fix(raft-drive-async): endpoint-quiescence for bug-1197 vote-ep teardown (task 3.4a)

When the (still-default-OFF) peer-control dispatch flag is ON,
raft_cb_become_follower runs as a vtask pool worker and frees every peer
vote-ep while other pool workers may still hold those eps' data — a UAF
(bug-1197). This makes that path quiescence-safe so the flag can later be
flipped ON (deferred to the 4.x prod-flip gate; production stays OFF here).

Mechanism (no libxce-wide endpoint refcount; reuses the existing per-ep FIFO +
busy serialization):

- Deferred teardown-as-vtask: the loser loop, when dispatch is ON and the pool
  is wired, does per vote-ep endpoint_mark_tearing_down -> iomgr_unmonitor ->
  enqueue a VTASK_IO/VTASK_LIVENESS teardown vtask bound to that ep. Per-ep busy
  FIFO makes it run strictly after any in-flight/queued same-ep peer-control
  vtask; the body NULLs iod->ep before endpoint_fini+free(ep) to disarm
  run_task's post-body endpoint_unblock_tasks (which would touch the freed
  ep->lock). OFF / pool-unwired path is byte-for-byte the legacy endpoint_fini.

- RCU-like drain closes the dispatch/teardown TOCTOU window: new _Atomic int
  endpoint_t.dispatch_inflight (+ tearing_down:1 bit). The ON dispatch brackets
  its flag-check+enqueue with fetch_add/fetch_sub (seq_cst);
  endpoint_mark_tearing_down sets the flag under ep->lock, releases, then spins
  until dispatch_inflight drains to 0. So every dispatch that read the flag
  false finishes its enqueue before mark returns (FIFO-before the teardown
  vtask); any later dispatch reads the flag set and drops the packet. ep->lock
  is deliberately NOT held across the enqueue (it inverts with the taskq lock in
  vtask_eligible); the counter is the lock-free handshake. Drain is bounded: the
  held window is just a calloc + non-blocking enqueue on the single io_worker.

Still open before the production flag flip (4.x): the borrowed-ep race in
snapshot_lane_fn/configpush_lane_fn is distinct (slowlane thread vs io_worker
RDHUP teardown, not the vtask-pool FIFO) and needs its own per-standby-ep
discipline.

Tests: test_ha_vep_teardown.c — 5 ASan tests (ordering, late-event drop, run_task
unblock disarm, OFF-path unchanged, deterministic mark-drains-inflight-dispatch).
xcelib_test 282/0; controller_unit_test EXIT=0; ASan-clean. Adversarial
cpp-review: no CRITICAL/HIGH; handshake formally sound, flag-ON path race-free.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01S6fDCtXkkEgupu7rUah1y2

src/include/netio.h

@@ -4,6 +4,7 @@
 
 #include <openssl/err.h>
 #include <openssl/ssl.h>
+#include <stdatomic.h>
 #include <stdint.h>
 #include <stdio.h>
 #include <sys/epoll.h>
@@ -72,7 +73,14 @@ typedef struct endpoint_t {
     int             idx;
     int             flags;
     unsigned int    busy               : 1;
+    unsigned int    tearing_down       : 1;
     unsigned int    ssl_handshake_done : 1;
+    /* bug-1197: count of peer-control dispatch attempts currently between the
+     * tearing_down flag-read and the completion of their pool enqueue.  Bitfields
+     * cannot be _Atomic, so this is a full atomic int.  endpoint_mark_tearing_down
+     * drains this to 0 (after setting the flag) so no dispatch can enqueue a
+     * same-ep vtask after teardown begins. */
+    _Atomic int     dispatch_inflight;
     dataq_t         outq;
     uint64_t        seq;
     conn_handler_t  conn_handler;
@@ -130,6 +138,11 @@ extern uint64_t     endpoint_getseq(endpoint_t*);
 extern void         endpoint_block_tasks(endpoint_t*);
 extern void         endpoint_unblock_tasks(endpoint_t*);
 extern int          endpoint_is_task_blocked(endpoint_t*);
+/* bug-1197: mark an endpoint as undergoing teardown so a late same-batch event
+ * delivered by the io_worker after iomgr_unmonitor is dropped instead of
+ * dispatched onto the vtask pool (which could outlive the ep). */
+extern void         endpoint_mark_tearing_down(endpoint_t*);
+extern int          endpoint_is_tearing_down(endpoint_t*);
 extern int          endpoint_set_owner(endpoint_t*, void*);
 extern int          endpoint_set_msg_handler(endpoint_t*, msg_handler_t);
 extern int          endpoint_set_err_handler(endpoint_t*, err_handler_t);