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  1. Core Server
  2. SERVER-46721

Step up may cause reads at PIT with holes after yielding

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    Details

    • Type: Bug
    • Status: Closed
    • Priority: Major - P3
    • Resolution: Fixed
    • Affects Version/s: None
    • Fix Version/s: 4.4.0-rc7, 4.2.9, 4.7.0
    • Component/s: Concurrency
    • Labels:
      None
    • Backwards Compatibility:
      Fully Compatible
    • Operating System:
      ALL
    • Backport Requested:
      v4.4, v4.2, v4.0
    • Sprint:
      Execution Team 2020-03-23, Execution Team 2020-04-06, Execution Team 2020-05-04, Execution Team 2020-05-18
    • Case:
    • Linked BF Score:
      0

      Description

      When we acquire collection locks with AutoGetCollectionForRead on a secondary, we set the TimestampReadSource on the recovery unit to kLastApplied if we were a secondary at the time. However, when we re-acquire locks after a yield in PlanExecutor, we use the same TimestampReadSource we had when we originally acquired those locks, even we've become a primary since. This is bad for the oplog specifically, because on a primary, the last applied time is not oplog-hole-free, meaning we can read past oplog holes. More in general, we should not read at a PIT if there are still write transactions that may commit at that PIT.

      Suggested fix: Have secondary reads use the kNoOverlap read source instead of kLastApplied, which is std::min(*lastApplied, allDurable). This is the same as kLastApplied while a node is remains secondary, but is equivalent to kAllDurableSnapshot after transition to primary.

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              Assignee:
              louis.williams Louis Williams
              Reporter:
              geert.bosch Geert Bosch
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                Updated:
                Resolved: