Persisting IdentityDictionary

[Chris Muller]

> when finding a key (#at), check if key has magmaOid first, if not,
> 'self errorKeyNotFound: key' because it doesn't exist in
> IdentityDictionary anyway.

..that won't work for objects that have been added to the dictionary,
but not yet committed.

> when storing a value (#at: put:); before finding its bucket, check if
> key has magmaOid, if not, store key in database so it will have a
> magmaOid.  I'm not sure what the code looks like on this last one, but
> it sounds good :)

This won't work because, maybe you are already in a transaction when
you decide to send #at:put:, but not yet ready to commit.  I'm not
sure if you meant to use #commit to "store key in database".

But you are evaluating exactly the right kinds of ideas, way to go.

With Magma's networking layer ("Ma client server", which is used
independently in other apps, without Magma, BTW), it is very easy to
define new types of requests and responses; they're all first-class
objects.  So I'm thinking of a new request-type that would be an
oid-reservation for any uncommitted object so that, if that object is
ever committed, that's the oid it will get.

The only downside to this is that the whole purpose of
MagmaPreallocatedDictionary is to be very fast, so we don't want to
require an extra trip to the server for each and every at:put:.  So
requests for oid reservations should bring back 100 at a time, and
sessions can then dole them out one by one as they're requested.  We
have plenty of oid space, there is no concern about wasting a few oids
if not all of them get used.

 - Chris