Commit 55261f31 authored by Kirill Smelkov's avatar Kirill Smelkov

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
parent 22d1405d
......@@ -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.
......
......@@ -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
......
......@@ -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:
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,
)
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:
# object was deleted
return None
# regular data record
return data, serial, next_serial
return result
def loadBlob(self, oid, serial):
try:
......
......@@ -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
......
......@@ -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)
......
......@@ -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:
......
......@@ -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
......
......@@ -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.
......
......@@ -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):
......
......@@ -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):
......
......@@ -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
......
......@@ -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',
)
......
......@@ -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):
......
......@@ -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
......@@ -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)]
......
......@@ -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):
......
......@@ -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)
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