Description
如果域名解析比较慢的话，那么在调用getaddrinfo的时候由于执行的是同步操作，导致整个nginx进程卡住。
0 0x00007ffff5d10680 in __poll_nocancel () from /lib64/libc.so.6
#1 0x00007ffff4a97dd4 in __libc_res_nsend () from /lib64/libresolv.so.2
#2 0x00007ffff4a95cce in __libc_res_nquery () from /lib64/libresolv.so.2
#3 0x00007ffff4a968b0 in __libc_res_nsearch () from /lib64/libresolv.so.2
#4 0x00007ffff26c6c53 in _nss_dns_gethostbyname4_r () from /lib64/libnss_dns.so.2
#5 0x00007ffff5d00c08 in gaih_inet () from /lib64/libc.so.6
#6 0x00007ffff5d042cd in getaddrinfo () from /lib64/libc.so.6
#7 0x000000000064441e in ngx_http_upsync_init_server (event=0x7ffff1f8e248) at ../modules/nginx-upsync-module/src/ngx_http_upsync_module.c:3042
#8 0x00000000006430ad in ngx_http_upsync_connect_handler (event=0x7ffff1f8e248) at ../modules/nginx-upsync-module/src/ngx_http_upsync_module.c:2536
#9 0x000000000064307f in ngx_http_upsync_begin_handler (event=0x7ffff1f8e248) at ../modules/nginx-upsync-module/src/ngx_http_upsync_module.c:2519
#10 0x000000000053fcc0 in ngx_event_expire_timers () at src/event/ngx_event_timer.c:94
#11 0x000000000053dd89 in ngx_process_events_and_timers (cycle=0x7ffff3394e10) at src/event/ngx_event.c:256
#12 0x000000000054836a in ngx_single_process_cycle (cycle=0x7ffff3394e10) at src/os/unix/ngx_process_cycle.c:332
#13 0x00000000005126bb in main (argc=1, argv=0x7fffffffe558) at src/core/nginx.c:364