fix(controller): size the config-push barrier for loaded hosts and admit its wait (bug-1914)

The configAck byte-delivery barrier waited 500 ms, below scheduling
delay on an oversubscribed host, so a push to a healthy majority was
rejected with -110 while its already-appended marker still committed.
CONF_PUSH_TIMEOUT_MS is now 2000 ms, the wait holds a commit-wait
admission slot (new commit_blocking_admit/release; a full budget fails
fast with not-ready), and the timeout reply no longer claims the change
was not committed.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

src/controller/commit.c

@@ -282,6 +282,27 @@ static void wait_release_locked(commit_engine_t* ce)
     }
 }
 
+int commit_blocking_admit(commit_engine_t* ce)
+{
+    int rc;
+
+    if (ce == NULL)
+        return -EINVAL;
+    pthread_mutex_lock(&ce->lock);
+    rc = wait_admit_locked(ce);
+    pthread_mutex_unlock(&ce->lock);
+    return rc;
+}
+
+void commit_blocking_release(commit_engine_t* ce)
+{
+    if (ce == NULL)
+        return;
+    pthread_mutex_lock(&ce->lock);
+    wait_release_locked(ce);
+    pthread_mutex_unlock(&ce->lock);
+}
+
 /* WAIT completion predicate (Req 1.1): own entry committed AND the
  * own-term gate (when set) reached. Caller holds ce->lock. */
 static int wait_done_locked(const commit_engine_t* ce, uint64_t lsn)

src/controller/config_repl.c

@@ -1839,9 +1839,13 @@ static int do_upgrade_config_with_bytes(cluster_t *c, endpoint_t *ep,
         /* Forward the barrier rc (-ETIMEDOUT / -EINVAL) as the reply
          * status so the admin tool sees WHY the commit failed (task 5.2
          * r1, reviewer finding 3). */
+        /* bug-1914: the marker may already be in the log (it is appended
+         * before the byte-delivery barrier), so a timeout here does not
+         * mean the change is void -- say so instead of "not committed". */
         send_upgrade_error(ep, req, c, rc,
-                           "Config marker failed to reach committed prefix"
-                           " -- upgrade not committed, no restart");
+                           "Config change not confirmed on a majority -- if it"
+                           " was already ordered in the log it may still"
+                           " activate; re-push the same config to confirm");
         return rc;
     }
 

src/controller/config_repl.h

@@ -28,9 +28,14 @@ typedef struct persist_s persist_t;
 #define CONF_ENABLE_UPGRADE_CONFIG 1
 #endif
 
-/* Push timeout for standby fan-out (ms) */
+/* Push timeout for standby fan-out and the configAck majority barrier (ms).
+ * bug-1914: 500 ms was below normal scheduling delay on an oversubscribed host
+ * (40 busy loops on 32 cores: the ACK landed ~575 ms in), so a push to a
+ * healthy majority was rejected while its marker still committed.  The
+ * barrier waits under the commit-wait admission reservation, so the longer
+ * bound cannot take workers reserved for liveness traffic. */
 #ifndef CONF_PUSH_TIMEOUT_MS
-#define CONF_PUSH_TIMEOUT_MS 500
+#define CONF_PUSH_TIMEOUT_MS 2000
 #endif
 
 /* Certificate expiry warning threshold (days) */

src/controller/include/commit.h

@@ -150,6 +150,16 @@ void commit_set_pool(commit_engine_t* ce, vtaskmgr_t* pool);
 int commit_submit(commit_engine_t* ce, uint64_t lsn, commit_mode_t mode,
                   uint32_t timeout_ms);
 
+/* commit_blocking_admit / commit_blocking_release (bug-1914): take / return
+ * one slot of the same blocking-wait admission budget commit_submit's WAIT
+ * mode uses, for a bounded blocking wait that is not itself a commit wait
+ * (the configAck byte-delivery barrier, repl_wait_majority_ack).  Bounded
+ * blocking on a vtask is allowed only under this reservation
+ * (ha-async-io.md).  admit returns 0 or -XCE_ERR_NOT_READY (budget
+ * exhausted, never blocks); every successful admit needs one release. */
+int  commit_blocking_admit(commit_engine_t* ce);
+void commit_blocking_release(commit_engine_t* ce);
+
 /* raft-apply-decoupling task 1.1: register a continuation for lsn instead of
  * parking the calling thread -- the event-driven equivalent of
  * commit_submit(..., COMMIT_MODE_WAIT, ...). Returns 0 on successful

src/controller/repl.c

@@ -641,6 +641,17 @@ int repl_wait_majority_ack(replicator_t *r, uint64_t entry_lsn,
         return 0;
     }
 
+    /* bug-1914: this is a bounded blocking wait on a vtask, so it runs under
+     * the same admission reservation as a commit WAIT (ha-async-io.md); a
+     * full budget rejects fast with -XCE_ERR_NOT_READY instead of taking a
+     * worker reserved for liveness traffic. */
+    int arc = commit_blocking_admit(r->ce);
+    if (arc != 0) {
+        WARN("repl_wait_majority_ack: configAck wait not admitted (rc=%d,"
+             " push_tag=%016" PRIx64 ")", arc, push_tag);
+        return arc;
+    }
+
     struct timespec deadline;
     clock_gettime(CLOCK_REALTIME, &deadline);
     deadline.tv_sec  += (long)(timeout_ms / 1000);
@@ -689,6 +700,7 @@ int repl_wait_majority_ack(replicator_t *r, uint64_t entry_lsn,
     }
 
     pthread_mutex_unlock(&r->ack_lock);
+    commit_blocking_release(r->ce);
 
     if (rc == 0)
         INFO("repl_wait_majority_ack: quorum reached (ack_op=%u)", ack_op);

src/controller/testsuite/test_commit_barrier_rollout.c

@@ -1185,6 +1185,50 @@ static void test_configack_self_plus_one_covering_is_majority_of_three(void)
     printf("test_configack_self_plus_one_covering_is_majority_of_three: PASS\n");
 }
 
+
+/* bug-1914: under heavy CPU load (40 busy loops on 32 cores) a standby's
+ * config ACK arrives a few hundred ms late; the 500 ms barrier rejected a push
+ * whose marker then committed anyway (client told -110, cluster activated
+ * it).  An ACK landing 700 ms in must still meet the production barrier
+ * bound, and while it waits the barrier holds a commit-wait admission slot
+ * (bounded blocking on a vtask is allowed only under that reservation). */
+static atomic_int g_1914_inflight_seen;
+
+static void* b1914_late_config_ack(void* arg)
+{
+    (void)arg;
+    usleep(100 * 1000);
+    pthread_mutex_lock(&g_bce.lock);
+    atomic_store(&g_1914_inflight_seen, g_bce.inflight_waits);
+    pthread_mutex_unlock(&g_bce.lock);
+    usleep(600 * 1000);
+    repl_on_config_ack(&g_bp.repl, &g_bep_a, 0x100);
+    return NULL;
+}
+
+static void test_configack_late_ack_under_load_meets_barrier(void)
+{
+    pthread_t th;
+    int       rc;
+
+    barrier_setup(3, 2, 0);
+    atomic_store(&g_1914_inflight_seen, -1);
+    TEST_ASSERT_EQUAL_INT(0, pthread_create(&th, NULL, b1914_late_config_ack,
+                                            NULL));
+    rc = repl_wait_majority_ack(&g_bp.repl, 0x100, CONF_PUSH_TIMEOUT_MS,
+                                (uint8_t)OPCode_upgradeConfigAck);
+    pthread_join(th, NULL);
+
+    TEST_ASSERT_EQUAL_INT_MESSAGE(0, rc,
+        "bug-1914: a config ACK 700 ms late (CPU-oversubscribed standby) must "
+        "still meet the config-push barrier");
+    TEST_ASSERT_EQUAL_INT_MESSAGE(1, atomic_load(&g_1914_inflight_seen),
+        "bug-1914: the configAck wait must hold a commit-wait admission slot");
+    TEST_ASSERT_EQUAL_INT(0, g_bce.inflight_waits);
+
+    barrier_teardown();
+    printf("test_configack_late_ack_under_load_meets_barrier: PASS\n");
+}
 /* n_peers=1: single-member cluster — immediate success without consuming
  * the timeout. PIN: green today (via the n_active == 0 short-circuit) and
  * green after task 5.2 (explicit n_peers <= 1 rule). */
@@ -2530,6 +2574,7 @@ void test_commit_barrier_rollout(void)
     RUN_TEST(test_configack_previous_tag_ack_does_not_count);
     RUN_TEST(test_configack_no_active_standbys_times_out);
     RUN_TEST(test_configack_self_plus_one_covering_is_majority_of_three);
+    RUN_TEST(test_configack_late_ack_under_load_meets_barrier);
     RUN_TEST(test_configack_single_member_immediate_success);
     RUN_TEST(test_push_config_withheld_acks_returns_etimedout);
     RUN_TEST(test_push_config_majority_acks_returns_zero_and_commits);