[queue runner] Fix db races in substituting steps.#1790
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stepnr is computed as MAX(stepnr) + 1 inside the insert. Transactions inserting steps for the same build at the same time pick the same number: the loser blocks on the winner's uncommitted unique-index entry, hits ON CONFLICT DO NOTHING once the winner commits and retries. The queue monitor substitutes outputs of a build concurrently (buffer_unordered(10), also across derivations of the same build), so this happened constantly. Each blocked insert held a connection from the pool. Observed while processing a 120-build nixos-small evaluation against an empty binary cache: 260 slow statement warnings, single inserts taking up to 15s and unrelated queries waiting 3s for a pool connection. Take pg_advisory_xact_lock(class, build_id) before the insert. Writers for the same build now queue up for the duration of a short transaction instead of fighting over the same stepnr.
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Ericson2314
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Conni2461
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| fn substitution_step(build_id: i32, drv_path: &StorePath) -> InsertBuildStep<'_> { |
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if we only need this function for test we can mark restrict it to test
| assert_eq!(result, Some(sp("final-result"))); | ||
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| async fn replica_conn(pool: &sqlx::PgPool) -> Connection { |
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if we only need this function for test we can mark restrict it to test
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I am currently testing nixos minimal in a private staging deployment.
I found this race in journal of my builder:
Still testing the fix.
Check the commits for more details.