At some circumstances unreachable poller stuck on snmp check with infinity timeout. An it lasts forever.

(gdb) bt
#0  0x00007f6f813a4844 in _GI__select (nfds=19, readfds=0x7ffea6a69bf0, writefds=0x0, exceptfds=0x0, timeout=0x0) at ../sysdeps/unix/sysv/linux/select.c:69
#1  0x00007f6f81ec33cb in netsnmp_large_fd_set_select () from /lib/x86_64-linux-gnu/libnetsnmp.so.40
#2  0x00007f6f81e7e047 in snmp_synch_response_cb () from /lib/x86_64-linux-gnu/libnetsnmp.so.40
#3  0x0000564a024a1039 in zbx_snmp_get_values.constprop ()
#4  0x0000564a024a1ade in zbx_snmp_process_standard.constprop ()
#5  0x0000564a024a51a7 in get_values_snmp ()
#6  0x0000564a024a78b7 in get_values ()
#7  0x0000564a024a7e77 in poller_thread ()
#8  0x0000564a026442d0 in zbx_thread_start ()
#9  0x0000564a0248bfde in server_startup.constprop ()
#10 0x0000564a0248da2b in MAIN_ZABBIX_ENTRY ()
#11 0x0000564a02482772 in main ()

1. strace -s 99 -ffp 4038660
   strace: Process 4038660 attached
   pselect6(19, [18], NULL, NULL, NULL, NULL^C

strace: Process 4038660 detached
 <detached ...>