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From Matt Benson <gudnabr...@yahoo.com>
Subject Re: Scanning Resource Collections Several Times
Date Fri, 21 Aug 2009 13:43:55 GMT


--- On Fri, 8/21/09, Stefan Bodewig <bodewig@apache.org> wrote:

> From: Stefan Bodewig <bodewig@apache.org>
> Subject: Scanning Resource Collections Several Times
> To: dev@ant.apache.org
> Date: Friday, August 21, 2009, 3:20 AM
> Hi,
> 
> I just closed a bug
> <https://issues.apache.org/bugzilla/show_bug.cgi?id=43574>
> in delete
> that really points to a deeper problem.
> 
> Delete was scanning the same fileset three times. 
> Once by innocently
> calling getDirectoryScanner (and discarding the results if
> includeEmptyDirs and removeNotFollowedSymlinks was false),
> once by
> calling isFileSystemOnly on Resources and once when
> iterating.
> 
> If the fileset contained a modified selector, things
> couldn't work since
> files that had been modified on the first pass wouldn't be
> modified on
> subsequent passes.
> 
> While I fixed it for filesets now (by caching the result of
> the first
> scan) it still remains for other resource collections
> (which might use
> modified as a resource selector) and it may very well
> happen in other
> tasks as well.
> 
> One culprit is Resources which will create a new
> MyCollection in
> isFileSystemOnly which again invokes size() on all nested
> resource
> collections, scanning them before any iteration is
> required.  Most other
> resource collections extend BaseResourceCollection which
> has a cache
> attribute that is true by default and should avoid the
> problem.
> 
> I'd like to modify Resources to
> 
>     (1) lazily initialize size() - Delete never
> asks for it, for
>         example.

Sounds good.

> 
>     (2) Cache the remaining iteration results
> when initializing size
> 

The only thing that concerns me about this is the fact that I did make 'cache' an attribute
on BaseRC because I thought there might be times caching was not wanted.  How would one defeat
caching of <resources>?

Thanks for all your work of late while the rest of us do other stuff, Stefan!

-Matt

> Does anybody see a problem with this?
> 
> There probably are more cases where resource collections
> could be
> scanned multiple times but I'm not sure a complete audit is
> warranted.
> 
> Stefan
> 
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