Checkout filtering wanted (badly!)

[Steve Johnson <st...@equilibrium.com>]

Greetings,

We've been playing with Subversion for a few months, and have just  
taken the plunge and converted over from CVS.  Svn is generally a  
huge improvement over CVS.  We do, however, have a few key features  
that we'd like to see implemented.  I'd like to discuss the first of  
these...

I've perused the mailing list and have read discussion threads  
regarding a number of requests/needs for checkout filtering.  From  
what I've seen, however, none of these discussions led to any real  
interest in actually implementing a filtering mechanism.  I'm  
wondering why this is, and I'm hoping to be able to change this.

Subversion's arbitrary properties would seem to me to lead naturally  
to a filtering mechanism.  What I want to see implemented is:

    checkout --filter '<name><op><value>'

where <name> is a subversion property name, <value> is a value to  
compare with the value of the <name> property at each node of a tree,  
and <op> defines the comparison.  Only when the comparison succeeded  
on a versioned object would that object be checked out.  For our  
purposes, this check could be done only on directories.  For  
generality, I could see extending it to files as well.  The 'update'  
action would have to take this argument as well.

For example (and to illustrate one of my company's needs), others  
have asked for a way to do a checkout that avoids directories of code  
specific to a platform other than the one currently targeted.  We  
currently support 6 platforms, and might add others.  Having  
developers pull down all the code and prebuilt libraries for all 6  
platforms is wasteful of both time and disk space.  Some have  
mentioned the work-around of separating all such code into totally  
separate major subtrees.  Don't most of you agree that this is  
absurd?  Good coding practices dictate organizing code by module, not  
by platform.  So it really does seem like a natural thing to want/ 
need to avoid irrelevant platform-specific directories sprinkled  
throughout a source tree.

Given the filtering enhancement mentioned above, one could simply tag  
their platform specific directories with say a "platform" property,  
and then do:

    checkout --filter 'platform=Mac'

to get a source tree applicable for building the mac version of a  
product.  For this to be really useful in this situation, the  
nonexistence of the referenced property should be treated as a match  
so that untagged directories are always grabbed.  I guess it would be  
nice to have the filter op able to work either way.

I would also use this feature for optimizing particular product  
builds.  Areas of the tree needed only to build specific products  
could similarly be tagged appropriately.  So, for example, to grab a  
tree with only the files sufficient to build DeBabelizer (that's our  
product) on Macintosh, you'd do:

    checkout --filter 'platform=Mac, product=Debab'

or something similar.

I want this feature so bad, I'm thinking of implementing it  
myself!!!  There must be others out there that would greatly benefit  
by this feature.  What do others think?  Is there any downside to  
such a feature?  Am I forgetting something that would make this  
feature "evil"?

Thanks in advance for any valuable discussion that might ensue.

Steve Johnson
Equilibrium


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