db: ignore writes below new block transaction on recovery #864
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asubiotto
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Data race in snapshot on recovery test. Will fix |
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Race should be fixed. I ended up having to store the next non-persisted txn in the wal proto, given the replay semantics had to otherwise be slightly modified with code to deduce this txn, so I decided it was easier to just store and retrieve it with minimal changes to the recovery code. I also added a second commit with a fix for duplicate writes found by DST due to an off-by-one WAL error. |
Previously, the recovery code was ignoring writes below the persist transaction of a table block. However, this could lead to dropped writes since the active block is swapped atomically on rotation *before* the old block is persisted. Writes in between these two events would be written to the new table block, but ignored on recovery given the recovery code assumed they were in the old table block.
Previously snapshots were performed using a write txn and the WAL was truncated so that that txn would be the first txn in the truncated WAL. However, snapshots were changed to use a read txn so if a write at txn k was included in the snapshot, a truncated WAL would still contain this write as the first write in the WAL, resulting in duplicate data after recovery.
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Previously, the recovery code was ignoring writes below the persist transaction of a table block. However, this could lead to dropped writes since the active block is swapped atomically on rotation before the old block is persisted. Writes in between these two events would be written to the new table block, but ignored on recovery given the recovery code assumed they were in the old table block.