1. 08 Nov, 2018 7 commits
    • Julien Muchembled's avatar
      storage: fix write-locking bug when a deadlock happens at the end of a replication · 7fff11f6
      Julien Muchembled authored
      During rebase, writes could stay lockless although the partition was
      replicated. Another transaction could then take locks prematurely, leading to
      the following crash:
      
        Traceback (most recent call last):
          File "neo/lib/handler.py", line 72, in dispatch
            method(conn, *args, **kw)
          File "neo/storage/handlers/master.py", line 36, in notifyUnlockInformation
            self.app.tm.unlock(ttid)
          File "neo/storage/transactions.py", line 329, in unlock
            self.abort(ttid, even_if_locked=True)
          File "neo/storage/transactions.py", line 573, in abort
            not self._replicated.get(self.getPartition(oid))), x
        AssertionError: ('\x00\x00\x00\x00\x00\x03\x03v', '\x03\xca\xb44J\x13\x99\x88', '\x03\xca\xb44J\xe0\xdcU', {}, set(['\x00\x00\x00\x00\x00\x03\x03v']))
      7fff11f6
    • Julien Muchembled's avatar
      client: log_flush most exceptions raised from Application to ZODB · efaae043
      Julien Muchembled authored
      Flushing logs will help fixing NEO bugs (e.g. failed assertions).
      efaae043
    • Julien Muchembled's avatar
      client: fix assertion failure in case of conflict + storage disconnection · a746f812
      Julien Muchembled authored
      This fixes:
      
        Traceback (innermost last):
          ...
          Module transaction._transaction, line 393, in _commitResources
            rm.tpc_vote(self)
          Module ZODB.Connection, line 797, in tpc_vote
            s = vote(transaction)
          Module neo.client.Storage, line 95, in tpc_vote
            return self.app.tpc_vote(transaction)
          Module neo.client.app, line 546, in tpc_vote
            self.waitStoreResponses(txn_context)
          Module neo.client.app, line 539, in waitStoreResponses
            _waitAnyTransactionMessage(txn_context)
          Module neo.client.app, line 160, in _waitAnyTransactionMessage
            self._handleConflicts(txn_context)
          Module neo.client.app, line 471, in _handleConflicts
            assert oid is None, (oid, serial)
        AssertionError: ('\x00\x00\x00\x00\x00\x02\n\xe3', '\x03\xca\xad\xcb!\x92\xb6\x9c')
      a746f812
    • Julien Muchembled's avatar
      client: simplify connection management in transaction contexts · 2851a274
      Julien Muchembled authored
      With previous commit, there's no point anymore to distinguish storage nodes
      for which we only check serials.
      2851a274
    • Julien Muchembled's avatar
      client: also vote to nodes that only check serials · ab435b28
      Julien Muchembled authored
      Not doing so was an incorrect optimization. Checking serials does take
      write-locks and they must not be released when a client-storage connection
      breaks between vote and lock, otherwise a concurrent transaction modifying such
      serials may finish before.
      ab435b28
    • Julien Muchembled's avatar
      qa: deindent code · d7245ee9
      Julien Muchembled authored
      d7245ee9
    • Julien Muchembled's avatar
      Bump protocol version · 9a5b46dd
      Julien Muchembled authored
      9a5b46dd
  2. 07 Nov, 2018 8 commits
    • Julien Muchembled's avatar
      client: fix undetected disconnections to storage nodes during commit · d68e9053
      Julien Muchembled authored
      When a client-storage connection breaks, the storage node discards data of all
      ongoing transactions by the client. Therefore, a reconnection within the
      context of the transaction is wrong, as it could lead to partially-written
      transactions.
      
      This fixes cases where such reconnection happened. The biggest issue was that
      the mechanism to dispatch disconnection events only works when waiting for an
      answer.
      
      The client can still reconnect for other purposes but the new connection won't
      be reused by transactions that already involved the storage node.
      d68e9053
    • Julien Muchembled's avatar
      Fix data corruption due to undetected conflicts after storage failures · 854a4920
      Julien Muchembled authored
      Without this new mechanism to detect oids that aren't write-locked,
      a transaction could be committed successfully without detecting conflicts.
      In the added test, the resulting value was 2, whereas it should be 5 if there
      was no node failure.
      854a4920
    • Julien Muchembled's avatar
      master: notify replicating nodes of aborted watched transactions · 59698faa
      Julien Muchembled authored
      This fixes stuck replication when a client loses connection to the master
      during a commit.
      59698faa
    • Julien Muchembled's avatar
    • Julien Muchembled's avatar
    • Julien Muchembled's avatar
      client: fix race condition when a storage connection is closed just after identification · bf6569d6
      Julien Muchembled authored
      The consequence was that the client never reconnected to that storage node.
      On commits, writes to that node always failed, causing the master to
      disconnect it.
      bf6569d6
    • Julien Muchembled's avatar
      storage: relax assertion · 21a61977
      Julien Muchembled authored
      Nothing wrong actually happens.
      
      Traceback (most recent call last):
        File "neo/scripts/neostorage.py", line 32, in main
          app.run()
        File "neo/storage/app.py", line 194, in run
          self._run()
        File "neo/storage/app.py", line 225, in _run
          self.doOperation()
        File "neo/storage/app.py", line 310, in doOperation
          poll()
        File "neo/storage/app.py", line 134, in _poll
          self.em.poll(1)
        File "neo/lib/event.py", line 168, in poll
          self._poll(0)
        File "neo/lib/event.py", line 220, in _poll
          if conn.readable():
        File "neo/lib/connection.py", line 483, in readable
          self._closure()
        File "neo/lib/connection.py", line 541, in _closure
          self.close()
        File "neo/lib/connection.py", line 533, in close
          handler.connectionClosed(self)
        File "neo/storage/handlers/client.py", line 46, in connectionClosed
          app.tm.abortFor(conn.getUUID())
        File "neo/storage/transactions.py", line 594, in abortFor
          self.abort(ttid)
        File "neo/storage/transactions.py", line 570, in abort
          self._replicated.get(self.getPartition(oid))), x
      AssertionError: ('\x00\x00\x00\x00\x00\x01a\xe5', '\x03\xcaZ\x04\x14o\x8e\xbb', '\x03\xcaZ\x04\x0eX{\xbb', {1: None, 21: '\x03\xcaZ\x04\x11\xc6\x94\xf6'}, set([]))
      21a61977
    • Julien Muchembled's avatar
      comments, unused import · 1551c4a9
      Julien Muchembled authored
      1551c4a9
  3. 05 Nov, 2018 2 commits
  4. 06 Sep, 2018 2 commits
    • Julien Muchembled's avatar
    • Julien Muchembled's avatar
      storage: fix assertion failure in case of connection reset with a client node · 652f1f0d
      Julien Muchembled authored
      Here is what happened after simulating a network failure between a client and
      a storage:
      
      C8
      
      DEBUG   recv failed for <SSLSocketConnectorIPv6 at 0x7f8198027f90 fileno 17 ('xxxx:xxxx:120:cd8::90a1', 53970), opened to ('xxxx:xxxx:60:4c2c::25c3', 39085)>: ECONNRESET (Connection reset by peer)
      DEBUG   connection closed for <MTClientConnection(uuid=S2, address=[xxxx:xxxx:60:4c2c::25c3]:39085, handler=StorageEventHandler, closed, client) at 7f81939a0950>
      DEBUG   connection started for <MTClientConnection(uuid=S2, address=[xxxx:xxxx:60:4c2c::25c3]:39085, handler=StorageEventHandler, fd=17, on_close=onConnectionClosed, connecting, client) at 7f8192eb17d0>
      PACKET  #0x0000 RequestIdentification          > S2 ([xxxx:xxxx:60:4c2c::25c3]:39085)        | (<EnumItem CLIENT (2)>, -536870904, None, '...', [], 1535555463.455761)
      DEBUG   SSL handshake done for <SSLSocketConnectorIPv6 at 0x7f8192eb1850 fileno 17 ('xxxx:xxxx:120:cd8::90a1', 54014), opened to ('xxxx:xxxx:60:4c2c::25c3', 39085)>: ECDHE-RSA-AES256-GCM-SHA384 256
      DEBUG   connection completed for <MTClientConnection(uuid=S2, address=[xxxx:xxxx:60:4c2c::25c3]:39085, handler=StorageEventHandler, fd=17, on_close=onConnectionClosed, client) at 7f8192eb17d0> (from xxxx:xxxx:120:cd8::90a1:54014)
      DEBUG   <SSLSocketConnectorIPv6 at 0x7f8192eb1850 fileno 17 ('xxxx:xxxx:120:cd8::90a1', 54014), opened to ('xxxx:xxxx:60:4c2c::25c3', 39085)> closed in recv
      DEBUG   connection closed for <MTClientConnection(uuid=S2, address=[xxxx:xxxx:60:4c2c::25c3]:39085, handler=StorageEventHandler, closed, client) at 7f8192eb17d0>
      ERROR   Connection to <StorageNode(uuid=S2, address=[xxxx:xxxx:60:4c2c::25c3]:39085, state=RUNNING, connection=None, not identified) at 7f81a8874690> failed
      
      S2
      
      DEBUG   accepted a connection from xxxx:xxxx:120:cd8::90a1:54014
      DEBUG   SSL handshake done for <SSLSocketConnectorIPv6 at 0x7f657144a910 fileno 22 ('xxxx:xxxx:60:4c2c::25c3', 39085), opened from ('xxxx:xxxx:120:cd8::90a1', 54014)>: ECDHE-RSA-AES256-GCM-SHA384 256
      DEBUG   connection completed for <ServerConnection(uuid=None, address=[xxxx:xxxx:120:cd8::90a1]:54014, handler=IdentificationHandler, fd=22, server) at 7f657144a090> (from xxxx:xxxx:60:4c2c::25c3:39085)
      PACKET  #0x0000 RequestIdentification          < None ([xxxx:xxxx:120:cd8::90a1]:54014)         | (<EnumItem CLIENT (2)>, -536870904, None, '...', [], 1535555463.455761)
      DEBUG   connection closed for <ServerConnection(uuid=None, address=[xxxx:xxxx:120:cd8::90a1]:54014, handler=IdentificationHandler, closed, server) at 7f657144a090>
      WARNING A connection was lost during identification
      ERROR   Pre-mortem data:
      ERROR   Traceback (most recent call last):
      ERROR     File "neo/storage/app.py", line 194, in run
      ERROR       self._run()
      ERROR     File "neo/storage/app.py", line 225, in _run
      ERROR       self.doOperation()
      ERROR     File "neo/storage/app.py", line 310, in doOperation
      ERROR       poll()
      ERROR     File "neo/storage/app.py", line 134, in _poll
      ERROR       self.em.poll(1)
      ERROR     File "neo/lib/event.py", line 160, in poll
      ERROR       to_process.process()
      ERROR     File "neo/lib/connection.py", line 499, in process
      ERROR       self._handlers.handle(self, self._queue.pop(0))
      ERROR     File "neo/lib/connection.py", line 85, in handle
      ERROR       self._handle(connection, packet)
      ERROR     File "neo/lib/connection.py", line 100, in _handle
      ERROR       pending[0][1].packetReceived(connection, packet)
      ERROR     File "neo/lib/handler.py", line 123, in packetReceived
      ERROR       self.dispatch(*args)
      ERROR     File "neo/lib/handler.py", line 72, in dispatch
      ERROR       method(conn, *args, **kw)
      ERROR     File "neo/storage/handlers/identification.py", line 56, in requestIdentification
      ERROR       assert not node.isConnected(), node
      ERROR   AssertionError: <ClientNode(uuid=C8, state=RUNNING, connection=<ServerConnection(uuid=C8, address=[xxxx:xxxx:120:cd8::90a1]:53970, handler=ClientOperationHandler, fd=18, on_close=onConnectionClosed, server) at 7f657147d7d0>) at 7f65714d6cd0>
      652f1f0d
  5. 03 Sep, 2018 1 commit
    • Julien Muchembled's avatar
      qa: document a rare random failure in testExport · b54c1c68
      Julien Muchembled authored
      Traceback (most recent call last):
        File "neo/tests/functional/testClient.py", line 241, in testExport
          self.assertEqual(dump, self.__dump(neo_db.storage, list))
        File "neo/tests/functional/testClient.py", line 210, in __dump
          for t in storage.iterator()}
        File "neo/tests/functional/testClient.py", line 207, in <dictcomp>
          return {u64(t.tid): sorted((u64(o.oid), o.data_txn and u64(o.data_txn),
        File "neo/client/iterator.py", line 69, in iterator
          max_tid, chunk = app.transactionLog(start, stop, CHUNK_LENGTH)
        File "neo/client/app.py", line 841, in transactionLog
          Packets.AskTIDsFrom(start, stop, limit, offset))
        File "neo/client/app.py", line 296, in _askStorageForRead
          return askStorage(conn, packet)
        File "neo/client/app.py", line 164, in _askStorage
          return self._ask(conn, packet, handler=self.storage_handler, **kw)
        File "neo/lib/threaded_app.py", line 144, in _ask
          _handlePacket(qconn, qpacket, kw, handler)
        File "neo/lib/threaded_app.py", line 133, in _handlePacket
          handler.dispatch(conn, packet, kw)
        File "neo/lib/handler.py", line 72, in dispatch
          method(conn, *args, **kw)
        File "neo/lib/handler.py", line 208, in error
          getattr(self, Errors[code])(conn, message)
        File "neo/lib/handler.py", line 227, in backendNotImplemented
          raise NotImplementedError(message)
      NotImplementedError: neo.storage.database.importer.ImporterDatabaseManager does not implement getReplicationTIDList
      b54c1c68
  6. 13 Aug, 2018 1 commit
  7. 07 Aug, 2018 2 commits
    • Julien Muchembled's avatar
      Use argparse instead of optparse · 9f1e4eef
      Julien Muchembled authored
      Besides the use of another module for option parsing, the main change is that
      there's no more Config class that mixes configuration for different components.
      Application classes now takes a simple 'dict' with parsed values.
      
      The changes in 'neoctl' are somewhat ugly, because command-line options are not
      defined on the command-line class, but this component is likely to disappear
      in the future.
      
      It remains possible to pass options via a configuration file. The code is a bit
      complex but isolated in neo.lib.config
      
      For SSL, the code may be simpler if we change for a single --ssl option that
      takes 3 paths. Not done to not break compatibility. Hence, the hack with
      an extra OptionList class in neo.lib.app
      
      A new functional test tests the 'neomigrate' script, instead of just the
      internal API to migrate data.
      9f1e4eef
    • Julien Muchembled's avatar
      56d0b764
  8. 06 Aug, 2018 1 commit
    • Julien Muchembled's avatar
      Add comment about dormant bug when sending a lot of data to a slow node · 05e19861
      Julien Muchembled authored
      This mainly concerns the storage node, and depending on how its polling loop is
      changed, the following crash could happen again during replication:
      
        File "neo/scripts/neostorage.py", line 66, in main
          app.run()
        File "neo/storage/app.py", line 147, in run
          self._run()
        File "neo/storage/app.py", line 178, in _run
          self.doOperation()
        File "neo/storage/app.py", line 258, in doOperation
          _poll(0)
        File "neo/lib/event.py", line 231, in _poll
          conn.writable()
        File "neo/lib/connection.py", line 418, in writable
          if self.connector.send():
        File "neo/lib/connector.py", line 179, in send
          n = self.socket.send(msg)
        File "ssl.py", line 719, in send
          v = self._sslobj.write(data)
      OverflowError: string longer than 2147483647 byte
      05e19861
  9. 30 Jul, 2018 1 commit
  10. 16 Jul, 2018 1 commit
  11. 22 Jun, 2018 2 commits
    • Julien Muchembled's avatar
      Maximize resiliency by taking into account the topology of storage nodes · 97af23cc
      Julien Muchembled authored
      This commit adds a contraint when tweaking the partition table with replicas,
      so that cells of each partition are assigned as far as possible from each
      other, e.g. not on the same machine even if each one has several disks, and
      in any case not on the same storage device.
      
      Currently, the topology path of each node is automatically calculated by the
      storage backend. Both MySQL and SQLite return a 2-tuple (host, st_dev).
      To be improved:
      - Add a storage option to override the path: the 'tweak' algorithm can already
        handle topology paths of any length, so something like (room, machine, disk)
        could be done easily.
      - Write OS-specific code to determine the real hardware behind st_dev
        (e.g. 2 different 'st_dev' values may actually refer to the same disk,
         because of layers like partitioning, device-mapper, loop, btrfs subvolumes,
         and so on).
      - Make 'neoctl' report in some way if the PT is optimal. Meanwhile,
        if it isn't, the master only logs a WARNING during tweak.
      97af23cc
    • Julien Muchembled's avatar
      storage: also commit updated cell TID at each replicated chunk of 'obj' records · d4ea398d
      Julien Muchembled authored
      This is a follow-up of commit b3dd6973
      ("Optimize resumption of replication by starting from a greater TID").
      I missed the case where a storage node is restarted while it is replicating:
      it lost the TID where it was interrupted.
      
      Although we commit after each replicated chunk, to avoid transferring again
      all the data from the beginning, it could still waste time to check that
      the data are already replicated.
      d4ea398d
  12. 21 Jun, 2018 1 commit
  13. 19 Jun, 2018 4 commits
  14. 04 Jun, 2018 1 commit
  15. 31 May, 2018 1 commit
    • Kirill Smelkov's avatar
      tests/cluster: speedup waiting a bit · d08c83d4
      Kirill Smelkov authored
      NEO functional tests use pdb.wait() in a few places, for example in
      NEOCluster .run(), .start() and .expectCondition(). The wait
      implementation uses polling with exponentially growing wait period.
      
      With the following instrumentation
      
      	--- a/neo/tests/cluster.py
      	+++ b/neo/tests/cluster.py
      	@@ -236,6 +236,7 @@ def wait(self, test, timeout):
      	                         return False
      	             finally:
      	                 cluster_dict.release()
      	+            print 'next_sleep:', next_sleep
      	             sleep(next_sleep)
      	         return True
      
      during execution of functional tests it is not uncommon to see the
      following sleep periods
      
      	next_sleep: 0.1
      	next_sleep: 0.1
      	next_sleep: 0.15
      	next_sleep: 0.225
      	next_sleep: 0.3375
      	next_sleep: 0.50625
      	next_sleep: 0.1
      	next_sleep: 0.1
      	next_sleep: 0.15
      	next_sleep: 0.225
      	next_sleep: 0.3375
      	next_sleep: 0.50625
      	next_sleep: 0.1
      	next_sleep: 0.1
      	next_sleep: 0.1
      	next_sleep: 0.15
      	next_sleep: 0.225
      	next_sleep: 0.3375
      	next_sleep: 0.1
      	next_sleep: 0.1
      	next_sleep: 0.1
      	next_sleep: 0.1
      	next_sleep: 0.1
      	next_sleep: 0.1
      	next_sleep: 0.1
      	next_sleep: 0.1
      	next_sleep: 0.1
      	next_sleep: 0.1
      	next_sleep: 0.15
      	next_sleep: 0.225
      	next_sleep: 0.3375
      	next_sleep: 0.50625
      
      .
      
      Without going into reworking the wait mechanism to use real
      notifications instead of polling, it was observed that the exponential
      progression tends to create too coarse sleeps. Initial 0.1s interval was
      found to be also too much.
      
      This patch remove the exponential period growth and reduces period by order
      of one magnitude. For functional tests timings on my computer it is thus:
      
      before patch:
      
      	Functional tests
      
      	28 Tests, 0 Failed
      
      	Title                     : TestRunner
      	Date                      : 2018-04-04
      	Node                      : deco
      	Machine                   : x86_64
      	System                    : Linux
      	Python                    : 2.7.14
      
      	Directory                 : /tmp/neo_tests/1522868674.115798
      	Status                    : 100.000%
      	NEO_TESTS_ADAPTER         : SQLite
      
      	                               NEO TESTS REPORT
      
      	              Test Module |  run  | unexpected | expected | skipped |  time
      	--------------------------+-------+------------+----------+---------+----------
      	                   Client |    6  |       .    |      .   |     .   |   8.51s
      	                  Cluster |    7  |       .    |      .   |     .   |   9.84s
      	                   Master |    4  |       .    |      .   |     .   |   9.68s
      	                  Storage |   11  |       .    |      .   |     .   |  20.76s
      	--------------------------+-------+------------+----------+---------+----------
      	     neo.tests.functional |       |            |          |         |
      	--------------------------+-------+------------+----------+---------+----------
      	                  Summary |   28  |       .    |      .   |     .   |  48.79s
      	--------------------------+-------+------------+----------+---------+----------
      
      after patch:
      
      	Functional tests
      
      	28 Tests, 0 Failed
      
      	Title                     : TestRunner
      	Date                      : 2018-04-04
      	Node                      : deco
      	Machine                   : x86_64
      	System                    : Linux
      	Python                    : 2.7.14
      
      	Directory                 : /tmp/neo_tests/1522868527.624376
      	Status                    : 100.000%
      	NEO_TESTS_ADAPTER         : SQLite
      
      	                               NEO TESTS REPORT
      
      	              Test Module |  run  | unexpected | expected | skipped |  time
      	--------------------------+-------+------------+----------+---------+----------
      	                   Client |    6  |       .    |      .   |     .   |   7.38s
      	                  Cluster |    7  |       .    |      .   |     .   |   7.05s
      	                   Master |    4  |       .    |      .   |     .   |   8.22s
      	                  Storage |   11  |       .    |      .   |     .   |  19.22s
      	--------------------------+-------+------------+----------+---------+----------
      	     neo.tests.functional |       |            |          |         |
      	--------------------------+-------+------------+----------+---------+----------
      	                  Summary |   28  |       .    |      .   |     .   |  41.87s
      	--------------------------+-------+------------+----------+---------+----------
      
      in other words ~ 10% improvement for the whole time to run functional tests.
      
      /reviewed-by @vpelletier, @jm
      /reviewed-on nexedi/neoppod!10
      d08c83d4
  16. 30 May, 2018 5 commits
    • Julien Muchembled's avatar
      protocol: update packet docstrings · 9f0f2afe
      Julien Muchembled authored
      /reviewed-on nexedi/neoppod!9
      9f0f2afe
    • Julien Muchembled's avatar
      Bump protocol version · f62f9bc9
      Julien Muchembled authored
      f62f9bc9
    • Julien Muchembled's avatar
    • Julien Muchembled's avatar
      protocol: small cleanup in packet registration · a00ab78b
      Julien Muchembled authored
      I made a mistake in commit 13a64cfe
      ("Simplify definition of packets by computing automatically their codes").
      My intention was that the code an answer packet continues to only differ by the
      highest bit, as implemented now by this commit.
      
      Before:
        0x0001, 0x8002   Ask1, Answer1
        0x0003           Notify2
        0x0004, 0x8005   Ask3, Answer3
        0x0006, 0x8007   Ask4, Answer4
      
      After:
        0x0001, 0x8001   Ask1, Answer1
        0x0002           Notify2
        0x0003, 0x8003   Ask3, Answer3
        0x0004, 0x8004   Ask4, Answer4
      
      This makes the protocol easier to document.
      
      And by not wasting the range of possible values, it seems we have enough
      space to shrink to a single byte.
      
      This also removes code that became meaningless since that codes are generated
      automatically.
      a00ab78b
    • Julien Muchembled's avatar
      Optimize resumption of replication by starting from a greater TID · b3dd6973
      Julien Muchembled authored
      Although data that are already transferred aren't transferred again, checking
      that the data are there for a whole partition can still be a lot of work for
      big databases. This commit is a major performance improvement in that a storage
      node that gets disconnected for a short time now gets fully operational quite
      instantaneously because it only has to replicate the new data. Before, the time
      to recover depended on the size of the DB.
      
      For OUT_OF_DATE cells, the difficult part was that they are writable and
      can then contain holes, so we can't just take the last TID in trans/obj
      (we wrongly did that at the beginning, and then committed
      6b1f198f as a workaround). We solve that
      by storing up to where it was up-to-date: this value is initialized from
      the last TIDs in trans/obj when the state switches from UP_TO_DATE/FEEDING.
      
      There's actually one such OUT_OF_DATE TID per assigned cell (backends store
      these values in the 'pt' table). Otherwise, a cell that still has a lot to
      replicate would still cause all other cells to resume from the a very small
      TID, or even ZERO_TID; the worse case is when a new cell is assigned to a node
      (as a result of tweak).
      
      For UP_TO_DATE cells of a backup cluster, replication was resumed from the
      maximum TID at which all assigned cells are known to be fully replicated.
      Like for OUT_OF_DATE cells, the presence of a late cell could cause a lot of
      extra work for others, the worst case being when setting up a backup cluster
      (it always restarted from ZERO_TID as long as at least 1 cell was still empty).
      Because UP_TO_DATE cells are guaranteed to have no holes, there's no need to
      store extra information: we simply look at the last TIDs in trans/obj.
      We even handle trans & obj independently, to minimize the work in 1 table
      (i.e. trans since it's processed first) if the other is late (obj).
      
      There's a small change in the protocol so that OUT_OF_DATE enum value equals 0.
      This way, backends can store the OUT_OF_DATE TID (verbatim) in the same column
      as the cell state.
      
      Note about MySQL changes in commit ca58ccd7:
      what we did as a workaround is not one any more. Now, we do so much on Python
      side that it's unlikely we could reduce the number of queries using GROUP BY.
      We even stopped doing that for SQLite.
      b3dd6973