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  1. Core Server
  2. SERVER-16570

writeConcerns could be erroneously satisfied after a rollback occurs

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    Details

    • Type: Bug
    • Status: Closed
    • Priority: Major - P3
    • Resolution: Gone away
    • Affects Version/s: 2.8.0-rc2
    • Fix Version/s: None
    • Component/s: Replication
    • Labels:
      None
    • Operating System:
      ALL

      Description

      After a rollback, a node's replication progress could be behind where other nodes believe it to be, because the writeConcern code does not have support for replication progress being undone. As a result of this, getLastError for ops that occurred between the point of diversion and the latestOp for the rolled-back node could be satisfied with one too few nodes.

      No test has been written for this case yet. But I believe one could be written that is a copy of replsets/rollback2.js where each node is replaced with a pair of nodes, and one of the to-be-rolled-back nodes does not apply operations it has copied from the PRIMARY (failpoint: rsSyncApplyStop), once they have been isolated. This node will have the replication progress of the to-be-rolled-back PRIMARY, but will not have the ops in its own oplog. Remove the PRIMARY, bring back the two not-going-to-roll-back nodes, wait for them to receive the replication progress of the to-be-rolled-back PRIMARY, then remove the other to-be-rolled-back node. After a new PRIMARY is elected, do one new write. We should then be able to erroneously satisfy writeConcern w:3 for any optime between the point of divergence and the to-be-rolled-back nodes' last optime.

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