Forum: CFEngine Help
Subject: Re: filecopy, transform with grep.
Author: davidlee
Link to topic: https://cfengine.com/forum/read.php?3,22985,23006#msg-23006
Nakarin,
Many thanks for your answer, which I appreciate. I'll give your suggestion a
try.
But looking more widely: two points; the first, short but pertinent; the
second, technical and constituting the remainder of this message. And both are
primarily addressed to the developers.
1. How do I sell this to "sceptical but prepared to be convinced if the
arguments are rock-solid" colleagues?
"Hey, guys, for this one simple, very clear 'grep' all we need is a twenty-two
line bloat of obscure cfengine-speak." I would find it hard to make such a
statement sound convincing. Who, really, could make a convincing defence of
the current necessity for such a verbose solution? (And that solution is
itself potentially technically flawed: see below.)
2. I realise that cf-3.1.4 might need such bloat. But this indicates a design
flaw in the current cfengine. It seems to have the components, but just hasn't
thought about gluing them together. It could get a major improvement, and
quite easily, by the developers giving a little though to this aspect.
All it needs is a specification like the following to be implemented:
==========
files:
"/path/to/resultfile"
create => true,
transformer_pipe => "/path/to/program /path/to/sourcefile";
==========
Much, much clearer, don't you think? Four lines instead of twenty-two. Put
the two side-by-side and compare for clarity.
"transformer_pipe" (a working name, feel free to think of another) would work
like "edit_line"/"delete_line"/"insert_line" etc.; it captures its standard
output as the potential new "resultfile" in a similar way to how "edit_line"
builds its potential new file; if this potential new "resultfile" differs from
the original, cfengine updates it, just as "edit_line" would.
Clean, concise; consistent with other parts of cfengine. No need for any of
the "do_update" stuff; it's just like "edit_line". No need for (non-)existence
checks; the "create" is just as in "copy_from" etc. No need for all that
painful stuff about handling an intermediate file (and the potential locking
problems, race conditions, etc.); use what cfengine already does in "edit_line".
An alternative design might be of the abstract form:
==========
files:
"/path/to/resultfile"
create => true,
transformer => "..."; # almost as existing 'transformer'
transformer_stdout => {"true"|"false"}; # default "false"; as per
existing cfengine.
==========
Existing uses of "transformer" would be unchanged.
Our particular "grep" example would become:
==========
files:
"/path/to/resultfile"
create => true,
transformer => "/bin/grep -v $(fq.uqhost)"; # command outputs to STDOUT
transformer_stdout => "true"; # capture STDOUT as potential new file
==========
(Might there also be a place for a "transformer_stdin"?)
So, how about it? Is there any reason why such an improvement could be
considered bad?
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