Fix for socket leak with "aio sendfile" and "limit_rate". Second aio post happened when timer set by limit_rate expired while we have aio request in flight, resulting in "second aio post" alert and socket leak. The patch adds actual protection from aio calls with r->aio already set to aio sendfile code in ngx_http_copy_filter(). This should fix other cases as well, e.g. when sending buffered to disk upstream replies while still talking to upstream. The ngx_http_writer() is also fixed to handle the above case (though it's mostly optimization now). Reported by Oleksandr V. Typlyns'kyi.