Optimized request<->app link allocation.

Only purpose of request<->app link instance is to be enqueued in application
requests queue.

It is possible to avoid request<->app link allocation from memory pool in
case when spare application port is available.  Instance from local stack
can be used to prepare and send message to application.

src/nxt_router.c

@@ -302,24 +302,14 @@ nxt_router_start_worker(nxt_task_t *task, nxt_app_t *app)
 }
 
 
-static nxt_req_app_link_t *
+nxt_inline void
-nxt_router_ra_create(nxt_task_t *task, nxt_req_conn_link_t *rc)
+nxt_router_ra_init(nxt_task_t *task, nxt_req_app_link_t *ra,
+    nxt_req_conn_link_t *rc)
 {
-    nxt_mp_t            *mp;
     nxt_event_engine_t  *engine;
-    nxt_req_app_link_t  *ra;
 
-    mp = rc->ap->mem_pool;
     engine = task->thread->engine;
 
-    ra = nxt_mp_retain(mp, sizeof(nxt_req_app_link_t));
-
-    if (nxt_slow_path(ra == NULL)) {
-        return NULL;
-    }
-
-    nxt_debug(task, "ra stream #%uD create", rc->stream);
-
     nxt_memzero(ra, sizeof(nxt_req_app_link_t));
 
     ra->stream = rc->stream;
@@ -327,13 +317,36 @@ nxt_router_ra_create(nxt_task_t *task, nxt_req_conn_link_t *rc)
     ra->rc = rc;
     rc->ra = ra;
     ra->reply_port = engine->port;
-
+    ra->ap = rc->ap;
-    ra->mem_pool = mp;
 
     ra->work.handler = NULL;
     ra->work.task = &engine->task;
     ra->work.obj = ra;
     ra->work.data = engine;
+}
+
+
+nxt_inline nxt_req_app_link_t *
+nxt_router_ra_create(nxt_task_t *task, nxt_req_app_link_t *ra_src)
+{
+    nxt_mp_t            *mp;
+    nxt_req_app_link_t  *ra;
+
+    mp = ra_src->ap->mem_pool;
+
+    ra = nxt_mp_retain(mp, sizeof(nxt_req_app_link_t));
+
+    if (nxt_slow_path(ra == NULL)) {
+
+        ra_src->rc->ra = NULL;
+        ra_src->rc = NULL;
+
+        return NULL;
+    }
+
+    nxt_router_ra_init(task, ra, ra_src->rc);
+
+    ra->mem_pool = mp;
 
     return ra;
 }
@@ -388,7 +401,9 @@ nxt_router_ra_release(nxt_task_t *task, void *obj, void *data)
         ra->app_port = NULL;
     }
 
+    if (ra->mem_pool != NULL) {
         nxt_mp_release(ra->mem_pool, ra);
+    }
 }
 
 
@@ -435,7 +450,9 @@ nxt_router_ra_abort(nxt_task_t *task, void *obj, void *data)
         ra->app_port = NULL;
     }
 
+    if (ra->mem_pool != NULL) {
         nxt_mp_release(ra->mem_pool, ra);
+    }
 }
 
 
@@ -494,7 +511,9 @@ nxt_router_ra_error(nxt_task_t *task, nxt_req_app_link_t *ra, int code,
         ra->app_port = NULL;
     }
 
+    if (ra->mem_pool != NULL) {
         nxt_mp_release(ra->mem_pool, ra);
+    }
 }
 
 
@@ -2807,14 +2826,17 @@ nxt_router_app_port(nxt_task_t *task, nxt_req_app_link_t *ra)
         nxt_timer_add(engine, &c->read_timer, app->timeout);
     }
 
+    can_start_worker = 0;
+
     nxt_thread_mutex_lock(&app->mutex);
 
     if (!nxt_queue_is_empty(&app->ports)) {
         port = nxt_router_app_get_port_unsafe(app, &use_delta);
 
-        can_start_worker = 0;
-
     } else {
+        ra = nxt_router_ra_create(task, ra);
+
+        if (nxt_fast_path(ra != NULL)) {
             nxt_queue_insert_tail(&app->requests, &ra->link);
 
             can_start_worker = (app->workers + app->pending_workers) <
@@ -2822,12 +2844,19 @@ nxt_router_app_port(nxt_task_t *task, nxt_req_app_link_t *ra)
             if (can_start_worker) {
                 app->pending_workers++;
             }
+        }
 
         port = NULL;
     }
 
     nxt_thread_mutex_unlock(&app->mutex);
 
+    if (nxt_slow_path(ra == NULL)) {
+        nxt_router_gen_error(task, c, 500, "Failed to allocate "
+                             "req<->app link");
+        return NXT_ERROR;
+    }
+
     if (port != NULL) {
         nxt_debug(task, "already have port for app '%V' %p ", &app->name, app);
 
@@ -2839,6 +2868,8 @@ nxt_router_app_port(nxt_task_t *task, nxt_req_app_link_t *ra)
         return NXT_OK;
     }
 
+    nxt_debug(task, "ra stream #%uD allocated", ra->stream);
+
     if (!can_start_worker) {
         nxt_debug(task, "app '%V' %p too many running or pending workers",
                   &app->name, app);
@@ -3060,7 +3091,7 @@ nxt_router_process_http_request(nxt_task_t *task, nxt_conn_t *c,
     nxt_int_t            res;
     nxt_port_t           *port;
     nxt_event_engine_t   *engine;
-    nxt_req_app_link_t   *ra;
+    nxt_req_app_link_t   ra_local, *ra;
     nxt_req_conn_link_t  *rc;
 
     engine = task->thread->engine;
@@ -3088,16 +3119,8 @@ nxt_router_process_http_request(nxt_task_t *task, nxt_conn_t *c,
     rc->ap = ap;
     c->socket.data = NULL;
 
-    ra = nxt_router_ra_create(task, rc);
+    ra = &ra_local;
-
+    nxt_router_ra_init(task, ra, rc);
-    if (nxt_slow_path(ra == NULL)) {
-        nxt_router_gen_error(task, c, 500, "Failed to allocate "
-                             "req<->app link");
-
-        return;
-    }
-
-    ra->ap = ap;
 
     res = nxt_router_app_port(task, ra);