[MERGE] Add an InventoryEntry cache for xml deserialization

[Robert Collins]

On Fri, 2008-12-12 at 15:16 -0600, John Arbash Meinel wrote:

> "source string" ? You mean the 'value' for split-inv, or the raw line of
> xml for others?

yes.

> Anyway, I ended up running into some cache-lifetime issues, and figured
> it would be reasonable to open it up for discussion how we want to
> handle it.
> 
> 1) Many, many, many tests re-use the same revision ids and file ids with
>    different actual values. This is pretty trivial to fix by adding a
>    "_entry_cache.clear()" as part of TestCase.setUp()

Or not using a global cache :>.

> 3) At one point I switched to having each XMLSerializer have its own
>    entry cache, in case that was actually necessary. ATM I think it is
>    actually a YAGNI.

Thats getting useful, because a repository will have lots of hits and
the lifetime is reasonably well defined, as long as repositories have
their own serializer instances, but IIRC we use singletons.

> 4) I've been considering restructuring things so that the repository
>    passes the entry_cache to self._serializer.deserialize_inventory(),
>    as this makes it obvious that the lifetime of the cache is bound to
>    the Repository. Which helps for us to know that the caching rules
>    won't be violated. The main downside is that something that ends up
>    dealing with two mostly-identical repositories will not benefit.
> 
>    The original use-case I was trying to handle is the "extract all
>    revision trees from the repository" which works just fine here, but
>    there are other use cases where an entry cache would be helpful.

I'd be inclined to start with 4) and see how it goes.

> I've gone down a couple different routes, and right now I'm leaning
> towards having a single entry cache, and just clearing it out at
> appropriate times during the test suite. In the "real-world" if we had
> revision_id collisions, we would have much more serious problems than
> just inventory entry caching, so I'm considering the problem as mostly
> just a test suite not being properly isolated between test cases issue.
> 
> Thoughts?

globals--

-Rob
-- 
GPG key available at: <http://www.robertcollins.net/keys.txt>.
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