Re: [dev] RFC IMVCCStorage everywhere (Move ZODB's MVCC to storage layer)

Ahhh tpc_finish !

Before today, I only tested NEO on the no-more-load branch before the adapters were merged. I have errors again. Some because tests do 'connection._storage.iterator', which I changed into 'connection.db().storage.iterator'

But what annoys me is the monkey-patch we have for tpc_finish, which would become more complicated, mainly because of missing 'return' in adapters[1], but also because Connection.tpc_finish differs between ZODB5 and older versions[2].

I thought I could leave this subject aside for ZODB5, but instead of wasting time inside NEO, I should rather work on pushing this ZODB before anything else.

Hmm... not sure everyone remembers exactly what I am talking about. The monkey-patch was sent long time ago on this mailing-list (early 2012). So, in NEO, we don't want to serialize of the second phase of 2PC, which means:
- the real tid is only known during tpc_finish
- before tpc_finish, NEO works with a temporary tid
- NEOStorage.tpc_vote only returns oids for which a conflict happened
- NEOSTorage.tpc_finish returns the committed tid
- and what tpc_vote usually does for other ZODB implementation has to be done at the end of Connection.tpc_finish

You can find the monkey-patch at
  https://lab.nexedi.com/nexedi/neoppod/blob/v1.6.3/neo/client/__init__.py#L35

https://lab.nexedi.com/nexedi/neoppod/blob/v1.6.3/ZODB.patch
contains other changes:
- update of IStorage.tpc_finish doc string
- update of existing unit tests

TODO:
- 2 lines are removed in tpc_vote, instead of being reworded
- write a new ZODB test to check this feature
- it's been a long time since the whole ZODB test suite was run with this patch

Julien

[1] Actually, I noticed the them after I updated neo/client/__init__.py for [2]. So I patched ZODB locally, and the whole NEO test suite passes.

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