Router: fixed race condition while access log reopening.

In order to reopen access log, the router process creates a memory pool
and allocates a buffer to send RPC message for the main process.

Previously, the memory pool was destroyed when RPC response handler was
called.  It appeared, that the buffer completion handler could be not
triggered by that time and still remained in a queue.

Now the memory pool is destroyed only after both events are happen.

src/nxt_router.c

@@ -220,6 +220,8 @@ static void nxt_router_access_log_error(nxt_task_t *task,
     nxt_port_recv_msg_t *msg, void *data);
 static void nxt_router_access_log_release(nxt_task_t *task,
     nxt_thread_spinlock_t *lock, nxt_router_access_log_t *access_log);
+static void nxt_router_access_log_reopen_completion(nxt_task_t *task, void *obj,
+    void *data);
 static void nxt_router_access_log_reopen_ready(nxt_task_t *task,
     nxt_port_recv_msg_t *msg, void *data);
 static void nxt_router_access_log_reopen_error(nxt_task_t *task,
@@ -2965,6 +2967,8 @@ nxt_router_access_log_reopen_handler(nxt_task_t *task, nxt_port_recv_msg_t *msg)
         goto fail;
     }
 
+    b->completion_handler = nxt_router_access_log_reopen_completion;
+
     nxt_buf_cpystr(b, &access_log->path);
     *b->mem.free++ = '\0';
 
@@ -2988,6 +2992,8 @@ nxt_router_access_log_reopen_handler(nxt_task_t *task, nxt_port_recv_msg_t *msg)
         goto fail;
     }
 
+    nxt_mp_retain(mp);
+
     return;
 
 fail:
@@ -2996,6 +3002,19 @@ fail:
 }
 
 
+static void
+nxt_router_access_log_reopen_completion(nxt_task_t *task, void *obj, void *data)
+{
+    nxt_mp_t   *mp;
+    nxt_buf_t  *b;
+
+    b = obj;
+    mp = b->data;
+
+    nxt_mp_release(mp);
+}
+
+
 static void
 nxt_router_access_log_reopen_ready(nxt_task_t *task, nxt_port_recv_msg_t *msg,
     void *data)
@@ -3016,7 +3035,7 @@ nxt_router_access_log_reopen_ready(nxt_task_t *task, nxt_port_recv_msg_t *msg,
     }
 
     nxt_fd_close(msg->fd);
-    nxt_mp_destroy(reopen->mem_pool);
+    nxt_mp_release(reopen->mem_pool);
 }
 
 
@@ -3028,7 +3047,7 @@ nxt_router_access_log_reopen_error(nxt_task_t *task, nxt_port_recv_msg_t *msg,
 
     reopen = data;
 
-    nxt_mp_destroy(reopen->mem_pool);
+    nxt_mp_release(reopen->mem_pool);
 }