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Re: Executing a file only on later passes


From: Chip Seraphine
Subject: Re: Executing a file only on later passes
Date: Mon, 03 Nov 2003 15:12:57 -0600
User-agent: Mozilla/5.0 (X11; U; SunOS i86pc; en-US; rv:1.2.1) Gecko/20030121

address@hidden 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





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