A primary never learns of a follower's higher term via AppendEntries-reject
responses: ha_raft_on_repl_ack only routes AE replies through
raft_on_append_entries_reply for success=1 ACKs, and that function itself
drops any reply whose term != s->term (raft.c stale-term drop) before it
could adopt a higher term anyway. Net effect: once a follower outpaces a
stale primary's term, every AppendEntries the primary sends is rejected
forever and it can never regain write-quorum — a permanent election livelock
(surfaced as "no primary elected at term>8" in HA soak testing).
Adds two fallback term-discovery vectors that fire regardless of AE-reject
outcome:
- StateReport heartbeat-reply now carries the standby's current term
(controller.capnp, typemap.h) so the primary learns a higher term on every
convergence cycle.
- New raft_on_higher_term_observed() mirrors the UpdateTerm step (strictly
greater-than only; touches term/voted_for/role; persists before notifying
on_become_follower) and is wired into both the heartbeat state-report path
and the WAL-NACK path in repl_ack_decode_and_dispatch.
Validated: 265 controller + 287 lib unit tests pass; 10-round sequential and
chaotic HA soaks both PASS with zero job loss and full leadership coverage.
Extend raft_sim.c's deterministic TLA+-conformance harness with two
fault-injectable checks: no reaction before commit, and idempotent
re-apply of an already-committed entry. Wired as dedicated tests only,
not into raft_sim_check_all's fixed four-invariant contract (owned by
a separate spec). RED/GREEN independently verified by adversarial
review; zero regression to existing scenario functions.
- sim_tla_action_name: drop brittle per-case 'raft.tla:NNN' line-number comments
(some had drifted a few lines off; the return string IS the stable raft.tla
Next-disjunct name, and the function header already cites raft.tla:454).
- test_sim_5_1_partition_heal: the per-follower convergence loop was vacuous —
it bounded by commit_index, which stays 0 for followers under the leader-only
commit model, so the loop never ran. Bound by log_count instead (followers DO
advance the replicated log); Log Matching still guarantees the shared-prefix
terms agree. Now non-vacuous; still passes.
Test/comment-only; no behavior change. Lib suite Ok:1 Fail:0.
Adds raft_sim_scenario_figure8: a previous-term entry on a quorum under a
new-term leader is NOT committed until a current-term entry commits (library-
gated by raft_advance_commit; verified by a mutation test). Also restores the
raft_sim_scenario_drop_dup_reorder implementation that commit e445404 (task 5.3)
silently dropped from raft_sim.c (a stray git checkout reverted the uncommitted
impl), which had left the lib build broken with an undefined reference.
Build green: test_sim_5_3_drop_dup_reorder + test_sim_5_4_figure8 both PASS.
raft_on_vote_reply double-counted duplicated vote-responses (counter ignored
from_idx) and failed to drop stale-term replies, allowing two leaders at the
same term (bug-851 split-brain family). Mirror the existing pre-vote per-peer
dedup pattern (new vote_replies[] set) and add DropStaleResponse (term <
currentTerm). Surfaced by the raft-sim-conformance oracle (task 4.2).
47 raft / 264 overall lib tests pass.
Harness surfaced a real upstream raft.c split-brain bug (bug-1095/1096,
bug-851 family) in raft_on_vote_reply: counter-based vote tally double-counts
a duplicated vote-response and no stale-term drop -> two leaders same term.
Out of this spec's boundary (raft.c protocol); recorded as Implementation Note.
logOk (raft.tla:285, closes G6): local_last_log_term = log_len>0 ? on_get_entry_term(log_len):0;
logOk = cand_last_log_term > local || (== && cand_last_log_index >= log_len). Lexicographic:
term dominates, tie decided by index. Shared raft_log_ok helper.
Vote (4.2): logOk deny inserted after stale-term/UpdateTerm/role-gate/one-vote-per-term and
BEFORE on_vote_eligible (denies even when the app gate would grant); persist-before-grant intact.
PreVote (5.2): identical logOk, fully side-effect-free (no term/vote/role/commit mutation, no
on_persist). Signatures extended (1.3) on raft_on_vote_request/pre_vote_request; all 39 lib call
sites + sim routing updated; sim captures the candidate's send-time last-log tuple (raft_sim_rpc_t).
Empty-log call sites pass (0,0) → logOk true → existing outcomes preserved. 9 freshness tests.
NOTE: controller cluster.c vote callers still use the old signatures and will NOT compile — an
accepted, design-documented revalidation trigger; the controller adapter plumbing is owned by the
downstream ha-term-safety spec (to be implemented next). Library suite ASAN green (239 tests).
AdvanceCommitIndex (raft.tla:259/270, closes G4): leader-only (-EPERM); Max(agreeIndexes)
= highest N with a quorum ((n_peers/2)+1) of match_index>=N, self counted as log_len;
scan log_len downward. CURRENT-TERM RULE: advance commit_index to N only when
on_get_entry_term(N)==s->term, else HOLD (Figure-8 safety — never commit a previous-term
entry on replication count alone; a later current-term commit indirectly commits it).
Monotonic; fires on_commit_advanced once per strict advance; commit_index<=log_len.
3.3: raft_on_append_entries_reply left untouched — verified it never advances commit
(separation, Req 2.6). 6.1: sim commit-driver helper raft_sim_advance_commit added.
Req 6.3 two-phase indirect-commit transition covered end-to-end by task 7.1 (sim).
ASAN green (230 tests).
1.1: raft_state_t.commit_index (monotonic, ==WAL LSN, <=log_len, advanced only via
raft_advance_commit, untouched by AE reply) + raft_config_t.initial_commit_index.
1.2: on_commit_advanced(ctx, commit_index) REQUIRED callback — fired only on strict
advance, a TRIGGER for the controller commit barrier (never does durability itself).
2.1: raft_init requires it (-EINVAL if NULL) + restores commit_index from
initial_commit_index. raft_advance_commit prototype declared (impl in 3.2).
Coupled callback wiring to keep the required-callback ripple green: sim wires
sim_on_commit_advanced (+per-node commit_index mirror & fire-count), all test cbs
builders set it, raft_sim_restart_node restores the durable commit position. Vote
signatures unchanged; no commit-advance logic yet. ASAN green (224 tests).
raft_sim_restart_node models a process restart: the durable in-memory log[]
(WAL) and persisted (term, voted_for) survive, while volatile consensus state
is cleared by re-running raft_init (returns the node to FOLLOWER). Callbacks,
ctx, idx, and partition flag are preserved.
test_sim_9_5_node_restart_resumes_replication restarts a follower mid-replication
(partial 2-of-3 log), asserts the log survived (load-bearing: log_count==2 would
fail if wiped), role/term restored, then replication resumes and converges with
Log Matching. Closes the 8.3 'node restart while replication proceeds' sub-clause
(8/8 requirement sections now fully covered). ASAN green (220 tests).
raft_sim_check_log_matching: for every node pair and every shared index i, if
both entries at i have equal term, assert the whole prefix 1..i is term-and-byte
identical (Req 8.4, raft.tla Log Matching). Read-only, framework-agnostic (returns
1/0), bounded. Self-tests: pass on a converged cluster, fail on a hand-divergent
pair sharing (i=2,term=7) but differing at the index-1 prefix. ASAN green.
raft_sim_deliver_append_entries drains the AE queue, routes each buffered AE to
the (non-partitioned) destination via raft_on_append_entries_request, and routes
the result back to the source via raft_on_append_entries_reply — mirroring the
vote-RPC routing; partitioned endpoints are skipped.
Review-applied (HIGH): raft_sim_ae_t now captures the leader's SEND-time term
(leader_term) at buffer time and delivery uses it instead of the live source
state.term — without this, reorder/churn delivery (9.3/9.4) would inject the
wrong term and defeat UpdateTerm/stale-drop (G5). Smoke test asserts replication
to a connected follower + isolation of a partitioned one. ASAN green.
HandleAppendEntriesResponse (raft.tla:393): stale-term drop first (applied only
when reply_term==s->term, raft.tla:415, Req 5.4 — both lower and higher dropped);
success -> next_index[peer]=match_index+1, match_index[peer]=match_index (Req
5.1-5.2); failure -> next_index[peer]=Max(next-1,1) floored, match_index unchanged
(Req 5.3); no commit advance. uint64 underflow blocked by the >1 floor guard.
3 tests incl. floor-at-1 and lower/higher stale. ASAN green.
Complete HandleAppendEntriesRequest (raft.tla:327) + UpdateTerm (raft.tla:406),
implemented as one function to avoid a misleading partial handler. Fixed step
order: UpdateTerm FIRST on every RPC incl. heartbeat (G5, strictly-greater,
clears voted_for, persists, leader step-down fires on_become_follower) ->
stale reject -> candidate step-down -> logOk (G2) -> reject if !logOk ->
accept (already-have / conflict-truncate-then-repersist (G3) / append).
Never advances commit, never does transport I/O.
Review-applied: logOk now includes the explicit prev_log_index <= log_len bound
(raft.tla:330 / Req 4.1) instead of relying solely on the 0-absent sentinel;
added a candidate-equal-term-logOk-fails step-ordering test. 15 tests, ASAN green.
Leader-only emit (raft.tla AppendEntries:204, Req 3.5 — no-op off leader);
prev_log_index=next_index[j]-1, prev_log_term via on_get_entry_term (0/unqueried
at index 0, Req 3.2-3.3); n_entries=1 when next_index[j]<=log_len else 0 heartbeat
(Req 3.4); leader_commit opaque passthrough. Trace fires only on successful send
(consistency with raft_log_append) and non-leader RAFT_OK asserted — both from
review. 4 spy-based tests. ASAN green.
Leader-only (-EPERM for follower/candidate, no mutation/callback); index=
log_len+1, term=currentTerm (raft.tla ClientRequest:246, Req 2.1-2.4);
persist-before-act: on_persist_entry called once BEFORE log_len advances,
non-zero return propagated with no phantom entry (Req 2.5); no commit-index
advance, no transport I/O. Spy-based tests for all four branches. ASAN green.
next_index[]/match_index[]/pre_vote_replies[] are fixed-size RAFT_MAX_PEERS
arrays indexed by peer; without an upper-bound check a caller passing
n_peers > 8 would OOB-write in the leader-progress and pre-vote loops.
Add the guard (returns -EINVAL, no mutation) + canary-checked test.
Latent since task 1.1; surfaced by task 2.2 review (first loop writes into
the new arrays). No current caller exceeds the bound (sim caps at 8, HA <= 4).
next_index[j]=log_len+1, match_index[j]=0 for every peer at both win paths
(single-node RAFT_ELECTED_IMMEDIATELY in raft_start_election; multi-node
RAFT_BECAME_LEADER in raft_on_vote_reply) via shared raft_init_leader_progress
helper (raft.tla BecomeLeader, Req 3.1). Suite green under ASAN.
2.1: raft_init returns -EINVAL when any of on_persist_entry/on_get_entry_term/
on_send_append_entries/on_truncate_from is NULL (Req 7.1-7.4), before any write
to *s; restores s->log_len = cfg->initial_log_len (Req 1.1).
8.1: raft_sim gains a per-node in-memory log (term+bytes per 1-based index) and
an AE buffer queue; wires all four callbacks (persist/get-term/truncate/send).
Pulled forward to land with 2.1 so the stricter init does not leave the
sim/proto suite red between tasks. AE delivery deferred to 8.2.
Suite green under ASAN (Ok:1 Fail:0).
raft_on_pre_vote_reply: per-peer-deduped tally returning WON at quorum
((n_peers/2)+1), LOST when all non-self peers reply without quorum, else
PENDING; bounds-guards peer_idx. Never fires become-leader/follower, never
mutates term/voted_for/role, never persists. 8 RED-first tests (3- and 5-node,
dedup, out-of-range, purity). Raft PreVote pure core complete.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
raft_begin_pre_vote: compute prospective term (term+1) locally without
persisting, arm the pre_vote_* tally with a self-grant, NULL-safely probe each
non-self peer, return RAFT_PRE_VOTE_WON for single-node else RAFT_OK. Stays
FOLLOWER, schedules no timeout, mutates no persistent state. 6 RED-first tests.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
raft_on_pre_vote_request: deny on stale term, else reuse on_vote_eligible
(primary/heartbeat-stickiness/catch-up gates). Never mutates term/voted_for/
role and never persists or adopts a higher term. 6 RED-first raft unit tests
assert the purity invariant; libxce suite green.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Election protocol fixes validated against raft.tla invariants:
- ha_init_raft(): one-time raft state init at startup (re-init would lose
durable election progress); n_peers = full cluster size including self
- handle_request_vote(): higher-term step-down moved BEFORE the role gate
(split-brain fix); standby vote-deny now requires a live primary control
channel (primary_ctrl_fd >= 0) AND a fresh heartbeat
- ha_heartbeat_is_fresh(): derive freshness from ha->last_activity (the
field production actually writes); the orphaned last_heartbeat timespec
had no production writer, leaving the vote-deny gate inert (bug-337);
field removed from ha_state_t
- handle_request_vote(): restore same-candidate idempotent re-grant —
deny only a DIFFERENT candidate in the same term, matching raft.c and
Raft §5.2 votedFor ∈ {Nil, candidateId} (bug-338)
- raft_init(): restore voted_for_term alongside a persisted vote; leaving
it 0 allowed a double vote in the same term across a restart (bug-339)
- myapp_run(): leader_known check treats a vote recorded under this node's
own name as a self-vote — production persists the real candidate name,
the "self" placeholder only appears in test mode
- raft_on_vote_request(): leader steps down on higher-term vote request
instead of denying it
Tests: test_proto_H6_restored_vote_blocks_second_vote added; H1 restore
assertion strengthened; test_ha_vote_gate.c driven through last_activity +
primary_ctrl_fd (the real production fields). Full /xce-build all green
(lib + controller + scheduler + Go suites, 0 failures, ASAN clean).
deploy-ha.sh: start.sh now forwards extra args to the controller binary.
