It wasn't really needed or used and caused a stupid ZEO test failure.

I do think the storage API needs to be thought about and refined at
some point, but now, it's not a priority.

I'd lazily assumed that undo and tpc_vote has the same sorts of
returns, but they don't.  When tpc_vote isn't None (ZEO), the retrun
is a list of oid and serial pairs.

To prevent modifying the set of instances while iterating over them.

That's applied to storages other than RelStorage.

See: https://groups.google.com/forum/#!topic/zodb/QJYmvF0eUUM

And in special case where a connectin is opened while a transaction is
active (Zope2).

It's too hard to remember to update the meta data (we've been in beta
for a long long time).

By resetting self.__onCloseCallbacks before calling the close
callbacks to forestall infinite recursion (as can be the case when a
close callback is the database close methos).

Changed it to the value corresponding to the maximim *signed*
big-endian 64-bit integer, because of LxBTrees.

Note that this value isn't used anywhere in ZODB yet.

Maybe it should be.

To mitigate errors during tearDown.

Lots of tests are sloppy about closing connection (because they could
be), but storages tend to get closed and transactions aborted. This
has the effedt of calling afterCompletion on connections which tries
to call storage lastTransaction, which causes errors.

Fixed: FileStorage loadBefore didn't handle deleted/undone data correctly

Enforce minimum required Python versions: 2.7, 3.3.

+1

Merge pull request #62 from zopefoundation/dont-depend-on-implementation-detail-in-tpc_finish-test-assertions

Fixed a test that depended on some off behavior of tpc_finish in some…

This and https://github.com/zodb/relstorage/pull/31 make RelStorage
pass with this branch (as much as they pass with master).

Closes #59.

This answers #13

- It failed with oids that already exist in the base storage.
- MappingStorage returned timestamps in wrong format.

On Python 2 and Mac OS X, checkFlushAfterTruncate was failing.

Even though the previous commit otherwise made MVCCMappingStorage and
IMVCCStorage handling better.

The previous commit, made in anger, made a test pass, but wasn't
really the right fix.

This commit fixes MVCCMappingStorage's loadBefore implementation by
fixing handling of the internal _ltid variable so that it's updated
during poll_invalidations.  This allowed the base class version of
loadBefore to be used and, I'm 97% sure has the right semantics.
Fixing this revealed a problem with the Connection changes.

Fixed Connection.py to poll for invalidations before computing
_txn_time by calling lastTransaction, so as to get a current value.
We still apply invalidations after computing _txn_time, so that
persistent classes can be loaded correctly when they are invalidated.
This was accomplished by weaving _flush_invalidations into
newTransaction.

Changed to always empty the file pool.  I doubt that just flushing on
Python 2 was much of an optimization, and ... Python 2. :)

This is needed because pack temporarily deletes data from the data
structure that loadBefore reads.

Note that in the new scheme of things, we avoid using load, but
instead use loadBefore.  This we bypass the _data_snapshot
shenanigans, which makes me think we aren't testing what we should be,
but I have no confidence that the machinery in MVCCMappingStorage
actually reflects anything meaningful.

This implements: https://github.com/zopefoundation/ZODB/issues/50

Rather than watching invalidations, simply use 1 + the storages
lastTransaction, which is equivalent to but much simpler than waiting
for the first invalidation after a transaction starts.

More importantly, it means we can always use loadBefore and get away
from load.  We no longer have to worry about ordering of invalidations
and load() results.

Much thanks to NEO for pointing the way toward this simplification!

Implementing this initially caused a deadlock, because DB.open()
called Connection.open() while holding a database lock and
Connection.open() now calls IStotage.lastTransaction(), which acquires a
storage lock. (It's not clear that lastTransaction() really needs a
storage lock.)  Meanwhile, IStotage.tpc_finish() calls a DB function
that requires the DB lock while holding the storage lock.  Fixing this
required moving the call to Connection.open() outside the region where
the DB lock was held.

To debug the problem above, I greatly improved lock-debugging
support. Now all of the ZODB code imports Lock, RLock and Condition
from ZODB.utils. If the DEBUG_LOCKING is set to a non-empty value,
then these are wrapped in such a way that debugging information is
printed as they are used. This made spotting the nature of the
deadlock easier.

Of course, a change this basic broke lots of tests. Most of the
breakage arises from the fact that connections now call
lastTransaction on storages at transaction boundaries.  Lots of tests
didn't clean up databases and connections properly.  I fixed many
tests, but ultimately gave up and added some extra cleanup code that
violated transaction-manager underware (and the underware's privates)
to clear transaction synchonizers in test setup and tear-down.  I plan
to add a transaction manager API for this and to use it in a
subsequent PR.

This tests makes database and connection hygiene a bit more important,
especially for tests, because a connection will continue to interact
with storages if it isn't properly closed, which can lead to errors if
the storage is closed.  I chose not to swallow these errors in
Connection, choosing rather to clean up tests.

The thread debugging and test changes make this PR larger than I would
have liked. Apologies in advance to the reviewers.