---
id: "bug-1426"
status: "fixed"
status_updated: "2026-09-24"
status_note: ""
component: "controller"
severity: "unknown"
timestamp: "2026-07-09T21:15:00.000Z"
last_seen: "2026-09-19T09:00:00Z"
occurrences: 1
files: ["src/controller/persist.c (trim_wal_segments", "apply_pump_read_entry cache)"]
tags: ["ha", "persistence", "wal-trim", "apply-pump", "low-priority", "raft-apply-decoupling-followup"]
related: []
legacy: true
---

# bug-1426: trim_wal_segments's retention floor has no awareness of the local apply pump's own lag — a…

## Description
trim_wal_segments's retention floor has no awareness of the local apply pump's own lag — a reachable scenario since apply-lag-behind-commit is raft-apply-decoupling's entire premise.

## Environment
_Not recorded (migrated from buglog.json)._

## Reproduction
_Not recorded (migrated from buglog.json)._

## Root cause
Identified during raft-apply-decoupling task 7.4's apply-pump segment-read cache fix (round-2 review finding): the new fast-path cache can (POSIX unlink-keeps-fd-alive) silently advance last_applied_lsn past already-deleted data if the pump is lagging when trim_wal_segments fires. Currently bounded by the pre-existing APPLY_DIVERGENT boot fence (deferred loud fencing on next restart, not silent corruption) — a real behavior change from the pre-cache code, not literally risk-free, but not an active defect either. Same family as an already-tracked standby-cursor variant (per task 7.4's own notes).

## Fix
FIXED (TDD RED->GREEN). trim_wal_segments() now caps its trim floor at the local apply pump's own progress (last_applied_lsn), not just remote standby-acked positions: floor = min(repl_trim_lsn's standby/disconnect-guard floor, last_applied_lsn+1). The +1 matters -- repl_trim_lsn's floor treats equality as 'still needed, preserve' (a standby acked through LSN N might still need that segment for retry/resync), but the LOCAL apply cursor's equality case means the opposite: last_applied_lsn==N means this process is DONE with entry N and will never re-read it, so a segment starting exactly at N is safe to delete. Feeding last_applied_lsn+1 into the same shared 'seg_start_lsn < floor' comparison gets correct delete-on-equal semantics for this term without a second, differently-shaped check. This closes the real gap: apply_pump_read_entry()'s fast-path cache holds an open fd into whatever segment it's currently reading; POSIX unlink-keeps-fd-alive meant a lagging pump could keep reading via that stale fd even after trim_wal_segments deleted the segment out from under it (no immediate corruption, since the fd's data was still valid) -- but a FRESH read of the same range (a restart's persist_recover() replay, or the pump's own cache falling back to its slow path after an unrelated miss) would find the segment already gone, producing a genuine APPLY_DIVERGENT-class gap between what the live process quietly caught up to and what a fresh read of disk state can see. Regression test (test_e2e_checkpoint.c's test_trim_guard_preserves_unapplied_local_segments): no standbys registered (so the pre-fix floor alone would allow trimming everything below the active segment) but last_applied_lsn deliberately lags behind one segment -- proves the already-applied segment IS trimmed (no unnecessary retention) while the not-yet-applied one is preserved (the actual guard). Also had to advance last_applied_lsn in the 3 pre-existing trim tests in the same file (test_checkpoint_wal_trim_after_checkpoint, test_trim_guard_preserves_standby_segments, test_trim_without_standbys_proceeds_normally), which call persist_wal_async directly (bypassing the whole apply path) and so never advance last_applied_lsn on their own -- without the added stamp, this fix would have correctly-but-unexpectedly blocked ALL trimming in those fixtures, since last_applied_lsn defaults to 0. Full controller_unit_test suite: 1099 tests, 2 pre-existing unrelated failures (bug-1561), no regressions.

[Original entry]
NOT FIXED — deliberately deferred. task 7.4's own scope explicitly excluded redesigning trim_wal_segments's retention-floor awareness of apply-pump lag; documented via an accurate code comment instead. A future fix would give trim_wal_segments a floor input derived from the local apply cursor, not just standby-acked LSNs.

## Verification
_Not recorded (migrated from buglog.json)._

## References
_Not recorded (migrated from buglog.json)._