 
added joke value here.

I think Eric said it very well, although he used verbage particular to 
the discussion at hand.  I think what really helped was that I finally 
had an example smacking me upside the head...  so I'll rephrase what he 
said as best I can and provide such an example.  What I've got below is 
awfully wordy, but I think this might be a sufficiently confusing 
subject to warrant the column-inches....

--

Cfengine attempts to optimize resource usage by ignoring code whose 
exeuction is dependent upon currently undefined classes.  This can cause 
problems if the class will be defined later.  In order to avoid this 
situation, the parser can be cued to avoid optimizing out such code by 
placing the controlling class(es) in the AddInstallable list.

If an action is controlled by an undefined class listed in the 
AddInstallable list, cfengine will defer execution of the controlled 
action in order to give the rest of the script an opportunity to define 
the class in question.  (Conversely, you may regard *not* marking a 
class Installable as telling cfengine to assume the class should be 
ignored unless already defined in previously parsed code.  This is 
probably only desirable if you are doing performance tuning.)

As an example, suppose a simple script intended to generate a message if 
the system has a file named /bin/sh:


control:
        ## AddInstallable = ( bin_sh_exists )
        actionsequence = ( files shellcommands )

shellcommands:
        bin_sh_exists:: "/bin/echo Bourne shell was found"

files:
        /bin/sh elsedefine=bin_sh_exists


The above example is broken, because the echo line is ignored by the 
parser; it sees that the "bin_sh_exists" class is not defined and 
optimizes away all code protected by that class.

Uncommenting the AddInstallable line will cause cfengine to defer the 
protected code instead of ignoring it.  After the files section has run, 
the code in shellcommands: protected by "bin_sh_exists" will be executed.

Note that this is of particular interest when importing files with the 
import: section, as the classes that control their importation must be 
flagged in AddInstallable unless they are previously defined in the 
parent script.

--

That sound good?

-Chip