---
id: "bug-991"
status: "fixed"
status_updated: "2026-09-24"
status_note: ""
component: "controller"
severity: "unknown"
timestamp: "2026-06-22T16:30:00.000Z"
last_seen: "2026-09-19T09:00:00Z"
occurrences: 1
files: ["src/controller/ha.c (ha_apply_term_entry_via_library ~2115; batch ~2205; single-entry ~2225)"]
tags: ["ha", "controller", "wal-catchup", "applied_lsn", "commit-stall", "off-by-one", "term-safety", "bug-991", "streamer-skip"]
related: ["bug-851", "bug-850", "bug-848", "bug-924", "bug-960"]
legacy: true
---

# bug-991: Standby WAL catch-up advanced applied_lsn to lsn+1 (next-to-apply) instead of lsn (last-held)

## Description
Standby WAL catch-up advanced applied_lsn to lsn+1 (next-to-apply) instead of lsn (last-held). The standby then advertised one LSN too high; the leader's catch-up streamer streamed (lsn+1, committed] and SKIPPED the real next-needed entry lsn+1, wedging the library logOk gate forever (no WalAck -> match_index never reaches -> raft commit_index frozen -> committed_lsn pinned at 1). Catch-up-path-specific; live repl_broadcast (absolute-LSN, no skip) commits fine, so failure was nondeterministic.

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

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

## Root cause
applied_lsn carried TWO conflicting semantics. Recovery (=durable_floor), checkpoint (=checkpoint_lsn), init (=0) and the leader's catch-up streamer + ha_evaluate_repl_action all treat it as 'last contiguously-held LSN' and stream (standby_lsn, committed]. But the three live/catch-up apply setters advanced to entry->lsn + 1 ('next-to-apply'). After a standby applied catch-up entry N it advertised N+1, so the leader's next stream began at N+2 and skipped N+1 -> logOk reject -> no WalAck.

## Fix
fix-1: the three live-apply setters now advance applied_lsn to the LAST-HELD LSN (e->lsn / last_good_lsn / entry->lsn), not +1, so applied_lsn means 'last contiguously-held LSN' everywhere. Reader audit (all consistent with last-held, no +1 reader needed): state report (advertised LSN, ha.c:2461) now matches streamer contract; convergence reported_lsn>=committed_lsn (2536) correct; snapshot/stream boundary standby_lsn+1<oldest_wal_lsn (2501) already assumed last-held; vote/prevote (5106/5183) feed applied_lsn only to the TAG fold, LSN axis overridden by ha_candidate_advertise_lsn(durable_floor); recovery ha_sync_applied_lsn_from_recovery already last-held. fix-B (source advertised LSN from raft.log_len) NOT landed: on the production catch-up road every streamed entry is term-carrying -> routed through the library, so raft.log_len already == applied_lsn after fix-1; raft.log_len legitimately differs from applied_lsn on the legacy term-0 direct-apply and recovery paths, so re-sourcing would regress bug-848 incremental catch-up -> fix-1 alone delivers the value the streamer+logOk need. Unit RED->GREEN: test_catchup_applied_lsn_is_last_held_no_skip (Expected 1 Was 2 pre-fix, PASS post-fix). 7 legacy test_wal_catchup_* expected values updated lsn+1 -> last-held. NOTE: the LIVE clean_failover_preservation STRICT_TERM commit=5 target was NOT reachable in the sandbox: committed_lsn stalls at 1 IDENTICALLY with this fix stashed (baseline experiment) -> that stall is a SEPARATE upstream blocker (library raft.log_len / commit-advance not advancing past the own-term noop; ha-term-safety task 3.1 area), NOT the bug-991 applied_lsn skip. waldump confirms all 3 members hold LSN 1..5 byte-identical (term 1) -> the replication half (which this fix corrects) is sound.

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

## References
- Related: bug-851, bug-850, bug-848, bug-924, bug-960