fix(orchestrator): never return an already-expired lock lease

[waldemort-auto]

Fixes #21.

Problem

fetch_orchestration_item could return an orchestration item whose lock lease had already expired at the moment it was returned. Every ack against that lease failed with Invalid lock token, so the turn's work was discarded, the message became visible again immediately, and the cycle repeated indefinitely.

The max_attempts poison path could not stop it either: poison marking, backoff, and the attempt-count increment all commit through an ack keyed on that same dead lease, so all three failed together. The message was never poisoned, never backed off, and never abandoned with a delay — its retry interval collapsed to the lock lease, the most aggressive retry the system can produce, applied to the message that most needed to be left alone.

Two defects combined to produce this, both in the stored procedure:

1. Deterministic candidate selection followed by a blocking advisory lock. ORDER BY q.visible_at, q.id LIMIT 1 is deterministic, so every dispatcher in the fleet independently computed the same candidate and then blocked on the same advisory key. The FOR UPDATE ... SKIP LOCKED re-verification could not prevent this — the blocking had already happened.

2. The lease was computed from a pre-wait timestamp. p_now_ms is minted by the Rust caller before the call and was never refreshed. When the advisory wait exceeded p_lock_timeout_ms, locked_until was written already in the past. The same stale value was also the liveness reference for candidate visibility and for stealing expired locks, so a long-running call became progressively less able to make progress.

These compound: a trapped message never commits, so its visible_at never advances, so it remains permanently the oldest row and is selected first by every dispatcher on every poll.

Changes

Migration 0024 rebuilds fetch_orchestration_item on top of the 0019 definition. Signature, return shape, and all downstream behavior (message batching, history/metadata fallback, KV snapshot) are unchanged.

Change	Rationale
pg_try_advisory_xact_lock replaces pg_advisory_xact_lock	Contended instances are skipped, not waited on, and excluded from re-selection within the call. Same-instance mutual exclusion is preserved — this is still the deadlock guard from the instance-level locking fix; only the waiting is removed.
Lease derived from v_now_ms = p_now_ms + elapsed-inside-call	The lease is always p_lock_timeout_ms in the future relative to when it is actually written.
Advisory key scoped to <schema>.<instance_id>	See below.
Retry loop bounded (50 iterations)	Exhausting it returns no item, which the dispatcher already treats as "nothing available"; the next poll starts with a fresh clock.

On the clock

Migrations 0006 and 0012 deliberately moved these procedures onto a Rust-supplied timestamp to avoid app/database clock skew. This change does not walk that back. Only an elapsed duration is taken from the server clock:

v_now_ms := p_now_ms + (EXTRACT(EPOCH FROM (clock_timestamp() - v_call_start)) * 1000)::BIGINT

No absolute server timestamp is ever used, so the caller's epoch remains the single time reference and no cross-clock skew is introduced against visible_at / locked_until values written elsewhere.

Second defect found while fixing the first

PostgreSQL advisory locks live in a single database-wide space, but the key was hashtext() of the instance id alone. Two duroxide schemas in the same database therefore serialized against each other whenever they happened to use the same instance id — entirely unrelated work contending on one key.

While acquisition blocked, this was invisible: the loser waited, then proceeded. With non-blocking acquisition it would surface as a spurious skip. It was caught here because duroxide's own provider_validation suite (atomicity_tests::test_concurrent_ack_prevention) began failing under parallel test execution, where each test uses its own schema but shares one database. It passes in isolation and passes now that the key is scoped.

Rolling-upgrade note: during a mixed deployment, nodes on the previous migration hash the unscoped key while upgraded nodes hash the scoped one, so the advisory layer does not mutually exclude across that boundary for the duration of the rollout. Correctness does not depend on it — instance_locks plus FOR UPDATE ... SKIP LOCKED still serialize the claim — and the provider already retries deadlock (40P01) errors, so the window degrades to the pre-advisory-lock behavior rather than to incorrect results.

Verification

Measured against disposable PostgreSQL 17.11 and 18.6 (the versions the downstream consumer supports; CI itself pins postgres:15). Contended fetch = a competing session holding the instance advisory lock for 3s while the fetch runs with a 1s lock timeout.

Measurement	Before	After
Contended fetch, time blocked	2.61 s	0.05 s
Contended fetch, lease returned	−1,658 ms (already expired)	none returned
Contended, with other work available	picked the contended instance after 2.61 s	picked the free instance in 0.06 s
Uncontended fetch, lease	4,958 ms	4,958 ms (unchanged)

Full suite, both versions: 353 passed / 0 failed / 9 ignored. cargo fmt --all --check clean.

The test proves the bug

tests/expired_lease_regression_test.rs fails with migration 0024 removed, on both 17.11 and 18.6:

fetch_orchestration_item blocked for 2.607683209s behind a contended instance
(limit 2s); the advisory lock is convoying

and passes with it. The competing session holds both the legacy unscoped key and the new scoped key, so the test reproduces contention against the procedure either side of this migration rather than silently failing to contend.

Notes for reviewers

• Repro A in fetch_orchestration_item writes an already-expired lock lease when the advisory lock wait exceeds the lock timeout #21 does not actually fire. Passing a deliberately stale p_now_ms also makes the row invisible via visible_at <= TO_TIMESTAMP(p_now_ms / 1000.0), so the function returns 0 rows. The real mechanism is a genuine advisory-lock wait with a fresh caller clock. I will correct the issue text.
• 0022 is intentionally skipped — reserved by the concurrent worker_queue dequeue PR (perf(worker_queue): partial index + two-phase fetch_work_item dequeue #18). This migration is independent and applies cleanly whether or not 0022 is present.
• include_dir! has no rerun-if-changed. Adding a migration file alone does not rebuild the crate, and CI restores a cargo build cache keyed on Cargo.lock, so a migration-only PR can silently test a stale migration set. This bit during development — the suite reported a failure against a binary that still had the old procedure embedded. Worth considering a build.rs emitting cargo:rerun-if-changed=migrations.
• cargo clippy fails on pre-existing code (src/provider.rs:2492, if_same_then_else) on unmodified main. CI does not run clippy; left alone here rather than mixing an unrelated change into this PR.
• The core-side half of this failure — poison disposition depending on the lease it is trying to escape — is tracked separately in Poison disposition rides the very lease it is trying to escape, so max_attempts cannot terminate a message with an expired lock duroxide#47. A provider that never returns an expired lease is necessary but not sufficient; the runtime should also survive being handed one.

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