A restarted standby could get permanently stuck unable to catch up,
with the primary re-streaming the identical doomed AppendEntries range
every heartbeat with zero progress -- discovered via a live 3-node
config-upgrade-snapshot soak. Liveness-only; no safety invariant
(Election Safety, Leader Completeness, State Machine Safety, Commit
Durability) was ever violated.
Three-layer root cause, all now closed:
1. ha_stream_wal_to_standby (config_repl.c) used to skip advancing its
prev_streamed_lsn/prev_streamed_term tracker across WAL_CHECKPOINT
entries (a local-only, never-replicated marker that still consumes
a real LSN slot). Now advances across them too, while still never
shipping a catchup entry for the checkpoint itself.
2. ha_apply_term_entry_via_library (ha.c) now JIT-resyncs
ha->raft.log_len from persist_durable_floor() before consuming an
AE, to correct for persist_write_checkpoint_marker advancing
current_lsn without going through raft_log_append/record_entry_term
(pre-existing gap, bug-1466, not itself fixed here).
3. The actual root cause, found via real instrumentation after layers
1-2 alone still left a regression test red: record_entry_term's
array-growth realloc zero-filled new slots, silently reclassifying
a checkpoint's deliberately-never-recorded gap slot from "absent"
to "present, term=0/WAL_LEGACY_TERM" the instant any later LSN got
recorded -- a time-dependent read where a permanent fact was
assumed. Fixed with a TERM_INDEX_UNSET (UINT64_MAX) sentinel that
growth now fills gap slots with instead of zero, so entry_term_locked
can distinguish "never explicitly written" from "explicitly
recorded as legacy term 0" regardless of later array growth.
The same determinism fix exposed a second, independent instance of the
identical wedge via the snapshot-install catch-up path instead of
WAL-streaming catch-up: persist_reconcile_checkpoint_term recorded a
wire checkpoint_term of 0 as a raw, explicit write (bypassing the
growth-fill sentinel entirely), so a snapshot-installed standby read
the checkpoint LSN back as WAL_LEGACY_TERM(1) forever while the leader
now correctly read its own checkpoint LSN as absent(0) -- the same
permanent logOk mismatch, a different code path. Fixed by substituting
TERM_INDEX_UNSET for a wire value of exactly 0 before recording; a
wire 0 is unambiguous (persist_entry_term's own contract never returns
raw 0 for a present entry, so 0 can only mean genuine absence).
Independently reviewed across three rounds: sentinel mechanism
verified via real RED->GREEN revert-and-rebuild; a documentation
claim that config_repl.c/ha.c were "comment-only" corrected; the
snapshot-path gap independently traced, confirmed real via its own
RED->GREEN cycle, and fixed; two stale doc comments (persist.h's
public contract, ha.c's call-site rationale) still describing the
disproven pre-fix legacy-0 convention corrected. Suite: 1028 -> 1035
tests, 0 failures.
Rongsong Shen
ce8df99037