Modify the releasing Zap endpoints from IO thread process

The patch separates the process of reducing the number of Zap endpoints
associated with an IO thread from the process of releasing the endpoint
from the IO thread. Additionally, it defers the nullification of
`zap->thread` until the endpoint reference reaches zero.

`zap_sock` and `zap_ugni` need to remove their events from epoll before
delivering the disconnected event to applications, preventing the
reception of additional epoll events. However, without this patch, this
results in resetting `zap->thread` to NULL. Thus, the applications
cannot retrive Zap thread information in the disconnected path. The
patch addresses this limitation.

lib/src/zap/fabric/zap_fabric.c

@@ -456,6 +456,8 @@ static void z_fi_destroy(zap_ep_t zep)
 
 	DLOG("rep %p has %d ctxts\n", rep, rep->num_ctxts);
 
+	zap_io_thread_ep_remove(zep);
+
 	/* Do this first. */
 	while (!LIST_EMPTY(&rep->ep.map_list)) {
 		map = (zap_map_t)LIST_FIRST(&rep->ep.map_list);

lib/src/zap/rdma/zap_rdma.c

@@ -499,8 +499,10 @@ static void __rdma_teardown_conn(struct z_rdma_ep *ep)
 static void z_rdma_destroy(zap_ep_t zep)
 {
 	struct z_rdma_ep *rep = (void*)zep;
-	if (zep->thread)
+	if (zep->thread) {
 		zap_io_thread_ep_release(zep);
+		zap_io_thread_ep_remove(zep);
+	}
 	pthread_mutex_lock(&rep->ep.lock);
 	__rdma_teardown_conn(rep);
 	pthread_mutex_unlock(&rep->ep.lock);

lib/src/zap/sock/zap_sock.c

@@ -1992,6 +1992,8 @@ static void z_sock_destroy(zap_ep_t ep)
 
 	DEBUG_LOG(sep, "%ld z_sock_destroy(%p)\n", GETTID(), sep);
 
+	zap_io_thread_ep_remove(ep);
+
 	while (!TAILQ_EMPTY(&sep->sq)) {
 		wr = TAILQ_FIRST(&sep->sq);
 		TAILQ_REMOVE(&sep->sq, wr, link);

lib/src/zap/ugni/zap_ugni.c

@@ -2235,6 +2235,9 @@ static void z_ugni_destroy(zap_ep_t ep)
 {
 	struct z_ugni_ep *uep = (void*)ep;
 	CONN_LOG("destroying endpoint %p\n", uep);
+
+	zap_io_thread_ep_remove(ep);
+
 	pthread_mutex_lock(&z_ugni_list_mutex);
 	ZUGNI_LIST_REMOVE(uep, link);
 	pthread_mutex_unlock(&z_ugni_list_mutex);

lib/src/zap/zap.c

@@ -910,15 +910,21 @@ zap_err_t zap_io_thread_ep_assign(zap_ep_t ep, int tpi)
 zap_err_t zap_io_thread_ep_release(zap_ep_t ep)
 {
 	zap_err_t zerr;
+	zerr = ep->z->io_thread_ep_release(ep->thread, ep);
+	__atomic_fetch_sub(&ep->thread->stat->sq_sz, ep->sq_sz, __ATOMIC_SEQ_CST);
+	return zerr;
+}
+
+zap_err_t zap_io_thread_ep_remove(zap_ep_t ep)
+{
+	zap_err_t zerr = 0;
 	zap_io_thread_t t = ep->thread;
 
 	pthread_mutex_lock(&t->mutex);
 	LIST_REMOVE(ep, _entry);
 	t->_n_ep--;
 	t->stat->n_eps = t->_n_ep;
 	pthread_mutex_unlock(&t->mutex);
-	zerr = ep->z->io_thread_ep_release(ep->thread, ep);
-	__atomic_fetch_sub(&ep->thread->stat->sq_sz, ep->sq_sz, __ATOMIC_SEQ_CST);
 	ep->thread = NULL;
 	return zerr;
 }

lib/src/zap/zap_priv.h

@@ -478,14 +478,23 @@ double zap_env_dbl(char *name, double default_value);
 zap_err_t zap_io_thread_ep_assign(zap_ep_t ep, int tpi);
 
 /**
- * Release \c ep from the zap io thread.
+ * Disable \c ep assigned to the zap io thread.
  *
  * The transport shall call this function to release an endpoint from the
  * associated io thread. \c zap.io_thread_ep_release() will also be called as a
  * subsequence.
  */
 zap_err_t zap_io_thread_ep_release(zap_ep_t ep);
 
+/**
+ * Free \c ep from the zap io thread.
+ *
+ * The transport shall call this function to release an endpoint from the
+ * associated io thread. \c zap.io_thread_ep_release() will also be called as a
+ * subsequence.
+ */
+zap_err_t zap_io_thread_ep_remove(zap_ep_t ep);
+
 /*
  * The zap_thrstat structure maintains state for
  * the Zap thread utilization tracking functions.