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fix(core): fix table reading timeout error after non-wal table dropped and re-created#6095

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ideoma merged 5 commits intomasterfrom
fix-reader-open-timeout-on-scoreboard-lock
Sep 2, 2025
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fix(core): fix table reading timeout error after non-wal table dropped and re-created#6095
ideoma merged 5 commits intomasterfrom
fix-reader-open-timeout-on-scoreboard-lock

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@ideoma ideoma commented Sep 1, 2025

Found by fuzz tests.
When a non-WAL table is dropped and re-created, purge jobs can push max txn using TxnScoreboardPoolV2.isRangeAvailable() and it can lead to a timeout on opening TableReader, where it cannot lock the latest transaction in the scoreboard.

The fix is not to modify the max in TxnScoreboardPoolV2.isRangeAvailable(), making it a read-only scoreboard operation.

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puzpuzpuz
puzpuzpuz reviewed Sep 1, 2025
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puzpuzpuz
puzpuzpuz reviewed Sep 2, 2025
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puzpuzpuz
puzpuzpuz reviewed Sep 2, 2025
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puzpuzpuz commented Sep 2, 2025

When a non-WAL table is dropped and re-created, purge jobs can push max txn using TxnScoreboardPoolV2.isRangeAvailable() and it can lead to a timeout on opening TableReader, where it cannot lock the latest transaction in the scoreboard.

Why exactly table readers time out? Aren't they supposed to spin with reading the latest txn and then trying to acquire it? Or somehow we end up with the max txn being incremented many times in this scenario?

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ideoma commented Sep 2, 2025

Why exactly table readers time out? Aren't they supposed to spin with reading the latest txn and then trying to acquire it? Or somehow we end up with the max txn being incremented many times in this scenario?

Purge job calls isRangeAvailable(0,13) because it tries to clean up dropped table with the same dir name (non-wal table), this pushes max txn to 13 in the score board. Readers then timeout because the spin to acquire txn 0 read from the _txn file and they cannot, because max is 13.

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puzpuzpuz commented Sep 2, 2025

because it tries to clean up dropped table with the same dir name (non-wal table)

Gotcha, that's the culprit!

@puzpuzpuz puzpuzpuz added Bug Incorrect or unexpected behavior Core Related to storage, data type, etc. labels Sep 2, 2025
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puzpuzpuz previously approved these changes Sep 2, 2025
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glasstiger commented Sep 2, 2025

[PR Coverage check]

😍 pass : 20 / 22 (90.91%)

file detail

path covered line new line coverage
🔵 io/questdb/tasks/ColumnPurgeTask.java 5 6 83.33%
🔵 io/questdb/cairo/ColumnPurgeOperator.java 10 11 90.91%
🔵 io/questdb/cairo/VacuumColumnVersions.java 1 1 100.00%
🔵 io/questdb/griffin/PurgingOperator.java 2 2 100.00%
🔵 io/questdb/cairo/CairoEngine.java 1 1 100.00%
🔵 io/questdb/cairo/ColumnPurgeJob.java 1 1 100.00%

@puzpuzpuz puzpuzpuz self-requested a review September 2, 2025 15:34
@ideoma ideoma merged commit e3c705c into master Sep 2, 2025
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@ideoma ideoma deleted the fix-reader-open-timeout-on-scoreboard-lock branch September 2, 2025 16:57
@ideoma ideoma mentioned this pull request Sep 3, 2025
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