loadAt

loadAt is new optional storage interface that is intended to replace loadBefore
with more clean and uniform semantic. Compared to loadBefore, loadAt:

1) returns data=None and serial of the removal, when loaded object was found to
   be deleted. loadBefore is returning only data=None in such case. This loadAt
   property allows to fix DemoStorage data corruption when whiteouts in overlay
   part were not previously correctly taken into account.

   https://github.com/zopefoundation/ZODB/issues/318

2) for regular data records, does not require storages to return next_serial,
   in addition to (data, serial). loadBefore requirement to return both
   serial and next_serial is constraining storages unnecessarily, and,
   while for FileStorage it is free to implement, for other storages it is
   not - for example for NEO and RelStorage, finding out next_serial, after
   looking up oid@at data record, costs one more SQL query:

   https://lab.nexedi.com/nexedi/neoppod/blob/fb746e6b/neo/storage/database/mysqldb.py#L484-508
   https://lab.nexedi.com/nexedi/neoppod/blob/fb746e6b/neo/storage/database/mysqldb.py#L477-482

   https://github.com/zodb/relstorage/blob/3.1.1-1-ge7628f9/src/relstorage/storage/load.py#L259-L264
   https://github.com/zodb/relstorage/blob/3.1.1-1-ge7628f9/src/relstorage/adapters/mover.py#L177-L199

   next_serial is not only about execution overhead - it is semantically
   redundant to be there and can be removed from load return. The reason
   I say that next_serial can be removed is that in ZODB/py the only place,
   that I could find, where next_serial is used on client side is in client
   cache (e.g. in NEO client cache), and that cache can be remade to
   work without using that next_serial at all. In simple words whenever
   after

     loadAt(oid, at)  ->  (data, serial)

   query, the cache can remember data for oid in [serial, at] range.

   Next, when invalidation message from server is received, cache entries,
   that had at == client_head, are extended (at -> new_head) for oids that
   are not present in invalidation message, while for oids that are present
   in invalidation message no such extension is done. This allows to
   maintain cache in correct state, invalidate it when there is a need to
   invalidate, and not to throw away cache entries that should remain live.
   This of course requires ZODB server to include both modified and
   just-created objects into invalidation messages

     ( https://github.com/zopefoundation/ZEO/pull/160 ,
       https://github.com/zopefoundation/ZODB/pull/319 ).

   Switching to loadAt should thus allow storages like NEO and, maybe,
   RelStorage, to do 2x less SQL queries on every object access.

   https://github.com/zopefoundation/ZODB/issues/318#issuecomment-657685745

In other words loadAt unifies return signature to always be

   (data, serial)

instead of

   POSKeyError				object does not exist at all
   None					object was removed
   (data, serial, next_serial)		regular data record

used by loadBefore.

This patch:

- introduces new interface.
- introduces ZODB.utils.loadAt helper, that uses either storage.loadAt,
  or, if the storage does not implement loadAt interface, tries to mimic
  loadAt semantic via storage.loadBefore to possible extent + emits
  corresponding warning.
- converts MVCCAdapter to use loadAt instead of loadBefore.
- changes DemoStorage to use loadAt, and this way fixes above-mentioned
  data corruption issue; adds corresponding test; converts
  DemoStorage.loadBefore to be a wrapper around DemoStorage.loadAt.
- adds loadAt implementation to FileStorage and MappingStorage.
- adapts other tests/code correspondingly.

/cc @jimfulton, @jamadden, @vpelletier, @jmuchemb, @arnaud-fontaine, @gidzit, @klawlf82, @hannosch

src/ZODB/BaseStorage.py

@@ -59,7 +59,7 @@ class BaseStorage(UndoLogCompatible):
 
     If it stores multiple revisions, it should implement
     loadSerial()
-    loadBefore()
+    loadAt()
 
     Each storage will have two locks that are accessed via lock
     acquire and release methods bound to the instance.  (Yuck.)
@@ -267,9 +267,8 @@ class BaseStorage(UndoLogCompatible):
         raise POSException.Unsupported(
             "Retrieval of historical revisions is not supported")
 
-    def loadBefore(self, oid, tid):
-        """Return most recent revision of oid before tid committed."""
-        return None
+    # do not provide loadAt/loadBefore here in BaseStorage - if child forgets
+    # to override it - storage will always return "no data" instead of failing.
 
     def copyTransactionsFrom(self, other, verbose=0):
         """Copy transactions from another storage.

src/ZODB/DB.py

@@ -726,8 +726,7 @@ class DB(object):
           - `before`: like `at`, but opens the readonly state before the
             tid or datetime.
         """
-        # `at` is normalized to `before`, since we use storage.loadBefore
-        # as the underlying implementation of both.
+        # `at` is normalized to `before`.
         before = getTID(at, before)
         if (before is not None and
             before > self.lastTransaction() and

src/ZODB/DemoStorage.py

@@ -24,6 +24,7 @@ import os
 import random
 import weakref
 import tempfile
+import warnings
 import ZODB.BaseStorage
 import ZODB.blob
 import ZODB.interfaces
@@ -217,45 +218,55 @@ class DemoStorage(ConflictResolvingStorage):
     # still want load for old clients (e.g. zeo servers)
     load = load_current
 
-    def loadBefore(self, oid, tid):
-        try:
-            result = self.changes.loadBefore(oid, tid)
-        except ZODB.POSException.POSKeyError:
-            # The oid isn't in the changes, so defer to base
-            return self.base.loadBefore(oid, tid)
+    def loadAt(self, oid, at):
+        data, serial = ZODB.utils.loadAt(self.changes, oid, at)
+        if (data is not None) or (serial != ZODB.utils.z64):
+            # object is present in changes either as data or deletion record.
+            return data, serial
 
-        if result is None:
-            # The oid *was* in the changes, but there aren't any
-            # earlier records. Maybe there are in the base.
-            try:
-                result = self.base.loadBefore(oid, tid)
-            except ZODB.POSException.POSKeyError:
-                # The oid isn't in the base, so None will be the right result
-                pass
+        # object is not present in changes at all - use base
+        return ZODB.utils.loadAt(self.base, oid, at)
+
+    def loadBefore(self, oid, before):
+        warnings.warn("loadBefore is deprecated - use loadAt instead",
+                DeprecationWarning, stacklevel=2)
+        p64 = ZODB.utils.p64
+        u64 = ZODB.utils.u64
+
+        if before in (maxtid, ZODB.utils.z64):
+            at = before
+        else:
+            at = p64(u64(before)-1)
+
+        data, serial = self.loadAt(oid, at)
+
+        # find out next_serial.
+        # it is ok to use dumb/slow implementation since loadBefore should not
+        # be used and is provided only for backward compatibility.
+        next_serial = maxtid
+        while 1:
+            _, s = self.loadAt(oid, p64(u64(next_serial)-1))
+            assert s >= serial
+            if s == serial:
+                # found - next_serial is serial of the next data record
+                break
+            next_serial = s
+
+        if next_serial == maxtid:
+            next_serial = None
+
+        # next_serial found -> return/raise what loadBefore users expect
+        if data is None:
+            if next_serial is None:
+                # object was never created
+                raise ZODB.POSException.POSKeyError(oid)
             else:
-                if result and not result[-1]:
-                    # The oid is current in the base.  We need to find
-                    # the end tid in the base by fining the first tid
-                    # in the changes. Unfortunately, there isn't an
-                    # api for this, so we have to walk back using
-                    # loadBefore.
-
-                    if tid == maxtid:
-                        # Special case: we were looking for the
-                        # current value. We won't find anything in
-                        # changes, so we're done.
-                        return result
-
-                    end_tid = maxtid
-                    t = self.changes.loadBefore(oid, end_tid)
-                    while t:
-                        end_tid = t[1]
-                        t = self.changes.loadBefore(oid, end_tid)
-                    result = result[:2] + (
-                        end_tid if end_tid != maxtid else None,
-                        )
-
-        return result
+                # object was deleted
+                return None
+
+        # regular data record
+        return data, serial, next_serial
+
 
     def loadBlob(self, oid, serial):
         try:

src/ZODB/FileStorage/FileStorage.py

@@ -21,6 +21,7 @@ import errno
 import logging
 import os
 import time
+import warnings
 from struct import pack
 from struct import unpack
 
@@ -57,6 +58,7 @@ from ZODB.interfaces import IStorageCurrentRecordIteration
 from ZODB.interfaces import IStorageIteration
 from ZODB.interfaces import IStorageRestoreable
 from ZODB.interfaces import IStorageUndoable
+from ZODB.interfaces import IStorageLoadAt
 from ZODB.POSException import ConflictError
 from ZODB.POSException import MultipleUndoErrors
 from ZODB.POSException import POSKeyError
@@ -133,6 +135,7 @@ class TempFormatter(FileStorageFormatter):
         IStorageCurrentRecordIteration,
         IExternalGC,
         IStorage,
+        IStorageLoadAt,
         )
 class FileStorage(
     FileStorageFormatter,
@@ -559,7 +562,40 @@ class FileStorage(
             else:
                 return self._loadBack_impl(oid, h.back)[0]
 
+    def loadAt(self, oid, at):
+        """loadAt implements IStorageLoadAt."""
+        with self._files.get() as _file:
+            try:
+                pos = self._lookup_pos(oid)
+            except POSKeyError:
+                # object does not exist
+                return None, z64
+
+            while 1:
+                h = self._read_data_header(pos, oid, _file)
+                if h.tid <= at:
+                    break
+                pos = h.prev
+                if not pos:
+                    # object not yet created as of at
+                    return None, z64
+
+            # h is the most recent DataHeader with .tid <= at
+            if h.plen:
+                # regular data record
+                return _file.read(h.plen), h.tid
+            elif h.back:
+                # backpointer
+                data, _, _, _ = self._loadBack_impl(oid, h.back,
+                                            fail=False, _file=_file)
+                return data, h.tid
+            else:
+                # deletion
+                return None, h.tid
+
     def loadBefore(self, oid, tid):
+        warnings.warn("loadBefore is deprecated - use loadAt instead",
+                DeprecationWarning, stacklevel=2)
         with self._files.get() as _file:
             pos = self._lookup_pos(oid)
             end_tid = None

src/ZODB/MappingStorage.py

@@ -19,6 +19,7 @@ storage without distracting storage details.
 
 import BTrees
 import time
+import warnings
 import ZODB.BaseStorage
 import ZODB.interfaces
 import ZODB.POSException
@@ -30,6 +31,7 @@ import zope.interface
 @zope.interface.implementer(
         ZODB.interfaces.IStorage,
         ZODB.interfaces.IStorageIteration,
+        ZODB.interfaces.IStorageLoadAt,
         )
 class MappingStorage(object):
     """In-memory storage implementation
@@ -148,9 +150,26 @@ class MappingStorage(object):
 
     load = ZODB.utils.load_current
 
+    # ZODB.interfaces.IStorageLoadAt
+    @ZODB.utils.locked(opened)
+    def loadAt(self, oid, at):
+        z64 = ZODB.utils.z64
+        tid_data = self._data.get(oid)
+        if not tid_data:
+            return None, z64
+        if at == z64:
+            return None, z64
+        tids_at = tid_data.keys(None, at)
+        if not tids_at:
+            return None, z64
+        serial = tids_at[-1]
+        return tid_data[serial], serial
+
     # ZODB.interfaces.IStorage
     @ZODB.utils.locked(opened)
     def loadBefore(self, oid, tid):
+        warnings.warn("loadBefore is deprecated - use loadAt instead",
+                DeprecationWarning, stacklevel=2)
         tid_data = self._data.get(oid)
         if tid_data:
             before = ZODB.utils.u64(tid)

src/ZODB/blob.py

@@ -866,10 +866,10 @@ class BlobStorage(BlobStorageMixin):
             for oid in self.fshelper.getOIDsForSerial(serial_id):
                 # we want to find the serial id of the previous revision
                 # of this blob object.
-                load_result = self.loadBefore(oid, serial_id)
-
-                if load_result is None:
+                at_before = utils.p64(utils.u64(serial_id)-1)
+                _, serial_before = utils.loadAt(self, oid, at_before)
 
+                if serial_before == utils.z64:
                     # There was no previous revision of this blob
                     # object.  The blob was created in the transaction
                     # represented by serial_id.  We copy the blob data
@@ -884,7 +884,6 @@ class BlobStorage(BlobStorageMixin):
                     # transaction implied by "serial_id".  We copy the blob
                     # data to a new file that references the undo transaction
                     # in case a user wishes to undo this undo.
-                    data, serial_before, serial_after = load_result
                     orig_fn = self.fshelper.getBlobFilename(oid, serial_before)
                     new_fn = self.fshelper.getBlobFilename(oid, undo_serial)
                 with open(orig_fn, "rb") as orig:

src/ZODB/historical_connections.rst

@@ -36,7 +36,7 @@ development continues on a "development" head.
 
 A database can be opened historically ``at`` or ``before`` a given transaction
 serial or datetime. Here's a simple example. It should work with any storage
-that supports ``loadBefore``.
+that supports ``loadAt`` or ``loadBefore``.
 
 We'll begin our example with a fairly standard set up.  We
 
@@ -138,10 +138,9 @@ root.
     >>> historical_conn.root()['first']['count']
     0
 
-In fact, ``at`` arguments are translated into ``before`` values because the
-underlying mechanism is a storage's loadBefore method.  When you look at a
-connection's ``before`` attribute, it is normalized into a ``before`` serial,
-no matter what you pass into ``db.open``.
+In fact, ``at`` arguments are translated into ``before`` values.
+When you look at a connection's ``before`` attribute, it is normalized into a
+``before`` serial, no matter what you pass into ``db.open``.
 
     >>> print(conn.before)
     None

src/ZODB/interfaces.py

@@ -710,6 +710,9 @@ class IStorage(Interface):
     def loadBefore(oid, tid):
         """Load the object data written before a transaction id
 
+        ( This method is deprecated and kept for backward-compatibility.
+          Please use loadAt instead. )
+
         If there isn't data before the object before the given
         transaction, then None is returned, otherwise three values are
         returned:
@@ -886,6 +889,24 @@ class IPrefetchStorage(IStorage):
         more than once.
         """
 
+class IStorageLoadAt(Interface):
+
+    def loadAt(oid, at): # -> (data, serial)
+        """Load object data as observed at given database state.
+
+        loadAt returns data for object with given object ID as observed by
+        database state ≤ at. Two values are returned:
+
+        - The data record,
+        - The transaction ID of the data record.
+
+        If the object does not exist, or is deleted as of `at` database state,
+        loadAt returns data=None, and serial indicates transaction ID of the
+        most recent deletion done in transaction with ID ≤ at, or null tid if
+        there is no such deletion.
+
+        Note: no POSKeyError is raised even if object id is not in the storage.
+        """
 
 class IMultiCommitStorage(IStorage):
     """A multi-commit storage can commit multiple transactions at once.

src/ZODB/mvccadapter.py

@@ -10,7 +10,7 @@ also simplifies the implementation of the DB and Connection classes.
 import zope.interface
 
 from . import interfaces, serialize, POSException
-from .utils import p64, u64, Lock, oid_repr, tid_repr
+from .utils import p64, u64, z64, maxtid, Lock, loadAt, oid_repr, tid_repr
 
 class Base(object):
 
@@ -99,7 +99,7 @@ class MVCCAdapterInstance(Base):
         'checkCurrentSerialInTransaction', 'tpc_abort',
         )
 
-    _start = None # Transaction start time
+    _start = None # Transaction start time (before)
     _ltid = b''   # Last storage transaction id
 
     def __init__(self, base):
@@ -151,8 +151,20 @@ class MVCCAdapterInstance(Base):
 
     def load(self, oid):
         assert self._start is not None
-        r = self._storage.loadBefore(oid, self._start)
-        if r is None:
+        at = p64(u64(self._start)-1)
+        data, serial = loadAt(self._storage, oid, at)
+        if data is None:
+            # raise POSKeyError if object does not exist at all
+            # TODO raise POSKeyError always and switch to raising ReadOnlyError only when
+            # actually detecting that load is being affected by simultaneous pack (see below).
+            if serial == z64:
+                # XXX second call to loadAt - it will become unneeded once we
+                # switch to raising POSKeyError.
+                _, serial_exists = loadAt(self._storage, oid, maxtid)
+                if serial_exists == z64:
+                    # object does not exist at all
+                    raise POSException.POSKeyError(oid)
+
             # object was deleted or not-yet-created.
             # raise ReadConflictError - not - POSKeyError due to backward
             # compatibility: a pack(t+δ) could be running simultaneously to our
@@ -174,11 +186,14 @@ class MVCCAdapterInstance(Base):
             # whether pack is/was actually running and its details, take that
             # into account, and raise ReadConflictError only in the presence of
             # database being simultaneously updated from back of its log.
+            #
+            # See https://github.com/zopefoundation/ZODB/pull/322 for
+            # preliminary steps in this direction.
             raise POSException.ReadConflictError(
                     "load %s @%s: object deleted, likely by simultaneous pack" %
                     (oid_repr(oid), tid_repr(p64(u64(self._start) - 1))))
 
-        return r[:2]
+        return data, serial
 
     def prefetch(self, oids):
         try:
@@ -257,10 +272,11 @@ class HistoricalStorageAdapter(Base):
     new_oid = pack = store = read_only_writer
 
     def load(self, oid, version=''):
-        r = self._storage.loadBefore(oid, self._before)
-        if r is None:
+        at = p64(u64(self._before)-1)
+        data, serial = loadAt(self._storage, oid, at)
+        if data is None:
             raise POSException.POSKeyError(oid)
-        return r[:2]
+        return data, serial
 
 
 class UndoAdapterInstance(Base):

src/ZODB/tests/IExternalGC.test

@@ -16,6 +16,7 @@ A create_storage function is provided that creates a storage.
     >>> transaction.commit()
     >>> oid0 = conn.root()[0]._p_oid
     >>> oid1 = conn.root()[1]._p_oid
+    >>> atLive = conn.root()._p_serial
     >>> del conn.root()[0]
     >>> del conn.root()[1]
     >>> transaction.commit()
@@ -66,9 +67,10 @@ Now if we try to load data for the objects, we get a POSKeyError:
 
 We can still get the data if we load before the time we deleted.
 
-    >>> storage.loadBefore(oid0, conn.root()._p_serial) == (p0, s0, tid)
+    >>> from ZODB.utils import loadAt, z64
+    >>> loadAt(storage, oid0, atLive) == (p0, s0)
     True
-    >>> storage.loadBefore(oid1, conn.root()._p_serial) == (p1, s1, tid)
+    >>> loadAt(storage, oid1, atLive) == (p1, s1)
     True
     >>> with open(storage.loadBlob(oid1, s1)) as fp: fp.read()
     'some data'
@@ -92,15 +94,11 @@ gone:
     ...
     POSKeyError: ...
 
-    >>> storage.loadBefore(oid0, conn.root()._p_serial) # doctest: +ELLIPSIS
-    Traceback (most recent call last):
-    ...
-    POSKeyError: ...
+    >>> loadAt(storage, oid0, atLive) == (None, z64)
+    True
 
-    >>> storage.loadBefore(oid1, conn.root()._p_serial) # doctest: +ELLIPSIS
-    Traceback (most recent call last):
-    ...
-    POSKeyError: ...
+    >>> loadAt(storage, oid1, atLive) == (None, z64)
+    True
 
     >>> storage.loadBlob(oid1, s1) # doctest: +ELLIPSIS
     Traceback (most recent call last):

src/ZODB/tests/MVCCMappingStorage.py

@@ -46,6 +46,7 @@ class MVCCMappingStorage(MappingStorage):
         inst.new_oid = self.new_oid
         inst.pack = self.pack
         inst.loadBefore = self.loadBefore
+        inst.loadAt = self.loadAt
         inst._ltid = self._ltid
         inst._main_lock = self._lock
         return inst

src/ZODB/tests/hexstorage.py

@@ -39,6 +39,13 @@ class HexStorage(object):
                 setattr(self, name, v)
 
         zope.interface.directlyProvides(self, zope.interface.providedBy(base))
+        if hasattr(base, 'loadAt') and 'loadAt' not in self.copied_methods:
+            def loadAt(oid, at):
+                data, serial = self.base.loadAt(oid, at)
+                if data is not None:
+                    data = unhexlify(data[2:])
+                return data, serial
+            self.loadAt = loadAt
 
     def __getattr__(self, name):
         return getattr(self.base, name)
@@ -130,7 +137,7 @@ class ServerHexStorage(HexStorage):
     """
 
     copied_methods = HexStorage.copied_methods + (
-        'load', 'loadBefore', 'loadSerial', 'store', 'restore',
+        'load', 'loadAt', 'loadBefore', 'loadSerial', 'store', 'restore',
         'iterator', 'storeBlob', 'restoreBlob', 'record_iternext',
         )
 

src/ZODB/tests/testConnection.py

@@ -1314,7 +1314,16 @@ class StubStorage(object):
             raise TypeError('StubStorage does not support versions.')
         return self._data[oid]
 
+    def loadAt(self, oid, at):
+        try:
+            data, serial = self._transdata[oid]
+        except KeyError:
+            return None, z64
+        return data, serial
+
     def loadBefore(self, oid, tid):
+        warnings.warn("loadBefore is deprecated - use loadAt instead",
+                DeprecationWarning, stacklevel=2)
         return self._data[oid] + (None, )
 
     def store(self, oid, serial, p, version, transaction):

src/ZODB/tests/testDemoStorage.py

@@ -23,6 +23,7 @@ from ZODB.tests import (
     StorageTestBase,
     Synchronization,
     )
+from ZODB.tests.MinPO import MinPO
 
 import os
 if os.environ.get('USE_ZOPE_TESTING_DOCTEST'):
@@ -33,7 +34,9 @@ import random
 import re
 import transaction
 import unittest
+import ZODB.Connection
 import ZODB.DemoStorage
+import ZODB.FileStorage
 import ZODB.tests.hexstorage
 import ZODB.tests.util
 import ZODB.utils
@@ -264,6 +267,101 @@ def load_before_base_storage_current():
     >>> base.close()
     """
 
+# additional DemoStorage tests that do not fit into common DemoStorageTests setup.
+class DemoStorageTests2(ZODB.tests.util.TestCase):
+    def checkLoadAfterDelete(self):
+        """Verify that DemoStorage correctly handles load requests for objects
+           deleted in read-write part of the storage.
+
+           https://github.com/zopefoundation/ZODB/issues/318
+        """
+        FileStorage = ZODB.FileStorage.FileStorage
+        DemoStorage = ZODB.DemoStorage.DemoStorage
+        TransactionMetaData = ZODB.Connection.TransactionMetaData
+
+        # mkbase prepares base part of the storage.
+        def mkbase(): # -> zbase
+            zbase = FileStorage("base.fs")
+            db    = DB(zbase)
+            conn  = db.open()
+            root  = conn.root()
+
+            root['obj'] = obj = MinPO(0)
+            transaction.commit()
+
+            obj.value += 1
+            transaction.commit()
+
+            conn.close()
+            db.close()
+            zbase.close()
+
+            zbase = FileStorage("base.fs", read_only=True)
+            return zbase
+
+        # prepare base + overlay
+        zbase    = mkbase()
+        zoverlay = FileStorage("overlay.fs")
+        zdemo    = DemoStorage(base=zbase, changes=zoverlay)
+
+        # overlay: modify obj and root
+        db   = DB(zdemo)
+        conn = db.open()
+        root = conn.root()
+        obj = root['obj']
+        oid = obj._p_oid
+        obj.value += 1
+        # modify root as well so that there is root revision saved in overlay that points to obj
+        root['x'] = 1
+        transaction.commit()
+        atLive = obj._p_serial
+
+        # overlay: delete obj from root making it a garbage
+        del root['obj']
+        transaction.commit()
+        atUnlink = root._p_serial
+
+        # unmount DemoStorage
+        conn.close()
+        db.close()
+        zdemo.close() # closes zbase and zoverlay as well
+        del zbase, zoverlay
+
+        # simulate GC on base+overlay
+        zoverlay = FileStorage("overlay.fs")
+        txn = transaction.get()
+        txn_meta = TransactionMetaData(txn.user, txn.description, txn.extension)
+        zoverlay.tpc_begin(txn_meta)
+        zoverlay.deleteObject(oid, atLive, txn_meta)
+        zoverlay.tpc_vote(txn_meta)
+        atGC = zoverlay.tpc_finish(txn_meta)
+
+        # remount base+overlay
+        zbase = FileStorage("base.fs", read_only=True)
+        zdemo = ZODB.DemoStorage.DemoStorage(base=zbase, changes=zoverlay)
+        db  = DB(zdemo)
+
+        # verify:
+        # load(obj, atLive)     -> 2
+        # load(obj, atUnlink)   -> 2  (garbage, but still in DB)
+        # load(obj, atGC)       -> POSKeyError, not 1 from base
+        def getObjAt(at):
+            conn = db.open(at=at)
+            obj = conn.get(oid)
+            self.assertIsInstance(obj, MinPO)
+            v = obj.value
+            conn.close()
+            return v
+
+        self.assertEqual(getObjAt(atLive),   2)
+        self.assertEqual(getObjAt(atUnlink), 2)
+        self.assertRaises(ZODB.POSException.POSKeyError, getObjAt, atGC)
+
+        # end
+        db.close()
+        zdemo.close() # closes zbase and zoverlay as well
+
+
 def test_suite():
     suite = unittest.TestSuite((
         doctest.DocTestSuite(
@@ -285,4 +383,5 @@ def test_suite():
                                      'check'))
     suite.addTest(unittest.makeSuite(DemoStorageWrappedAroundHexMappingStorage,
                                      'check'))
+    suite.addTest(unittest.makeSuite(DemoStorageTests2, 'check'))
     return suite

src/ZODB/tests/test_storage.py

@@ -20,10 +20,11 @@ storage tests against the test storage.
 """
 import bisect
 import unittest
+import warnings
 
 from ZODB.BaseStorage import BaseStorage
 from ZODB import POSException
-from ZODB.utils import z64
+from ZODB.utils import p64, u64, z64
 
 from ZODB.tests import StorageTestBase
 from ZODB.tests import BasicStorage, MTStorage, Synchronization
@@ -105,6 +106,11 @@ class MinimalMemoryStorage(BaseStorage, object):
             self._ltid = self._tid
 
     def loadBefore(self, the_oid, the_tid):
+        warnings.warn("loadBefore is deprecated - use loadAt instead",
+                DeprecationWarning, stacklevel=2)
+        return self._loadBefore(the_oid, the_tid)
+
+    def _loadBefore(self, the_oid, the_tid):
         # It's okay if loadBefore() is really expensive, because this
         # storage is just used for testing.
         with self._lock:
@@ -126,6 +132,17 @@ class MinimalMemoryStorage(BaseStorage, object):
 
             return self._index[(the_oid, tid)], tid, end_tid
 
+    def loadAt(self, oid, at):
+        try:
+            r = self._loadBefore(oid, p64(u64(at)+1))
+        except KeyError:
+            return None, z64
+        if r is None:
+            # not-yet created (deleteObject not supported -> serial=0)
+            return None, z64
+        data, serial, _ = r
+        return data, serial
+
     def loadSerial(self, oid, serial):
         return self._index[(oid, serial)]
 

src/ZODB/tests/testmvcc.py

@@ -35,7 +35,7 @@ MinimalMemoryStorage that implements MVCC support, but not much else.
 ***IMPORTANT***: The MVCC approach has changed since these tests were
 originally written. The new approach is much simpler because we no
 longer call load to get the current state of an object. We call
-loadBefore instead, having gotten a transaction time at the start of a
+loadAt instead, having gotten a transaction time at the start of a
 transaction.  As a result, the rhythm of the tests is a little odd,
 because we no longer need to probe a complex dance that doesn't exist any more.
 
@@ -290,7 +290,7 @@ first connection's state for b "old".
 
 Now deactivate "b" in the first connection, and (re)fetch it.  The first
 connection should still see 1, due to MVCC, but to get this old state
-TmpStore needs to handle the loadBefore() method.
+TmpStore needs to handle the loadAt() or loadBefore() methods.
 
 >>> r1["b"]._p_deactivate()
 
@@ -322,7 +322,7 @@ why ZODB no-longer calls load. :)
 
 Rather than add all the complexity of ZEO to these tests, the
 MinimalMemoryStorage has a hook.  We'll write a subclass that will
-deliver an invalidation when it loads (or loadBefore's) an object.
+deliver an invalidation when it loads (or loadAt's) an object.
 The hook allows us to test the Connection code.
 
 >>> class TestStorage(MinimalMemoryStorage):

src/ZODB/utils.py

@@ -17,6 +17,7 @@ import struct
 import sys
 import time
 import threading
+import warnings
 from binascii import hexlify, unhexlify
 
 from tempfile import mkstemp
@@ -382,8 +383,51 @@ def load_current(storage, oid, version=''):
     some time in the future.
     """
     assert not version
-    r = storage.loadBefore(oid, maxtid)
-    if r is None:
+    data, serial = loadAt(storage, oid, maxtid)
+    if data is None:
         raise ZODB.POSException.POSKeyError(oid)
-    assert r[2] is None
-    return r[:2]
+    return data, serial
+
+
+_loadAtWarned = set() # of storage class
+def loadAt(storage, oid, at):
+    """loadAt provides IStorageLoadAt semantic for all storages.
+
+    Storages that do not implement loadAt are served via loadBefore.
+    """
+    load_at = getattr(storage, 'loadAt', None)
+    if load_at is not None:
+        return load_at(oid, at)
+
+    # storage does not provide IStorageLoadAt - warn + fall back to loadBefore
+    if type(storage) not in _loadAtWarned:
+        # there is potential race around _loadAtWarned access, but due to the
+        # GIL this race cannot result in that set corruption, and can only lead
+        # to us emitting the warning twice instead of just once.
+        # -> do not spend CPU on lock and just ignore it.
+        warnings.warn(
+            "FIXME %s does not provide loadAt - emulating it via loadBefore, but ...\n"
+            "\t... 1) access will be potentially slower, and\n"
+            "\t... 2) not full semantic of loadAt could be provided.\n"
+            "\t...    this can lead to data corruption.\n"
+            "\t... -> please see https://github.com/zopefoundation/ZODB/issues/318 for details." %
+            type(storage), DeprecationWarning)
+        _loadAtWarned.add(type(storage))
+
+    if at == maxtid:
+        before = at
+    else:
+        before = p64(u64(at)+1)
+
+    try:
+        r = storage.loadBefore(oid, before)
+    except ZODB.POSException.POSKeyError:
+        return (None, z64)  # object does not exist at all
+
+    if r is None:
+        # object was removed; however loadBefore does not tell when.
+        # return serial=0 - this is the "data corruption" case talked about above.
+        return (None, z64)
+
+    data, serial, next_serial = r
+    return (data, serial)