Implement deadlock avoidance

This is a first version with several optimizations possible:
- improve EventQueue (or implement a specific queue) to minimize deadlocks
- turn the RebaseObject packet into a notification

Sorting oids could also be useful to reduce the probability of deadlocks,
but that would never be enough to avoid them completely, even if there's a
single storage. For example:

1. C1 does a first store (x or y)
2. C2 stores x and y; one is delayed
3. C1 stores the other -> deadlock
   When solving the deadlock, the data of the first store may only
   exist on the storage.

2 functional tests are removed because they're redundant,
either with ZODB tests or with the new threaded tests.

BUGS.rst

@@ -4,18 +4,6 @@ or promised features of NEO (marked with N).
 
 All the listed bugs will be fixed with high priority.
 
-(Z) Conflict resolution not fully implemented
----------------------------------------------
-
-Even with a single storage node, so-called 'deadlock avoidance' may
-happen to in order to resolve conflicts. In such cases, conflicts will not be
-resolved even if your _p_resolveConflict() method would succeed, leading to a
-normal ConflictError.
-
-Although this should happen rarely enough not to affect performance, this can
-be an issue if your application can't afford restarting the transaction,
-e.g. because it interacted with external environment.
-
 (N) Storage failure or update may lead to POSException or break undoLog()
 -------------------------------------------------------------------------
 

TODO

@@ -125,6 +125,7 @@
     - Add ctl command to list last transactions, like fstail for FileStorage.
 
     Tests
+    - Split neo/tests/threaded/test.py
     - Use another mock library: Python 3.3+ has unittest.mock, which is
       available for earlier versions at https://pypi.python.org/pypi/mock
 

neo/client/app.py

@@ -417,24 +417,16 @@ class Application(ThreadedApplication):
         while 1:
             # We iterate over conflict_dict, and clear it,
             # because new items may be added by calls to _store.
+            # This is also done atomically, to avoid race conditions
+            # with PrimaryNotificationsHandler.notifyDeadlock
             try:
                 oid, (serial, conflict_serial) = pop_conflict()
             except KeyError:
                 return
-            if conflict_serial == MAX_TID:
-              if 1:
-                # XXX: disable deadlock avoidance code until it is fixed
-                logging.info('Deadlock avoidance on %r:%r',
-                    dump(oid), dump(serial))
-                # 'data' parameter of ConflictError is only used to report the
-                # class of the object. It doesn't matter if 'data' is None
-                # because the transaction is too big.
-                try:
-                    data = data_dict[oid][0]
-                except KeyError:
-                    # succesfully stored on another storage node
-                    data = txn_context.cache_dict[oid]
-              else:
+            try:
+                data = data_dict.pop(oid)[0]
+            except KeyError:
+                assert oid is conflict_serial is None, (oid, conflict_serial)
                 # Storage refused us from taking object lock, to avoid a
                 # possible deadlock. TID is actually used for some kind of
                 # "locking priority": when a higher value has the lock,
@@ -443,11 +435,15 @@ class Application(ThreadedApplication):
                 # To recover, we must ask storages to release locks we
                 # hold (to let possibly-competing transactions acquire
                 # them), and requeue our already-sent store requests.
-                logging.info('Deadlock avoidance triggered on %r:%r',
-                    dump(oid), dump(serial))
-                raise NotImplementedError
+                ttid = txn_context.ttid
+                logging.info('Deadlock avoidance triggered for TXN %s'
+                  ' with new locking TID %s', dump(ttid), dump(serial))
+                txn_context.locking_tid = serial
+                packet = Packets.AskRebaseTransaction(ttid, serial)
+                for uuid, status in txn_context.involved_nodes.iteritems():
+                    if status < 2:
+                        self._askStorageForWrite(txn_context, uuid, packet)
             else:
-                data = data_dict.pop(oid)[0]
                 if data is CHECKED_SERIAL:
                     raise ReadConflictError(oid=oid, serials=(conflict_serial,
                         serial))
@@ -457,12 +453,17 @@ class Application(ThreadedApplication):
                 if self.last_tid < conflict_serial:
                     self.sync() # possible late invalidation (very rare)
                 try:
-                    new_data = tryToResolveConflict(oid, conflict_serial,
+                    data = tryToResolveConflict(oid, conflict_serial,
                         serial, data)
                 except ConflictError:
                     logging.info('Conflict resolution failed for '
                         '%r:%r with %r', dump(oid), dump(serial),
                         dump(conflict_serial))
+                    # With recent ZODB, get_pickle_metadata (from ZODB.utils)
+                    # does not support empty values, so do not pass 'data'
+                    # in this case.
+                    raise ConflictError(oid=oid, serials=(conflict_serial,
+                        serial), data=data or None)
                 else:
                     logging.info('Conflict resolution succeeded for '
                         '%r:%r with %r', dump(oid), dump(serial),
@@ -470,12 +471,18 @@ class Application(ThreadedApplication):
                     # Mark this conflict as resolved
                     resolved_dict[oid] = conflict_serial
                     # Try to store again
-                    self._store(txn_context, oid, conflict_serial, new_data)
-                    continue
-            # With recent ZODB, get_pickle_metadata (from ZODB.utils) does
-            # not support empty values, so do not pass 'data' in this case.
-            raise ConflictError(oid=oid, serials=(conflict_serial,
-                serial), data=data or None)
+                    self._store(txn_context, oid, conflict_serial, data)
+
+    def _askStorageForWrite(self, txn_context, uuid, packet):
+          node = self.nm.getByUUID(uuid)
+          if node is not None:
+              conn = self.cp.getConnForNode(node)
+              if conn is not None:
+                  try:
+                      return conn.ask(packet, queue=txn_context.queue)
+                  except ConnectionClosed:
+                      pass
+          txn_context.involved_nodes[uuid] = 2
 
     def waitResponses(self, queue):
         """Wait for all requests to be answered (or their connection to be
@@ -510,16 +517,7 @@ class Application(ThreadedApplication):
         packet = Packets.AskVoteTransaction(ttid)
         for uuid, status in involved_nodes.iteritems():
             if status == 1 and uuid not in trans_nodes:
-                node = self.nm.getByUUID(uuid)
-                if node is not None:
-                    conn = self.cp.getConnForNode(node)
-                    if conn is not None:
-                        try:
-                            conn.ask(packet, queue=queue)
-                            continue
-                        except ConnectionClosed:
-                            pass
-                involved_nodes[uuid] = 2
+                self._askStorageForWrite(txn_context, uuid, packet)
         self.waitResponses(txn_context.queue)
         # If there are failed nodes, ask the master whether they can be
         # disconnected while keeping the cluster operational. If possible,

neo/client/handlers/master.py

@@ -194,6 +194,13 @@ class PrimaryNotificationsHandler(MTEventHandler):
                 if node and node.isConnected():
                     node.getConnection().close()
 
+    def notifyDeadlock(self, conn, ttid, locking_tid):
+        for txn_context in self.app.txn_contexts():
+            if txn_context.ttid == ttid:
+                txn_context.conflict_dict[None] = locking_tid, None
+                txn_context.wakeup(conn)
+                break
+
 class PrimaryAnswersHandler(AnswerBaseHandler):
     """ Handle that process expected packets from the primary master """
 

neo/client/handlers/storage.py

@@ -14,11 +14,12 @@
 # You should have received a copy of the GNU General Public License
 # along with this program.  If not, see <http://www.gnu.org/licenses/>.
 
+from zlib import decompress
 from ZODB.TimeStamp import TimeStamp
 
 from neo.lib import logging
-from neo.lib.protocol import MAX_TID
-from neo.lib.util import dump
+from neo.lib.protocol import Packets
+from neo.lib.util import dump, makeChecksum
 from neo.lib.exception import NodeNotReady
 from neo.lib.handler import MTEventHandler
 from . import AnswerBaseHandler
@@ -75,17 +76,75 @@ class StorageAnswersHandler(AnswerBaseHandler):
             # receive the conflict answer from the first store on S2.
             logging.info('%r report a conflict for %r with %r',
                          conn, dump(oid), dump(conflict))
-            if conflict != MAX_TID:
-                # If this conflict is not already resolved, mark it for
-                # resolution.
-                if conflict <= txn_context.resolved_dict.get(oid, ''):
-                    return
-            txn_context.conflict_dict[oid] = serial, conflict
+            # If this conflict is not already resolved, mark it for
+            # resolution.
+            if  txn_context.resolved_dict.get(oid, '') < conflict:
+                txn_context.conflict_dict[oid] = serial, conflict
         else:
             txn_context.written(self.app, conn.getUUID(), oid)
 
     answerCheckCurrentSerial = answerStoreObject
 
+    def answerRebaseTransaction(self, conn, oid_list):
+        txn_context = self.app.getHandlerData()
+        ttid = txn_context.ttid
+        queue = txn_context.queue
+        try:
+            for oid in oid_list:
+                # We could have an extra parameter to tell the storage if we
+                # still have the data, and in this case revert what was done
+                # in Transaction.written. This would save bandwidth in case of
+                # conflict.
+                conn.ask(Packets.AskRebaseObject(ttid, oid),
+                         queue=queue, oid=oid)
+        except ConnectionClosed:
+            txn_context.involved_nodes[conn.getUUID()] = 2
+
+    def answerRebaseObject(self, conn, conflict, oid):
+        if conflict:
+            txn_context = self.app.getHandlerData()
+            serial, conflict, data = conflict
+            assert serial and serial < conflict, (serial, conflict)
+            resolved = conflict <= txn_context.resolved_dict.get(oid, '')
+            try:
+                cached = txn_context.cache_dict.pop(oid)
+            except KeyError:
+                if resolved:
+                    # We should still be waiting for an answer from this node.
+                    assert conn.uuid in txn_context.data_dict[oid][1]
+                    return
+                assert oid in txn_context.data_dict
+                if oid in txn_context.conflict_dict:
+                    # Another node already reported the conflict, by answering
+                    # to this rebase or to the previous store.
+                    # Filling conflict_dict again would be a no-op.
+                    assert txn_context.conflict_dict[oid] == (serial, conflict)
+                    return
+                # A node has not answered yet to a previous store. Do not wait
+                # it to report the conflict because it may fail before.
+            else:
+                # The data for this oid are now back on client side.
+                # Revert what was done in Transaction.written
+                assert not resolved
+                if data is None: # undo or CHECKED_SERIAL
+                    data = cached
+                else:
+                    compression, checksum, data = data
+                    if checksum != makeChecksum(data):
+                        raise NEOStorageError(
+                            'wrong checksum while getting back data for'
+                            ' object %s during rebase of transaction %s'
+                            % (dump(oid), dump(txn_context.ttid)))
+                    if compression:
+                        data = decompress(data)
+                    size = len(data)
+                    txn_context.data_size += size
+                    if cached:
+                        assert cached == data
+                        txn_context.cache_size -= size
+                txn_context.data_dict[oid] = data, None
+            txn_context.conflict_dict[oid] = serial, conflict
+
     def answerStoreTransaction(self, conn):
         pass
 

neo/client/transactions.py

@@ -17,14 +17,22 @@
 from ZODB.POSException import StorageTransactionError
 from neo.lib.connection import ConnectionClosed
 from neo.lib.locking import SimpleQueue
+from neo.lib.protocol import Packets
 from .exception import NEOStorageError
 
+@apply
+class _WakeupPacket(object):
+
+    handler_method_name = 'pong'
+    decode = tuple
+    getId = int
 
 class Transaction(object):
 
     cache_size = 0  # size of data in cache_dict
     data_size = 0   # size of data in data_dict
     error = None
+    locking_tid = None
     voted = False
     ttid = None     # XXX: useless, except for testBackupReadOnlyAccess
 
@@ -45,6 +53,9 @@ class Transaction(object):
         # status: 0 -> check only, 1 -> store, 2 -> failed
         self.involved_nodes = {}                 # {node_id: status}
 
+    def wakeup(self, conn):
+        self.queue.put((conn, _WakeupPacket, {}))
+
     def write(self, app, packet, object_id, store=1, **kw):
         uuid_list = []
         pt = app.pt
@@ -53,7 +64,7 @@ class Transaction(object):
         for cell in pt.getCellList(object_id):
             node = cell.getNode()
             uuid = node.getUUID()
-            status = involved.setdefault(uuid, store)
+            status = involved.get(uuid, -1)
             if status < store:
                 involved[uuid] = store
             elif status > 1:
@@ -61,6 +72,13 @@ class Transaction(object):
             conn = app.cp.getConnForNode(node)
             if conn is not None:
                 try:
+                    if status < 0 and self.locking_tid and 'oid' in kw:
+                        # A deadlock happened but this node is not aware of it.
+                        # Tell it to write-lock with the same locking tid as
+                        # for the other nodes. The condition on kw is because
+                        # we don't need that for transaction metadata.
+                        conn.ask(Packets.AskRebaseTransaction(
+                            self.ttid, self.locking_tid), queue=self.queue)
                     conn.ask(packet, queue=self.queue, **kw)
                     uuid_list.append(uuid)
                     continue

neo/lib/protocol.py

@@ -398,6 +398,19 @@ class PStructItemOrNone(PStructItem):
         value = reader(self.size)
         return None if value == self._None else self.unpack(value)[0]
 
+class POption(PStruct):
+
+    def _encode(self, writer, value):
+        if value is None:
+            writer('\0')
+        else:
+            writer('\1')
+            PStruct._encode(self, writer, value)
+
+    def _decode(self, reader):
+        if '\0\1'.index(reader(1)):
+            return PStruct._decode(self, reader)
+
 class PList(PStructItem):
     """
         A list of homogeneous items
@@ -949,14 +962,60 @@ class GenerateOIDs(Packet):
         PFOidList,
     )
 
+class Deadlock(Packet):
+    """
+    Ask master to generate a new TTID that will be used by the client
+    to rebase a transaction. S -> PM -> C
+    """
+    _fmt = PStruct('notify_deadlock',
+        PTID('ttid'),
+        PTID('locking_tid'),
+    )
+
+class RebaseTransaction(Packet):
+    """
+    Rebase transaction. C -> S.
+    """
+    _fmt = PStruct('ask_rebase_transaction',
+        PTID('ttid'),
+        PTID('locking_tid'),
+    )
+
+    _answer = PStruct('answer_rebase_transaction',
+        PFOidList,
+    )
+
+class RebaseObject(Packet):
+    """
+    Rebase object. C -> S.
+
+    XXX: It is a request packet to simplify the implementation. For more
+         efficiency, this should be turned into a notification, and the
+         RebaseTransaction should answered once all objects are rebased
+         (so that the client can still wait on something).
+    """
+    _fmt = PStruct('ask_rebase_object',
+        PTID('ttid'),
+        PTID('oid'),
+    )
+
+    _answer = PStruct('answer_rebase_object',
+        POption('conflict',
+            PTID('serial'),
+            PTID('conflict_serial'),
+            POption('data',
+                PBoolean('compression'),
+                PChecksum('checksum'),
+                PString('data'),
+            ),
+        )
+    )
+
 class StoreObject(Packet):
     """
     Ask to store an object. Send an OID, an original serial, a current
     transaction ID, and data. C -> S.
     As for IStorage, 'serial' is ZERO_TID for new objects.
-    Answered 'conflict' value means:
-    - MAX_TID: deadlock
-    - else: conflict
     """
     _fmt = PStruct('ask_store_object',
         POID('oid'),
@@ -1658,6 +1717,12 @@ class Packets(dict):
                     UnlockInformation)
     AskNewOIDs, AnswerNewOIDs = register(
                     GenerateOIDs)
+    NotifyDeadlock = register(
+                    Deadlock)
+    AskRebaseTransaction, AnswerRebaseTransaction = register(
+                    RebaseTransaction)
+    AskRebaseObject, AnswerRebaseObject = register(
+                    RebaseObject)
     AskStoreObject, AnswerStoreObject = register(
                     StoreObject)
     AbortTransaction = register(

neo/master/handlers/storage.py

@@ -61,6 +61,9 @@ class StorageServiceHandler(BaseServiceHandler):
         p = Packets.AnswerUnfinishedTransactions(last_tid, pending_list)
         conn.answer(p)
 
+    def notifyDeadlock(self, conn, *args):
+        self.app.tm.deadlock(conn.getUUID(), *args)
+
     def answerInformationLocked(self, conn, ttid):
         self.app.tm.lock(ttid, conn.getUUID())
 

neo/master/transactions.py

@@ -19,13 +19,15 @@ from time import time
 from struct import pack, unpack
 from neo.lib import logging
 from neo.lib.handler import DelayEvent, EventQueue
-from neo.lib.protocol import ProtocolError, uuid_str, ZERO_OID, ZERO_TID
+from neo.lib.protocol import Packets, ProtocolError, uuid_str, \
+    ZERO_OID, ZERO_TID
 from neo.lib.util import dump, u64, addTID, tidFromTime
 
 class Transaction(object):
     """
         A pending transaction
     """
+    locking_tid = ZERO_TID
     _tid = None
     _msg_id = None
     _oid_list = None
@@ -303,6 +305,19 @@ class TransactionManager(EventQueue):
         logging.debug('Begin %s', txn)
         return tid
 
+    def deadlock(self, storage_id, ttid, locking_tid):
+        try:
+            txn = self._ttid_dict[ttid]
+        except KeyError:
+            return
+        if txn.locking_tid <= locking_tid:
+            client = txn.getNode()
+            txn.locking_tid = locking_tid = self._nextTID()
+            logging.info('Deadlock avoidance triggered by %s for %s:'
+                ' new locking tid for TXN %s is %s', uuid_str(storage_id),
+                uuid_str(client.getUUID()), dump(ttid), dump(locking_tid))
+            client.notify(Packets.NotifyDeadlock(ttid, locking_tid))
+
     def vote(self, app, ttid, uuid_list):
         """
             Check that the transaction can be voted

neo/storage/database/importer.py

@@ -304,7 +304,7 @@ class ImporterDatabaseManager(DatabaseManager):
                     getPartitionTable changePartitionTable
                     getUnfinishedTIDDict dropUnfinishedData abortTransaction
                     storeTransaction lockTransaction unlockTransaction
-                    storeData getOrphanList _pruneData deferCommit
+                    loadData storeData getOrphanList _pruneData deferCommit
                  """.split():
             setattr(self, x, getattr(self.db, x))
 

neo/storage/database/manager.py

@@ -463,6 +463,11 @@ class DatabaseManager(object):
         no hash collision.
         """
 
+    @abstract
+    def loadData(self, data_id):
+        """Inverse of storeData
+        """
+
     def holdData(self, checksum_or_id, *args):
         """Store raw data of temporary object
 

neo/storage/database/mysqldb.py

@@ -541,6 +541,15 @@ class MySQLDatabaseManager(DatabaseManager):
             raise
         return self.conn.insert_id()
 
+    def loadData(self, data_id):
+        compression, hash, value = self.query(
+            "SELECT compression, hash, value FROM data where id=%s"
+            % data_id)[0]
+        if compression and compression & 0x80:
+            compression &= 0x7f
+            data = ''.join(self._bigData(data))
+        return compression, hash, value
+
     del _structLL
 
     def _getDataTID(self, oid, tid=None, before_tid=None):

neo/storage/database/sqlite.py

@@ -404,6 +404,10 @@ class SQLiteDatabaseManager(DatabaseManager):
                     return r
             raise
 
+    def loadData(self, data_id):
+        return self.query("SELECT compression, hash, value"
+                          " FROM data where id=?", (data_id,)).fetchone()
+
     def _getDataTID(self, oid, tid=None, before_tid=None):
         partition = self._getPartition(oid)
         sql = 'SELECT tid, value_tid FROM obj' \

neo/storage/handlers/client.py

@@ -110,6 +110,26 @@ class ClientOperationHandler(EventHandler):
             self.app.tm.queueEvent(self._askStoreObject, conn, (oid, serial,
                 compression, checksum, data, data_serial, ttid, time.time()))
 
+    def askRebaseTransaction(self, conn, *args):
+        conn.answer(Packets.AnswerRebaseTransaction(
+            self.app.tm.rebase(conn, *args)))
+
+    def askRebaseObject(self, conn, ttid, oid):
+        try:
+            self._askRebaseObject(conn, ttid, oid, None)
+        except DelayEvent:
+            # locked by a previous transaction, retry later
+            self.app.tm.queueEvent(self._askRebaseObject,
+                conn, (ttid, oid, time.time()))
+
+    def _askRebaseObject(self, conn, ttid, oid, request_time):
+        conflict = self.app.tm.rebaseObject(ttid, oid)
+        if request_time and SLOW_STORE is not None:
+            duration = time.time() - request_time
+            if duration > SLOW_STORE:
+                logging.info('RebaseObject delay: %.02fs', duration)
+        conn.answer(Packets.AnswerRebaseObject(conflict))
+
     def askTIDsFrom(self, conn, min_tid, max_tid, length, partition):
         conn.answer(Packets.AnswerTIDsFrom(self.app.dm.getReplicationTIDList(
             min_tid, max_tid, length, partition)))
@@ -204,6 +224,8 @@ class ClientReadOnlyOperationHandler(ClientOperationHandler):
     askVoteTransaction      = _readOnly
     askStoreObject          = _readOnly
     askFinalTID             = _readOnly
+    askRebaseObject         = _readOnly
+    askRebaseTransaction    = _readOnly
     # takes write lock & is only used when going to commit
     askCheckCurrentSerial   = _readOnly
 

neo/tests/functional/testClient.py

@@ -23,7 +23,6 @@ import socket
 from struct import pack
 from neo.lib.util import makeChecksum, u64
 from ZODB.FileStorage import FileStorage
-from ZODB.POSException import ConflictError
 from ZODB.tests.StorageTestBase import zodb_pickle
 from persistent import Persistent
 from . import NEOCluster, NEOFunctionalTest
@@ -41,25 +40,6 @@ class Tree(Persistent):
         self.right = Tree(depth)
         self.left = Tree(depth)
 
-
-# simple persistent object with conflict resolution
-class PCounter(Persistent):
-
-    _value = 0
-
-    def value(self):
-        return self._value
-
-    def inc(self):
-        self._value += 1
-
-
-class PCounterWithResolution(PCounter):
-
-    def _p_resolveConflict(self, old, saved, new):
-        new['_value'] = saved['_value'] + new['_value']
-        return new
-
 class PObject(Persistent):
     pass
 
@@ -93,29 +73,6 @@ class ClientTests(NEOFunctionalTest):
         conn = self.db.open(transaction_manager=txn)
         return (txn, conn)
 
-    def testConflictResolutionTriggered1(self):
-        """ Check that ConflictError is raised on write conflict """
-        # create the initial objects
-        self.__setup()
-        t, c = self.makeTransaction()
-        c.root()['without_resolution'] = PCounter()
-        t.commit()
-
-        # first with no conflict resolution
-        t1, c1 = self.makeTransaction()
-        t2, c2 = self.makeTransaction()
-        o1 = c1.root()['without_resolution']
-        o2 = c2.root()['without_resolution']
-        self.assertEqual(o1.value(), 0)
-        self.assertEqual(o2.value(), 0)
-        o1.inc()
-        o2.inc()
-        o2.inc()
-        t1.commit()
-        self.assertEqual(o1.value(), 1)
-        self.assertEqual(o2.value(), 2)
-        self.assertRaises(ConflictError, t2.commit)
-
     def testIsolationAtZopeLevel(self):
         """ Check transaction isolation within zope connection """
         self.__setup()
@@ -270,51 +227,6 @@ class ClientTests(NEOFunctionalTest):
             db2, conn2 = self.neo.getZODBConnection()
         self.runWithTimeout(40, test)
 
-    def testDelayedLocksCancelled(self):
-        """
-            Hold a lock on an object, try to get another lock on the same
-            object to delay it. Then cancel the second transaction and check
-            that the lock is not hold when the first transaction ends
-        """
-        def test():
-            self.neo = NEOCluster(['test_neo1'], replicas=0,
-                temp_dir=self.getTempDirectory())
-            self.neo.start()
-            db1, conn1 = self.neo.getZODBConnection()
-            db2, conn2 = self.neo.getZODBConnection()
-            st1, st2 = conn1._storage, conn2._storage
-            t1, t2 = transaction.Transaction(), transaction.Transaction()
-            t1.user = t2.user = u'user'
-            t1.description = t2.description = u'desc'
-            oid = st1.new_oid()
-            rev = '\0' * 8
-            data = zodb_pickle(PObject())
-            st1.tpc_begin(t1)
-            st2.tpc_begin(t2)
-            # t1 own the lock
-            st1.store(oid, rev, data, '', t1)
-            # t2 store is delayed
-            st2.store(oid, rev, data, '', t2)
-            # cancel t2, should cancel the store too
-            st2.tpc_abort(t2)
-            # finish t1, should release the lock
-            st1.tpc_vote(t1)
-            st1.tpc_finish(t1)
-            db3, conn3 = self.neo.getZODBConnection()
-            st3 = conn3._storage
-            t3 = transaction.Transaction()
-            t3.user = u'user'
-            t3.description = u'desc'
-            st3.tpc_begin(t3)
-            # retrieve the last revision
-            data, serial = st3.load(oid)
-            # try to store again, should not be delayed
-            st3.store(oid, serial, data, '', t3)
-            # the vote should not timeout
-            st3.tpc_vote(t3)
-            st3.tpc_finish(t3)
-        self.runWithTimeout(10, test)
-
     def testGreaterOIDSaved(self):
         """
             Store an object with an OID greater than the last generated by the

neo/tests/threaded/__init__.py

@@ -413,6 +413,9 @@ class ClientApplication(Node, neo.client.app.Application):
     def __init__(self, master_nodes, name, **kw):
         super(ClientApplication, self).__init__(master_nodes, name, **kw)
         self.poll_thread.node_name = name
+        # Smaller cache to speed up tests that checks behaviour when it's too
+        # small. See also NEOCluster.cache_size
+        self._cache._max_size //= 1024
 
     def _run(self):
         try:
@@ -433,6 +436,10 @@ class ClientApplication(Node, neo.client.app.Application):
                 conn = self.cp.getConnForNode(self.nm.getByUUID(peer.uuid))
             yield conn
 
+    def extraCellSortKey(self, key):
+        return Patch(self.cp, getCellSortKey=lambda orig, cell:
+            (orig(cell), key(cell)))
+
 class NeoCTL(neo.neoctl.app.NeoCTL):
 
     def __init__(self, *args, **kw):
@@ -885,10 +892,6 @@ class NEOCluster(object):
         txn = transaction.TransactionManager()
         return txn, (self.db if db is None else db).open(txn)
 
-    def extraCellSortKey(self, key):
-        return Patch(self.client.cp, getCellSortKey=lambda orig, cell:
-            (orig(cell), key(cell)))
-
     def moduloTID(self, partition):
         """Force generation of TIDs that will be stored in given partition"""
         partition = p64(partition)
@@ -974,6 +977,8 @@ class NEOThreadedTest(NeoTestBase):
                 self.__exc_info = None
             except:
                 self.__exc_info = sys.exc_info()
+                if self.__exc_info[0] is NEOThreadedTest.failureException:
+                    traceback.print_exception(*self.__exc_info)
 
         def join(self, timeout=None):
             threading.Thread.join(self, timeout)
@@ -1003,6 +1008,44 @@ class NEOThreadedTest(NeoTestBase):
         return Patch(jar.db().storage.app.cp, getConnForNode=lambda orig, node:
             None if node.getUUID() == storage.uuid else orig(node))
 
+    @staticmethod
+    def readCurrent(ob):
+        ob._p_activate()
+        ob._p_jar.readCurrent(ob)
+
+
+class ThreadId(list):
+
+    def __call__(self):
+        try:
+            return self.index(thread.get_ident())
+        except ValueError:
+            i = len(self)
+            self.append(thread.get_ident())
+            return i
+
+
+@apply
+class RandomConflictDict(dict):
+    # One must not depend on how Python iterates over dict keys, because this
+    # is implementation-defined behaviour. This patch makes sure of that when
+    # resolving conflicts.
+
+    def __new__(cls):
+        from neo.client.transactions import Transaction
+        def __init__(orig, self, *args):
+            orig(self, *args)
+            assert self.conflict_dict == {}
+            self.conflict_dict = dict.__new__(cls)
+        return Patch(Transaction, __init__=__init__)
+
+    def popitem(self):
+        try:
+            k = random.choice(list(self))
+        except IndexError:
+            raise KeyError
+        return k, self.pop(k)
+
 
 def predictable_random(seed=None):
     # Because we have 2 running threads when client works, we can't