Merge pull request #66 from zopefoundation/MVCCAdapter

Abstract ZODB's MVCC implementation into a storage adapter.

setup.py

@@ -20,7 +20,7 @@ to application logic.  ZODB includes features such as a plugable storage
 interface, rich transaction support, and undo.
 """
 
-version = "4.3.0.dev0"
+version = "5.0.dev0"
 
 import os
 from setuptools import setup, find_packages

src/ZODB/Connection.py

@@ -27,7 +27,6 @@ from persistent import PickleCache
 from persistent.interfaces import IPersistentDataManager
 from ZODB.interfaces import IConnection
 from ZODB.interfaces import IBlobStorage
-from ZODB.interfaces import IMVCCStorage
 from ZODB.blob import Blob, rename_or_copy_blob, remove_committed_dir
 from transaction.interfaces import ISavepointDataManager
 from transaction.interfaces import IDataManagerSavepoint
@@ -50,6 +49,8 @@ from ZODB.utils import p64, u64, z64, oid_repr, positive_id
 from ZODB import utils
 import six
 
+from .mvccadapter import HistoricalStorageAdapter
+
 global_reset_counter = 0
 
 noop = lambda : None
@@ -102,13 +103,17 @@ class Connection(ExportImport, object):
         # Multi-database support
         self.connections = {self._db.database_name: self}
 
-        storage = db.storage
-        if IMVCCStorage.providedBy(storage):
-            # Use a connection-specific storage instance.
-            self._mvcc_storage = True
-            storage = storage.new_instance()
+        storage = db._mvcc_storage
+        if before:
+            try:
+                before_instance = storage.before_instance
+            except AttributeError:
+                def before_instance(before):
+                    return HistoricalStorageAdapter(
+                        storage.new_instance(), before)
+            storage = before_instance(before)
         else:
-            self._mvcc_storage = False
+            storage = storage.new_instance()
 
         self._normal_storage = self._storage = storage
         self.new_oid = db.new_oid
@@ -138,17 +143,17 @@ class Connection(ExportImport, object):
         # ReadConflictError (just to be able to clean them up from the
         # cache on abort with the other modified objects). All objects
         # of this list are either in _cache or in _added.
-        self._registered_objects = []
+        self._registered_objects = [] # [object]
 
         # ids and serials of objects for which readCurrent was called
         # in a transaction.
-        self._readCurrent = {}
+        self._readCurrent = {} # {oid ->serial}
 
         # Dict of oid->obj added explicitly through add(). Used as a
         # preliminary cache until commit time when objects are all moved
         # to the real _cache. The objects are moved to _creating at
         # commit time.
-        self._added = {}
+        self._added = {} # {oid -> object}
 
         # During commit this is turned into a list, which receives
         # objects added as a side-effect of storing a modified object.
@@ -162,32 +167,11 @@ class Connection(ExportImport, object):
         # adding. Used during abort to remove created objects from the
         # _cache, and by persistent_id to check that a new object isn't
         # reachable from multiple databases.
-        self._creating = {}
+        self._creating = {} # {oid -> implicitly_added_flag}
 
         # List of oids of modified objects, which have to be invalidated
         # in the cache on abort and in other connections on finish.
-        self._modified = []
-
-        # _invalidated queues invalidate messages delivered from the DB
-        # _inv_lock prevents one thread from modifying the set while
-        # another is processing invalidations.  All the invalidations
-        # from a single transaction should be applied atomically, so
-        # the lock must be held when reading _invalidated.
-
-        # It sucks that we have to hold the lock to read _invalidated.
-        # Normally, _invalidated is written by calling dict.update, which
-        # will execute atomically by virtue of the GIL.  But some storage
-        # might generate oids where hash or compare invokes Python code.  In
-        # that case, the GIL can't save us.
-        # Note:  since that was written, it was officially declared that the
-        # type of an oid is str.  TODO:  remove the related now-unnecessary
-        # critical sections (if any -- this needs careful thought).
-
-        self._inv_lock = utils.Lock()
-        self._invalidated = set()
-
-        # Flag indicating whether the cache has been invalidated:
-        self._invalidatedCache = False
+        self._modified = [] # [oid]
 
         # We intend to prevent committing a transaction in which
         # ReadConflictError occurs.  _conflicts is the set of oids that
@@ -198,12 +182,6 @@ class Connection(ExportImport, object):
         # _conflicts).
         self._conflicts = {}
 
-        # _txn_time stores the upper bound on transactions visible to
-        # this connection. That is, all object revisions must be
-        # written before _txn_time. If it is None, then the current
-        # revisions are acceptable.
-        self._txn_time = None
-
         # To support importFile(), implemented in the ExportImport base
         # class, we need to run _importDuringCommit() from our commit()
         # method.  If _import is not None, it is a two-tuple of arguments
@@ -252,17 +230,7 @@ class Connection(ExportImport, object):
         if obj is not None:
             return obj
 
-        before = self.before
-        if before is None:
-            # Normal case
-            before = self._txn_time
-
-        data = self._storage.loadBefore(oid, before)
-        if data is None:
-            # see the comment in setstate
-            raise ReadConflictError()
-
-        p, _, _ = data
+        p, _ = self._storage.load(oid, '')
         obj = self._reader.getGhost(p)
 
         # Avoid infiniate loop if obj tries to load its state before
@@ -320,9 +288,6 @@ class Connection(ExportImport, object):
         if self.opened:
             self.transaction_manager.unregisterSynch(self)
 
-        if self._mvcc_storage:
-            self._storage.sync(force=False)
-
         if primary:
             for connection in self.connections.values():
                 if connection is not self:
@@ -351,23 +316,7 @@ class Connection(ExportImport, object):
         """Returns True if this connection is read only."""
         if self.opened is None:
             raise ConnectionStateError("The database connection is closed")
-        return self.before is not None or self._storage.isReadOnly()
-
-    def invalidate(self, tid, oids):
-        """Notify the Connection that transaction 'tid' invalidated oids."""
-        if self.before is not None:
-            # This is a historical connection.  Invalidations are irrelevant.
-            return
-
-        with self._inv_lock:
-            self._invalidated.update(oids)
-
-    def invalidateCache(self):
-        self._inv_lock.acquire()
-        try:
-            self._invalidatedCache = True
-        finally:
-            self._inv_lock.release()
+        return self._storage.isReadOnly()
 
     @property
     def root(self):
@@ -545,9 +494,6 @@ class Connection(ExportImport, object):
 
         self._added_during_commit = []
 
-        if self._invalidatedCache:
-            raise ConflictError()
-
         for obj in self._registered_objects:
             oid = obj._p_oid
             assert oid
@@ -559,10 +505,6 @@ class Connection(ExportImport, object):
             elif oid in self._added:
                 assert obj._p_serial == z64
             elif obj._p_changed:
-                if oid in self._invalidated:
-                    resolve = getattr(obj, "_p_resolveConflict", None)
-                    if resolve is None:
-                        raise ConflictError(object=obj)
                 self._modified.append(oid)
             else:
                 # Nothing to do.  It's been said that it's legal, e.g., for
@@ -600,9 +542,6 @@ class Connection(ExportImport, object):
                 self._creating[oid] = implicitly_adding
 
             else:
-                if (oid in self._invalidated
-                    and not hasattr(obj, '_p_resolveConflict')):
-                    raise ConflictError(object=obj)
                 self._modified.append(oid)
 
             p = writer.serialize(obj)  # This calls __getstate__ of obj
@@ -739,21 +678,11 @@ class Connection(ExportImport, object):
                 self._handle_serial(oid, serial)
 
     def tpc_finish(self, transaction):
-        """Indicate confirmation that the transaction is done."""
-
-        def callback(tid):
-            if self._mvcc_storage:
-                # Inter-connection invalidation is not needed when the
-                # storage provides MVCC.
-                return
-            d = dict.fromkeys(self._modified)
-            self._db.invalidate(tid, d, self)
-#       It's important that the storage calls the passed function
-#       while it still has its lock.  We don't want another thread
-#       to be able to read any updated data until we've had a chance
-#       to send an invalidation message to all of the other
-#       connections!
-        self._storage.tpc_finish(transaction, callback)
+        """Indicate confirmation that the transaction is done.
+        """
+        # XXX someday, we'll care about the *real* tid we get back via
+        # the callback (that we're no longer calling).
+        self._storage.tpc_finish(transaction)
         self._tpc_cleanup()
 
     def sortKey(self):
@@ -773,67 +702,16 @@ class Connection(ExportImport, object):
     def newTransaction(self, transaction, sync=True):
         self._readCurrent.clear()
 
-        if self._mvcc_storage:
-            # XXX It's not clear if we should make calling sync
-            # dependent on the sync flag here.  RelStorage's sync semantics
-            # are a bit different, plus RelStorage doesn't actually
-            # work with this code. :/ This will probably be sorted when
-            # we do:
-            # https://groups.google.com/forum/#!topic/zodb/QJYmvF0eUUM
-            self._storage.sync(True)
-            # Poll the storage for invalidations.
-            mvc_invalidated = self._storage.poll_invalidations()
-            if mvc_invalidated is None:
+        try:
+            self._storage.sync(sync)
+            invalidated = self._storage.poll_invalidations()
+            if invalidated is None:
                 # special value: the transaction is so old that
                 # we need to flush the whole cache.
-                self._invalidatedCache = True
-        else:
-            if sync:
-                getattr(self._storage, 'sync', noop)()
-            mvc_invalidated = None
-
-        if self.opened:
-            self._txn_time = p64(u64(self._storage.lastTransaction()) + 1)
-
-        with self._inv_lock:
-            # Non-ghostifiable objects may need to read when they are
-            # invalidated, so we'll quickly just replace the
-            # invalidating dict with a new one.  We'll then process
-            # the invalidations after freeing the lock *and* after
-            # resetting the time.  This means that invalidations will
-            # happen after the start of the transactions.  They are
-            # subject to conflict errors and to reading old data.
-
-            # TODO: There is a potential problem lurking for persistent
-            # classes.  Suppose we have an invalidation of a persistent
-            # class and of an instance.  If the instance is
-            # invalidated first and if the invalidation logic uses
-            # data read from the class, then the invalidation could
-            # be performed with stale data.  Or, suppose that there
-            # are instances of the class that are freed as a result of
-            # invalidating some object.  Perhaps code in their __del__
-            # uses class data.  Really, the only way to properly fix
-            # this is to, in fact, make classes ghostifiable.  Then
-            # we'd have to reimplement attribute lookup to check the
-            # class state and, if necessary, activate the class.  It's
-            # much worse than that though, because we'd also need to
-            # deal with slots.  When a class is ghostified, we'd need
-            # to replace all of the slot operations with versions that
-            # reloaded the object when called. It's hard to say which
-            # is better or worse.  For now, it seems the risk of
-            # using a class while objects are being invalidated seems
-            # small enough to be acceptable.
-
-            if self._invalidatedCache:
-                self._invalidatedCache = False
                 invalidated = self._cache.cache_data.copy()
-            else:
-                invalidated = dict.fromkeys(self._invalidated)
-            self._invalidated = set()
-
-        if mvc_invalidated:
-            self._cache.invalidate(mvc_invalidated)
-        self._cache.invalidate(invalidated)
+            self._cache.invalidate(invalidated)
+        except AttributeError:
+            assert self._storage is None
 
         # Now is a good time to collect some garbage.
         self._cache.incrgc()
@@ -877,24 +755,8 @@ class Connection(ExportImport, object):
                 raise
 
         try:
+            p, serial = self._storage.load(oid, '')
 
-            before = self.before
-            if before is None:
-                # Normal case
-                if self._invalidatedCache:
-                    raise ReadConflictError()
-                before = self._txn_time
-
-            data = self._storage.loadBefore(oid, before)
-            if data is None:
-                # We had data (by definition, since we have a
-                # reference to it), but it's gone.  It must have
-                # updated since this transaction, and been packed
-                # away, cuz the tests are mean. The best we can do is
-                # raise a ReadConflictError and try again
-                raise ReadConflictError()
-
-            p, serial, _ = data
             self._load_count += 1
 
             self._reader.setGhostState(obj, p)
@@ -1033,8 +895,6 @@ class Connection(ExportImport, object):
         See the docstring for the resetCaches() function.
         """
         self._reset_counter = global_reset_counter
-        self._invalidated.clear()
-        self._invalidatedCache = False
         cache_size = self._cache.cache_size
         cache_size_bytes = self._cache.cache_size_bytes
         self._cache = cache = PickleCache(self, cache_size, cache_size_bytes)
@@ -1042,11 +902,9 @@ class Connection(ExportImport, object):
             self._reader._cache = cache
 
     def _release_resources(self):
-        assert not self.opened
         for c in six.itervalues(self.connections):
-            if c._mvcc_storage:
-                if c._storage is not None:
-                    c._storage.release()
+            if c._storage is not None:
+                c._storage.release()
             c._storage = c._normal_storage = None
             c._cache = PickleCache(self, 0, 0)
 
@@ -1089,7 +947,7 @@ class Connection(ExportImport, object):
 
     def savepoint(self):
         if self._savepoint_storage is None:
-            tmpstore = TmpStore(self._normal_storage, self._txn_time)
+            tmpstore = TmpStore(self._normal_storage)
             self._savepoint_storage = tmpstore
             self._storage = self._savepoint_storage
 
@@ -1111,7 +969,7 @@ class Connection(ExportImport, object):
         self.cacheGC()
         return result
 
-    def _rollback(self, state):
+    def _rollback_savepoint(self, state):
         self._abort()
         self._registered_objects = []
         src = self._storage
@@ -1138,7 +996,7 @@ class Connection(ExportImport, object):
             self._creating.update(src.creating)
 
             for oid in oids:
-                data, serial, _ = src.loadBefore(oid, self._txn_time)
+                data, serial = src.load(oid)
                 obj = self._cache.get(oid, None)
                 if obj is not None:
                     self._cache.update_object_size_estimation(
@@ -1153,7 +1011,7 @@ class Connection(ExportImport, object):
                     # that that the next attribute access of its name
                     # unghostify it, which will cause its blob data
                     # to be reattached "cleanly"
-                    self.invalidate(None, (oid, ))
+                    self._cache.invalidate(oid)
                 else:
                     s = self._storage.store(oid, serial, data,
                                             '', transaction)
@@ -1195,26 +1053,29 @@ class Connection(ExportImport, object):
     # Savepoint support
     #####################################################################
 
+
 @implementer(IDataManagerSavepoint)
 class Savepoint:
 
-
     def __init__(self, datamanager, state):
         self.datamanager = datamanager
         self.state = state
 
     def rollback(self):
-        self.datamanager._rollback(self.state)
+        self.datamanager._rollback_savepoint(self.state)
+
 
 @implementer(IBlobStorage)
 class TmpStore:
     """A storage-like thing to support savepoints."""
 
 
-    def __init__(self, storage, before):
+    def __init__(self, storage):
         self._storage = storage
-        self._before = before
-        for method in 'getName', 'new_oid', 'getSize', 'sortKey', 'isReadOnly':
+        for method in (
+            'getName', 'new_oid', 'getSize', 'sortKey',
+            'isReadOnly'
+            ):
             setattr(self, method, getattr(storage, method))
 
         self._file = tempfile.TemporaryFile(prefix='TmpStore')
@@ -1236,11 +1097,10 @@ class TmpStore:
             remove_committed_dir(self._blob_dir)
             self._blob_dir = None
 
-    def loadBefore(self, oid, before):
-        assert before == self._before
+    def load(self, oid, version=''):
         pos = self.index.get(oid)
         if pos is None:
-            return self._storage.loadBefore(oid, before)
+            return self._storage.load(oid, '')
         self._file.seek(pos)
         h = self._file.read(8)
         oidlen = u64(h)
@@ -1250,7 +1110,7 @@ class TmpStore:
         h = self._file.read(16)
         size = u64(h[8:])
         serial = h[:8]
-        return self._file.read(size), serial, None
+        return self._file.read(size), serial
 
     def store(self, oid, serial, data, version, transaction):
         # we have this funny signature so we can reuse the normal non-commit
@@ -1331,6 +1191,7 @@ class TmpStore:
         self.index = index.copy()
         self.creating = creating
 
+
 class RootConvenience(object):
 
     def __init__(self, root):

src/ZODB/DB.py

@@ -401,12 +401,6 @@ class DB(object):
           - `xrefs` - Boolian flag indicating whether implicit cross-database
             references are allowed
         """
-        if isinstance(storage, six.string_types):
-            from ZODB import FileStorage
-            storage = ZODB.FileStorage.FileStorage(storage, **storage_args)
-        elif storage is None:
-            from ZODB import MappingStorage
-            storage = ZODB.MappingStorage.MappingStorage(**storage_args)
 
         # Allocate lock.
         x = utils.RLock()
@@ -423,12 +417,24 @@ class DB(object):
         self._historical_cache_size_bytes = historical_cache_size_bytes
 
         # Setup storage
+        if isinstance(storage, six.string_types):
+            from ZODB import FileStorage
+            storage = ZODB.FileStorage.FileStorage(storage, **storage_args)
+        elif storage is None:
+            from ZODB import MappingStorage
+            storage = ZODB.MappingStorage.MappingStorage(**storage_args)
+        else:
+            assert not storage_args
+
         self.storage = storage
+
+        if IMVCCStorage.providedBy(storage):
+            self._mvcc_storage = storage
+        else:
+            from .mvccadapter import MVCCAdapter
+            self._mvcc_storage = MVCCAdapter(storage)
+
         self.references = ZODB.serialize.referencesf
-        try:
-            storage.registerDB(self)
-        except TypeError:
-            storage.registerDB(self, None) # Backward compat
 
         if (not hasattr(storage, 'tpc_vote')) and not storage.isReadOnly():
             warnings.warn(
@@ -438,12 +444,10 @@ class DB(object):
                 DeprecationWarning, 2)
             storage.tpc_vote = lambda *args: None
 
-        if IMVCCStorage.providedBy(storage):
-            temp_storage = storage.new_instance()
-        else:
-            temp_storage = storage
+        temp_storage = self._mvcc_storage.new_instance()
         try:
             try:
+                temp_storage.poll_invalidations()
                 temp_storage.load(z64, '')
             except KeyError:
                 # Create the database's root in the storage if it doesn't exist
@@ -462,8 +466,7 @@ class DB(object):
                 temp_storage.tpc_vote(t)
                 temp_storage.tpc_finish(t)
         finally:
-            if IMVCCStorage.providedBy(temp_storage):
-                temp_storage.release()
+            temp_storage.release()
 
         # Multi-database setup.
         if databases is None:
@@ -634,16 +637,13 @@ class DB(object):
 
         @self._connectionMap
         def _(c):
-            if c.opened:
-                c.transaction_manager.abort()
-                # Note that this will modify our pool, but this is safe, because
-                # _connectionMap makes a list of the pool to iterate over
-                c.close()
+            c.transaction_manager.abort()
             c.afterCompletion = c.newTransaction = c.close = noop
             c._release_resources()
 
-        self.storage.close()
+        self._mvcc_storage.close()
         del self.storage
+        del self._mvcc_storage
 
     def getCacheSize(self):
         return self._cache_size
@@ -675,27 +675,6 @@ class DB(object):
     def getHistoricalTimeout(self):
         return self.historical_pool.timeout
 
-    def invalidate(self, tid, oids, connection=None, version=''):
-        """Invalidate references to a given oid.
-
-        This is used to indicate that one of the connections has committed a
-        change to the object.  The connection commiting the change should be
-        passed in to prevent useless (but harmless) messages to the
-        connection.
-        """
-        # Storages, esp. ZEO tests, need the version argument still. :-/
-        assert version==''
-        # Notify connections.
-        def inval(c):
-            if c is not connection:
-                c.invalidate(tid, oids)
-        self._connectionMap(inval)
-
-    def invalidateCache(self):
-        """Invalidate each of the connection caches
-        """
-        self._connectionMap(lambda c: c.invalidateCache())
-
     transform_record_data = untransform_record_data = lambda self, data: data
 
     def objectCount(self):
@@ -1001,9 +980,9 @@ class TransactionalUndo(object):
 
     def __init__(self, db, tids):
         self._db = db
-        self._storage = db.storage
+        self._storage = getattr(
+            db._mvcc_storage, 'undo_instance', db._mvcc_storage.new_instance)()
         self._tids = tids
-        self._oids = set()
 
     def abort(self, transaction):
         pass
@@ -1013,19 +992,13 @@ class TransactionalUndo(object):
 
     def commit(self, transaction):
         for tid in self._tids:
-            result = self._storage.undo(tid, transaction)
-            if result:
-                self._oids.update(result[1])
+            self._storage.undo(tid, transaction)
 
     def tpc_vote(self, transaction):
-        for oid, _ in  self._storage.tpc_vote(transaction) or ():
-            self._oids.add(oid)
+        self._storage.tpc_vote(transaction)
 
     def tpc_finish(self, transaction):
-        self._storage.tpc_finish(
-            transaction,
-            lambda tid: self._db.invalidate(tid, self._oids)
-            )
+        self._storage.tpc_finish(transaction)
 
     def tpc_abort(self, transaction):
         self._storage.tpc_abort(transaction)

src/ZODB/historical_connections.txt

@@ -273,8 +273,6 @@ test.
     >>> conn.root()['first']['count'] += 1
     >>> conn.root()['third'] = persistent.mapping.PersistentMapping()
     >>> transaction.commit()
-    >>> len(historical_conn._invalidated)
-    0
     >>> historical_conn.close()
 
 Note that if you try to open an historical connection to a time in the future,

src/ZODB/interfaces.py

@@ -201,21 +201,6 @@ class IConnection(Interface):
     def isReadOnly():
         """Returns True if the storage for this connection is read only."""
 
-    def invalidate(tid, oids):
-        """Notify the Connection that transaction 'tid' invalidated oids.
-
-        When the next transaction boundary is reached, objects will be
-        invalidated.  If any of the invalidated objects are accessed by the
-        current transaction, the revision written before Connection.tid will be
-        used.
-
-        The DB calls this method, even when the Connection is closed.
-
-        Parameters:
-        tid: the storage-level id of the transaction that committed
-        oids: oids is an iterable of oids.
-        """
-
     def root():
         """Return the database root object.
 
@@ -278,14 +263,6 @@ class IConnection(Interface):
         If clear is True, reset the counters.
         """
 
-    def invalidateCache():
-        """Invalidate the connection cache
-
-        This invalidates *all* objects in the cache. If the connection
-        is open, subsequent reads will fail until a new transaction
-        begins or until the connection os reopned.
-        """
-
     def readCurrent(obj):
         """Make sure an object being read is current
 
@@ -625,19 +602,6 @@ class IStorage(Interface):
         otherwise, POSKeyError is raised.
         """
 
-#     The following two methods are effectively part of the interface,
-#     as they are generally needed when one storage wraps
-#     another. This deserves some thought, at probably debate, before
-#     adding them.
-#
-#     def _lock_acquire():
-#         """Acquire the storage lock
-#         """
-
-#     def _lock_release():
-#         """Release the storage lock
-#         """
-
     def new_oid():
         """Allocate a new object id.
 
@@ -675,11 +639,7 @@ class IStorage(Interface):
         The passed object is a wrapper object that provides an upcall
         interface to support composition.
 
-        Note that, for historical reasons, an implementation may
-        require a second argument, however, if required, the None will
-        be passed as the second argument.
-
-        Also, for historical reasons, this is called registerDB rather
+        Note that, for historical reasons, this is called registerDB rather
         than register_wrapper.
         """
 
@@ -818,7 +778,6 @@ class IStorage(Interface):
 
         """
 
-
 class IStorageRestoreable(IStorage):
     """Copying Transactions
 
@@ -1110,11 +1069,9 @@ class IMVCCStorage(IStorage):
         """
 
     def release():
-        """Release all persistent sessions used by this storage instance.
+        """Release resources held by the storage instance.
 
-        After this call, the storage instance can still be used;
-        calling methods that use persistent sessions will cause the
-        persistent sessions to be reopened.
+        The storage instance won't be used again after this call.
         """
 
     def poll_invalidations():

src/ZODB/mvccadapter.py

+"""Adapt IStorage objects to IMVCCStorage
+
+This is a largely internal implementation of ZODB, especially DB and
+Connection.  It takes the MVCC implementation involving invalidations
+and start time and moves it into a storage adapter.  This allows ZODB
+to treat Relstoage and other storages in pretty much the same way and
+also simplifies the implementation of the DB and Connection classes.
+"""
+import zope.interface
+
+from . import interfaces, serialize, POSException
+from .utils import p64, u64, Lock
+
+class Base(object):
+
+    _copy_methods = (
+        'getName', 'getSize', 'history', 'lastTransaction', 'sortKey',
+        'loadBlob', 'openCommittedBlobFile',
+        'isReadOnly', 'supportsUndo', 'undoLog', 'undoInfo',
+        'temporaryDirectory',
+        )
+
+    def __init__(self, storage):
+        self._storage = storage
+        if interfaces.IBlobStorage.providedBy(storage):
+            zope.interface.alsoProvides(self, interfaces.IBlobStorage)
+
+    def __getattr__(self, name):
+        if name in self._copy_methods:
+            if hasattr(self._storage, name):
+                m = getattr(self._storage, name)
+                setattr(self, name, m)
+                return m
+
+        raise AttributeError(name)
+
+    def __len__():
+        return len(self._storage)
+
+class MVCCAdapter(Base):
+
+    def __init__(self, storage):
+        Base.__init__(self, storage)
+        self._instances = set()
+        self._lock = Lock()
+        if hasattr(storage, 'registerDB'):
+            storage.registerDB(self)
+
+    def new_instance(self):
+        instance = MVCCAdapterInstance(self)
+        with self._lock:
+            self._instances.add(instance)
+        return instance
+
+    def before_instance(self, before=None):
+        return HistoricalStorageAdapter(self._storage, before)
+
+    def undo_instance(self):
+        return UndoAdapterInstance(self)
+
+    def _release(self, instance):
+        with self._lock:
+            self._instances.remove(instance)
+
+    closed = False
+    def close(self):
+        if not self.closed:
+            self.closed = True
+            self._storage.close()
+            del self._instances
+            del self._storage
+
+    def invalidateCache(self):
+        with self._lock:
+            for instance in self._instances:
+                instance._invalidateCache()
+
+    def invalidate(self, transaction_id, oids, version=''):
+        with self._lock:
+            for instance in self._instances:
+                instance._invalidate(oids)
+
+    def _invalidate_finish(self, oids, committing_instance):
+        with self._lock:
+            for instance in self._instances:
+                if instance is not committing_instance:
+                    instance._invalidate(oids)
+
+    references = serialize.referencesf
+    transform_record_data = untransform_record_data = lambda self, data: data
+
+    def pack(self, pack_time, referencesf):
+        return self._storage.pack(pack_time, referencesf)
+
+class MVCCAdapterInstance(Base):
+
+    _copy_methods = Base._copy_methods + (
+        'loadSerial', 'new_oid', 'tpc_vote',
+        'checkCurrentSerialInTransaction', 'tpc_abort',
+        )
+
+    def __init__(self, base):
+        self._base = base
+        Base.__init__(self, base._storage)
+        self._lock = Lock()
+        self._invalidations = set()
+        self._start = None # Transaction start time
+        self._sync = getattr(self._storage, 'sync', lambda : None)
+
+    def release(self):
+        self._base._release(self)
+
+    close = release
+
+    def _invalidateCache(self):
+        with self._lock:
+            self._invalidations = None
+
+    def _invalidate(self, oids):
+        with self._lock:
+            try:
+                self._invalidations.update(oids)
+            except AttributeError:
+                if self._invalidations is not None:
+                    raise
+
+    def sync(self, force=True):
+        if force:
+            self._sync()
+
+    def poll_invalidations(self):
+        self._start = p64(u64(self._storage.lastTransaction()) + 1)
+        with self._lock:
+            if self._invalidations is None:
+                self._invalidations = set()
+                return None
+            else:
+                result = list(self._invalidations)
+                self._invalidations.clear()
+                return result
+
+    def load(self, oid, version=''):
+        assert self._start is not None
+        r = self._storage.loadBefore(oid, self._start)
+        if r is None:
+            raise POSException.ReadConflictError(repr(oid))
+        return r[:2]
+
+    _modified = None # Used to keep track of oids modified within a
+                     # transaction, so we can invalidate them later.
+
+    def tpc_begin(self, transaction):
+        self._storage.tpc_begin(transaction)
+        self._modified = set()
+
+    def store(self, oid, serial, data, version, transaction):
+        s = self._storage.store(oid, serial, data, version, transaction)
+        self._modified.add(oid)
+        return s
+
+    def storeBlob(self, oid, serial, data, blobfilename, version, transaction):
+        s = self._storage.storeBlob(
+            oid, serial, data, blobfilename, '', transaction)
+        self._modified.add(oid)
+        return s
+
+    def tpc_finish(self, transaction, func = lambda tid: None):
+        modified = self._modified
+        self._modified = None
+
+        def invalidate_finish(tid):
+            self._base._invalidate_finish(modified, self)
+            func(tid)
+
+        self._storage.tpc_finish(transaction, invalidate_finish)
+
+def read_only_writer(self, *a, **kw):
+    raise POSException.ReadOnlyError
+
+class HistoricalStorageAdapter(Base):
+    """Adapt a storage to a historical storage
+    """
+
+    _copy_methods = Base._copy_methods + (
+        'loadSerial', 'tpc_begin', 'tpc_finish', 'tpc_abort', 'tpc_vote',
+        'checkCurrentSerialInTransaction',
+        )
+
+    def __init__(self, storage, before=None):
+        Base.__init__(self, storage)
+        self._before = before
+
+    def isReadOnly(self):
+        return True
+
+    def supportsUndo(self):
+        return False
+
+    def release(self):
+        pass
+
+    close = release
+
+    def sync(self, force=True):
+        pass
+
+    def poll_invalidations(self):
+        return []
+
+    new_oid = pack = store = read_only_writer
+
+    def load(self, oid, version=''):
+        r = self._storage.loadBefore(oid, self._before)
+        if r is None:
+            raise POSException.POSKeyError(oid)
+        return r[:2]
+
+
+class UndoAdapterInstance(Base):
+
+    _copy_methods = Base._copy_methods + (
+        'tpc_abort',
+        )
+
+    def __init__(self, base):
+        self._base = base
+        Base.__init__(self, base._storage)
+
+    def release(self):
+        pass
+
+    close = release
+
+    def tpc_begin(self, transaction):
+        self._storage.tpc_begin(transaction)
+        self._undone = set()
+
+    def undo(self, transaction_id, transaction):
+        result = self._storage.undo(transaction_id, transaction)
+        if result:
+            self._undone.update(result[1])
+        return result
+
+    def tpc_vote(self, transaction):
+        result = self._storage.tpc_vote(transaction)
+        if result:
+            for oid, serial in result:
+                self._undone.add(oid)
+
+    def tpc_finish(self, transaction, func = lambda tid: None):
+
+        def invalidate_finish(tid):
+            self._base._invalidate_finish(self._undone, None)
+            func(tid)
+
+        self._storage.tpc_finish(transaction, invalidate_finish)

src/ZODB/tests/blob_connection.txt

@@ -74,7 +74,7 @@ You can't put blobs into a database that has uses a Non-Blob-Storage, though:
     >>> transaction2.commit()        # doctest: +ELLIPSIS
     Traceback (most recent call last):
         ...
-    Unsupported: Storing Blobs in <ZODB.MappingStorage.MappingStorage object at ...> is not supported.
+    Unsupported: Storing Blobs in ...
 
     >>> transaction2.abort()
     >>> connection2.close()

src/ZODB/tests/testConnection.py

@@ -27,6 +27,8 @@ from persistent import Persistent
 from zope.interface.verify import verifyObject
 from zope.testing import loggingsupport, renormalizing
 
+from .. import mvccadapter
+
 checker = renormalizing.RENormalizing([
     # Python 3 bytes add a "b".
     (re.compile("b('.*?')"), r"\1"),
@@ -154,7 +156,8 @@ class ConnectionDotAdd(ZODB.tests.util.TestCase):
         self.datamgr.add(obj)
         self.datamgr.tpc_begin(self.transaction)
         self.datamgr.tpc_finish(self.transaction)
-        self.assertTrue(obj._p_oid not in self.datamgr._storage._stored)
+        self.assertTrue(obj._p_oid not in
+                        self.datamgr._storage._storage._stored)
 
     def test__resetCacheResetsReader(self):
         # https://bugs.launchpad.net/zodb/+bug/142667
@@ -435,8 +438,11 @@ class UserMethodTests(unittest.TestCase):
           ...
         ConnectionStateError: The database connection is closed
 
+        >>> db.close()
+
         An expedient way to create a read-only storage:
 
+        >>> db = databaseFromString("<zodb>\n<mappingstorage/>\n</zodb>")
         >>> db.storage.isReadOnly = lambda: True
         >>> cn = db.open()
         >>> cn.isReadOnly()
@@ -510,7 +516,9 @@ class InvalidationTests(unittest.TestCase):
         they have the expected effect.
 
         >>> db = databaseFromString("<zodb>\n<mappingstorage/>\n</zodb>")
+        >>> mvcc_storage = db._mvcc_storage
         >>> cn = db.open()
+        >>> mvcc_instance = cn._storage
         >>> p1 = Persistent()
         >>> p2 = Persistent()
         >>> p3 = Persistent()
@@ -521,29 +529,18 @@ class InvalidationTests(unittest.TestCase):
         Transaction ids are 8-byte strings, just like oids; p64() will
         create one from an int.
 
-        >>> cn.invalidate(p64(1), {p1._p_oid: 1})
+        >>> mvcc_storage.invalidate(p64(1), {p1._p_oid: 1})
 
         Transaction start times are based on storage's last
         transaction. (Previousely, they were based on the first
         invalidation seen in a transaction.)
 
-        >>> cn._txn_time == p64(u64(db.storage.lastTransaction()) + 1)
+        >>> mvcc_instance.poll_invalidations() == [p1._p_oid]
         True
-
-        >>> p1._p_oid in cn._invalidated
-        True
-        >>> p2._p_oid in cn._invalidated
-        False
-
-        >>> cn.invalidate(p64(10), {p2._p_oid: 1, p64(76): 1})
-
-        >>> cn._txn_time == p64(u64(db.storage.lastTransaction()) + 1)
+        >>> mvcc_instance._start == p64(u64(db.storage.lastTransaction()) + 1)
         True
 
-        >>> p1._p_oid in cn._invalidated
-        True
-        >>> p2._p_oid in cn._invalidated
-        True
+        >>> mvcc_storage.invalidate(p64(10), {p2._p_oid: 1, p64(76): 1})
 
         Calling invalidate() doesn't affect the object state until
         a transaction boundary.
@@ -560,24 +557,24 @@ class InvalidationTests(unittest.TestCase):
 
         >>> cn.sync()
         >>> p1._p_state
-        -1
+        0
         >>> p2._p_state
         -1
         >>> p3._p_state
         0
-        >>> cn._invalidated
-        set([])
 
         >>> db.close()
         """
 
 def doctest_invalidateCache():
-    """The invalidateCache method invalidates a connection's cache.  It also
-    prevents reads until the end of a transaction::
+    """The invalidateCache method invalidates a connection's cache.
+
+    It also prevents reads until the end of a transaction::
 
         >>> from ZODB.tests.util import DB
         >>> import transaction
         >>> db = DB()
+        >>> mvcc_storage = db._mvcc_storage
         >>> tm = transaction.TransactionManager()
         >>> connection = db.open(transaction_manager=tm)
         >>> connection.root()['a'] = StubObject()
@@ -593,53 +590,33 @@ def doctest_invalidateCache():
     So we have a connection and an active transaction with some modifications.
     Lets call invalidateCache:
 
-        >>> connection.invalidateCache()
-
-    Now, if we try to load an object, we'll get a read conflict:
-
-        >>> connection.root()['b'].x
-        Traceback (most recent call last):
-        ...
-        ReadConflictError: database read conflict error
-
-    If we try to commit the transaction, we'll get a conflict error:
+        >>> mvcc_storage.invalidateCache()
 
-        >>> tm.commit()
-        Traceback (most recent call last):
-        ...
-        ConflictError: database conflict error
+    This won't have any effect until the next transaction:
 
-    and the cache will have been cleared:
+        >>> connection.root()['a']._p_changed
+        0
+        >>> connection.root()['b']._p_changed
+        >>> connection.root()['c']._p_changed
+        1
 
-        >>> print(connection.root()['a']._p_changed)
-        None
-        >>> print(connection.root()['b']._p_changed)
-        None
-        >>> print(connection.root()['c']._p_changed)
-        None
+    But if we sync():
 
-    But we'll be able to access data again:
+        >>> connection.sync()
 
-        >>> connection.root()['b'].x
-        1
+    All of our data was invalidated:
 
-    Aborting a transaction after a read conflict also lets us read data and go
-    on about our business:
+        >>> connection.root()['a']._p_changed
+        >>> connection.root()['b']._p_changed
+        >>> connection.root()['c']._p_changed
 
-        >>> connection.invalidateCache()
+    But we can load data as usual:
 
-        >>> connection.root()['c'].x
-        Traceback (most recent call last):
-        ...
-        ReadConflictError: database read conflict error
+    Now, if we try to load an object, we'll get a read conflict:
 
-        >>> tm.abort()
-        >>> connection.root()['c'].x
+        >>> connection.root()['b'].x
         1
 
-        >>> connection.root()['c'].x = 2
-        >>> tm.commit()
-
         >>> db.close()
     """
 
@@ -1333,6 +1310,7 @@ class StubDatabase:
 
     def __init__(self):
         self.storage = StubStorage()
+        self._mvcc_storage = mvccadapter.MVCCAdapter(self.storage)
         self.new_oid = self.storage.new_oid
 
     classFactory = None

src/ZODB/tests/testDB.py

@@ -83,34 +83,40 @@ def test_invalidateCache():
         >>> from ZODB.tests.util import DB
         >>> import transaction
         >>> db = DB()
+        >>> mvcc_storage = db._mvcc_storage
         >>> tm1 = transaction.TransactionManager()
         >>> c1 = db.open(transaction_manager=tm1)
         >>> c1.root()['a'] = MinPO(1)
         >>> tm1.commit()
         >>> tm2 = transaction.TransactionManager()
         >>> c2 = db.open(transaction_manager=tm2)
-        >>> c1.root()['a']._p_deactivate()
+        >>> c2.root()['a'].value
+        1
         >>> tm3 = transaction.TransactionManager()
         >>> c3 = db.open(transaction_manager=tm3)
         >>> c3.root()['a'].value
         1
         >>> c3.close()
-        >>> db.invalidateCache()
-
-        >>> c1.root()['a'].value
-        Traceback (most recent call last):
-        ...
-        ReadConflictError: database read conflict error
-
-        >>> c2.root()['a'].value
-        Traceback (most recent call last):
-        ...
-        ReadConflictError: database read conflict error
 
+        >>> mvcc_storage.invalidateCache()
+        >>> c1.root.a._p_changed
+        0
+        >>> c1.sync()
+        >>> c1.root.a._p_changed
+        >>> c2.root.a._p_changed
+        0
+        >>> c2.sync()
+        >>> c2.root.a._p_changed
         >>> c3 is db.open(transaction_manager=tm3)
         True
-        >>> print(c3.root()['a']._p_changed)
-        None
+        >>> c3.root.a._p_changed
+
+        >>> c1.root()['a'].value
+        1
+        >>> c2.root()['a'].value
+        1
+        >>> c3.root()['a'].value
+        1
 
         >>> db.close()
     """

src/ZODB/tests/testblob.py

@@ -547,14 +547,13 @@ def loadblob_tmpstore():
     >>> transaction.commit()
     >>> blob_oid = root['blob']._p_oid
     >>> tid = connection._storage.lastTransaction()
-    >>> _txn_time = connection._txn_time
 
     Now we open a database with a TmpStore in front:
 
     >>> database.close()
 
     >>> from ZODB.Connection import TmpStore
-    >>> tmpstore = TmpStore(blob_storage, _txn_time)
+    >>> tmpstore = TmpStore(blob_storage)
 
     We can access the blob correctly:
 

src/ZODB/tests/testmvcc.py

@@ -68,9 +68,9 @@ Now open a second connection.
 >>> tm2 = transaction.TransactionManager()
 >>> cn2 = db.open(transaction_manager=tm2)
 >>> from ZODB.utils import p64, u64
->>> cn2._txn_time == p64(u64(st.lastTransaction()) + 1)
+>>> cn2._storage._start == p64(u64(st.lastTransaction()) + 1)
 True
->>> txn_time2  = cn2._txn_time
+>>> txn_time2  = cn2._storage._start
 
 Connection high-water mark
 --------------------------
@@ -85,13 +85,13 @@ storage has seen.
 
 >>> cn = db.open()
 
->>> cn._txn_time == p64(u64(st.lastTransaction()) + 1)
+>>> cn._storage._start == p64(u64(st.lastTransaction()) + 1)
 True
->>> cn.invalidate(100, dict.fromkeys([1, 2]))
->>> cn._txn_time == p64(u64(st.lastTransaction()) + 1)
+>>> cn.db()._mvcc_storage.invalidate(100, dict.fromkeys([1, 2]))
+>>> cn._storage._start == p64(u64(st.lastTransaction()) + 1)
 True
->>> cn.invalidate(200, dict.fromkeys([1, 2]))
->>> cn._txn_time == p64(u64(st.lastTransaction()) + 1)
+>>> cn.db()._mvcc_storage.invalidate(200, dict.fromkeys([1, 2]))
+>>> cn._storage._start == p64(u64(st.lastTransaction()) + 1)
 True
 
 A connection's high-water mark is set to the transaction id taken from
@@ -105,7 +105,7 @@ but that doesn't work unless an object is modified.  sync() will abort
 a transaction and process invalidations.
 
 >>> cn.sync()
->>> cn._txn_time == p64(u64(st.lastTransaction()) + 1)
+>>> cn._storage._start == p64(u64(st.lastTransaction()) + 1)
 True
 
 Basic functionality
@@ -121,14 +121,14 @@ will modify "a."  The other transaction will then modify "b" and commit.
 
 The second connection already has its high-water mark set.
 
->>> cn2._txn_time == txn_time2
+>>> cn2._storage._start == txn_time2
 True
 
 It is safe to read "b," because it was not modified by the concurrent
 transaction.
 
 >>> r2 = cn2.root()
->>> r2["b"]._p_serial < cn2._txn_time
+>>> r2["b"]._p_serial < cn2._storage._start
 True
 >>> r2["b"].value
 1
@@ -140,7 +140,7 @@ non-current version.
 
 >>> r2["a"].value
 1
->>> r2["a"]._p_serial < cn2._txn_time
+>>> r2["a"]._p_serial < cn2._storage._start
 True
 
 We can confirm that we have a non-current revision by asking the
@@ -153,32 +153,32 @@ It's possible to modify "a", but we get a conflict error when we
 commit the transaction.
 
 >>> r2["a"].value = 3
->>> tm2.get().commit()
+>>> tm2.get().commit() # doctest: +ELLIPSIS
 Traceback (most recent call last):
  ...
-ConflictError: database conflict error (oid 0x01, class ZODB.tests.MinPO.MinPO)
+ConflictError: database conflict error (oid 0x01, class ZODB.tests.MinPO...
 
 >>> tm2.get().abort()
 
 This example will demonstrate that we can commit a transaction if we only
 modify current revisions.
 
->>> cn2._txn_time == p64(u64(st.lastTransaction()) + 1)
+>>> cn2._storage._start == p64(u64(st.lastTransaction()) + 1)
 True
->>> txn_time2  = cn2._txn_time
+>>> txn_time2  = cn2._storage._start
 
 >>> r1 = cn1.root()
 >>> r1["a"].value = 3
 >>> tm1.get().commit()
 >>> txn = db.lastTransaction()
->>> cn2._txn_time == txn_time2
+>>> cn2._storage._start == txn_time2
 True
 
 >>> r2["b"].value = r2["a"].value + 1
 >>> r2["b"].value
 3
 >>> tm2.get().commit()
->>> cn2._txn_time == p64(u64(st.lastTransaction()) + 1)
+>>> cn2._storage._start == p64(u64(st.lastTransaction()) + 1)
 True
 
 Object cache
@@ -362,18 +362,18 @@ This test is kinda screwy because it depends on an old approach that
 has changed.  We'll hack the _txn_time to get the original expected
 result, even though what's going on now is much simpler.
 
->>> cn1._txn_time = ts.lastTransaction()
+>>> cn1._storage._start = ts.lastTransaction()
 
 Once the oid is hooked, an invalidation will be delivered the next
 time it is activated.  The code below activates the object, then
 confirms that the hook worked and that the old state was retrieved.
 
->>> oid in cn1._invalidated
+>>> oid in cn1._storage._invalidations
 False
 >>> r1["b"]._p_state
 -1
 >>> r1["b"]._p_activate()
->>> oid in cn1._invalidated
+>>> oid in cn1._storage._invalidations
 True
 >>> ts.count
 1
@@ -406,15 +406,15 @@ Again, once the oid is hooked, an invalidation will be delivered the next
 time it is activated.  The code below activates the object, but unlike the
 section above, this is no older state to retrieve.
 
->>> oid in cn1._invalidated
+>>> oid in cn1._storage._invalidations
 False
 >>> r1["b"]._p_state
 -1
->>> cn1._txn_time = ts.lastTransaction()
->>> r1["b"]._p_activate()
+>>> cn1._storage._start = ts.lastTransaction()
+>>> r1["b"]._p_activate() # doctest: +ELLIPSIS
 Traceback (most recent call last):
  ...
-ReadConflictError: database read conflict error
+ReadConflictError: ...
 
 >>> db.close()
 """