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PostgreSQL: DeleteTable's first catalog select has no lock timeout and waits behind CreateBackup

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Valutazione

Difficoltà
4/5
Tempo stimato
3-5 giorni
Idoneità per principianti
48/100
Tipo di issue
Bug
Chiarezza
Abbastanza chiara
Stato di attività
Attiva
Stack tecnologico
postgresql, rust
Ambito
backend, databases

Direzione di ricerca

Read crates/storage-postgres/src/delete_table.rs around the initial FOR UPDATE select, then inspect the FOR SHARE rationale in crates/storage-postgres/src/backup_engine.rs and the timeout handling from PR #504 in worker_store.rs. First measure how DynamoDB handles DeleteTable during CreateBackup, as the issue says this behavior has not been re-measured. Add a test that holds the row FOR SHARE on a second connection; done means DeleteTable returns within the timeout with the response matching the measured service behavior.

Scritto dal modello di indicizzazione a partire dal testo della issue.

Descrizione

backend:postgresql bug

Where

crates/storage-postgres/src/delete_table.rs, the SELECT ... FROM tables WHERE account_id = $1 AND table_name = $2 AND table_status IN ('ACTIVE', 'CREATING') FOR UPDATE that starts every DeleteTable, on main at abf8082f (lines 27 to 41).

What happens

The select runs on a catalog transaction with no lock_timeout. It waits for any transaction that holds the table's row:

  • CreateBackup holds the row FOR SHARE from its first catalog read until the data snapshot has been taken (crates/storage-postgres/src/backup_engine.rs, the comment above the FOR SHARE select explains why), so a DeleteTable issued during a backup of a large table waits for the whole snapshot.
  • UpdateTable holds the row FOR UPDATE for the length of its catalog transaction (crates/storage-postgres/src/update_table.rs).

While it waits, the request holds one connection of the catalog pool. With pool_size concurrent DeleteTable calls against tables being backed up, the pool is exhausted for other requests until the backups finish or sqlx's 30 s acquire timeout fires for the later callers.

The same shape was bounded for the control-plane pass in PR #504 (a transaction-local lock_timeout on the catalog transaction, and a defined answer when it fires). This select was left as it was because it is on main and not part of that fix.

Service behaviour

On Amazon DynamoDB, DeleteTable during a CreateBackup of the same table answers 200 with TableStatus: DELETING and the backup completes (the backup is of the table state at request time). This was not re-measured for this issue; the row for it should be measured before the fix is written.

Suggested fix

Set a transaction-local lock_timeout on the DeleteTable catalog transaction before the select (the constant PR #504 introduces in worker_store.rs can move to a shared place). On lock_not_available, decide the answer from the measurement: if the service serves the delete during a backup, the fix is to not block on the backup's FOR SHARE at all (for example by reading the row without the lock and marking it DELETING with a conditional UPDATE, letting the backup's definition read stay consistent through its own snapshot); otherwise answer ResourceInUseException with the measured sentence.

A test: hold the row FOR SHARE from a second connection, call DeleteTable, expect it to return within the timeout rather than wait for the release.

Lingua principale
Rust
Stelle
517
Fork
60
Merge medio
2g 23h
PR unite (30g)
25

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