feat(ha-convergence-recovery): quorum transition logging — CAS edge-detect + 4 call sites (task 5.2)

src/controller/ha_convergence.c

@@ -383,21 +383,72 @@ void cluster_apply_ctrlhello_reply(cluster_t *c, uint64_t committed_lsn,
 }
 
 /*
- * ha_log_quorum_transition — RED scaffold (ha-convergence-recovery task 5.1).
+ * ha_log_quorum_transition — write-quorum loss/restore edge logger
+ * (ha-convergence-recovery task 5.2, Req 8.1, 8.2, 8.3).
  *
- * Task 5.2 replaces this body with the real edge detector: evaluate the current
- * write-quorum state (ha_has_write_quorum, ha.c), compare against ha->quorum_lost,
- * and on a transition CAS-flip the flag and emit exactly one WARN with the
- * connected/required counts (Req 8.1, 8.2), invoking ha_quorum_transition_log_hook
- * once under XCE_TESTING.  Repeated calls with no state change emit nothing
- * (Req 8.3); a later loss/restore cycle logs again (edge detector, not a latch).
+ * Evaluate the current write-quorum state and compare it against the
+ * ha_state_t.quorum_lost edge-detector flag.  On a TRANSITION — and only on a
+ * transition — atomically flip the flag via CAS and emit exactly one WARN
+ * carrying the connected/required member counts:
+ *   - loss:    "HA quorum lost: connected=%d required=%d"     (Req 8.1)
+ *   - restore: "HA quorum restored: connected=%d required=%d" (Req 8.2)
  *
- * This RED stub does NOTHING: no edge detection, no flag flip, no WARN, no hook
- * invocation — so the task 5.1 quorum-transition tests FAIL until 5.2 lands.
+ * The flag makes the logging an edge detector, NOT a latch: repeated calls while
+ * the state is unchanged emit nothing (Req 8.3 — no log storms across the
+ * dispatch-gate backstop's per-rejection calls), but a later loss/restore cycle
+ * logs again.  The CAS guarantees once-per-transition under concurrency: only the
+ * caller that actually flips the flag emits the WARN.
+ *
+ * Observability only — this never returns a status and never gates an operation
+ * outcome; the dispatch gate's existing -XCE_ERR_NOT_READY behavior is unchanged.
+ *
+ * connected/required mirror the ha_has_write_quorum BODY (ha.c): with the repl
+ * back-pointer wired, connected = repl_active_standby_count(repl) and the
+ * threshold required = n_peers/2 (integer division — for n_peers=3 → 1).  When
+ * the back-pointer is NULL (legacy contexts), connected falls back to the
+ * standby_endpoints count, matching the gate's own fallback exactly.  The quorum
+ * decision math is NOT changed here — this only reports what that body decides.
  */
 void ha_log_quorum_transition(ha_state_t *ha)
 {
-    (void)ha; /* RED scaffold — task 5.2 implements the edge detection. */
+    if (ha == NULL)
+        return;
+
+    /* Single-node clusters always hold quorum (ha_has_write_quorum returns true
+     * for n_peers <= 1); there is no transition to observe. */
+    if (ha->n_peers <= 1)
+        return;
+
+    /* Mirror ha_has_write_quorum's connected-count derivation (ha.c body). */
+    int connected;
+    if (ha->repl != NULL)
+        connected = repl_active_standby_count(ha->repl);
+    else
+        connected = ha->standby_endpoints ? ha->standby_endpoints->count : 0;
+
+    int required    = (int)(ha->n_peers / 2);
+    int lost_now    = (connected < required) ? 1 : 0;
+
+    /* Edge detection: only the caller that flips quorum_lost from its prior value
+     * to lost_now emits.  CAS expected=!lost_now → desired=lost_now; if the flag
+     * already equals lost_now (no transition) the CAS fails and we return
+     * silently (Req 8.3). */
+    int expected = lost_now ? 0 : 1;
+    if (!atomic_compare_exchange_strong_explicit(
+            &ha->quorum_lost, &expected, lost_now,
+            memory_order_acq_rel, memory_order_acquire))
+        return; /* no edge (or another thread already logged it) — no WARN */
+
+    if (lost_now)
+        WARN("HA quorum lost: connected=%d required=%d", connected, required);
+    else
+        WARN("HA quorum restored: connected=%d required=%d", connected, required);
+
+#ifdef XCE_TESTING
+    /* Emission-count seam (Req 8.1-8.3 observable): invoked once per WARN. */
+    if (ha_quorum_transition_log_hook != NULL)
+        ha_quorum_transition_log_hook(lost_now, connected, required);
+#endif
 }
 
 #ifdef XCE_TESTING