[MERGE/RFC] catch-all handler for filter stack lookup

[Jelmer Vernooij]

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Hi Ian,

Ian Clatworthy wrote:
> Jelmer wants to transparently map svn properties to bzr
> content filters. Our current filter stack registration
> function supports a dictionary mapping from
> 
>   supported value -> filter stack
> 
> but that doesn't scale to stuff like ...
> 
>   keywords = Date, Version
> 
> where the value is arbitrarily complex. This patch adds
> an optional fallback handler - a callable invoked if an
> unknown value is encountered. That should be good enough
> I think for Jelmer to weave his magic. :-)
> 
> I'm making this an RFC because I want Jelmer to confirm
> this meets his needs and because I'm not sure how best
> to expand the lazy registration function Jelmer added.
> Perhaps we want to register a compound object rather
> than a (map,fallback) tuple? If so, we could point to that
> object. Or we could point lazily to both a map and a fallback
> callable?
Thanks! I can confirm this would meet my needs, but it would also be
nice to load the svn keywords support in bzr-svn lazily.

Perhaps just the fallback parameter could be supported for lazy loading?
The dictionary lookup can be done inside of the fallback function as well.

Cheers,

Jelmer
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