Created attachment 1398219 [details]
engine log

Description of problem:
[DB] [DNS] - Updating the host's capabilities while running a VM may cause 'ERROR: duplicate key value violates unique constraint "name_server_pkey"'

The issue is the dns_configuration of the host is part of the vds_dynamic table.
If the table is updated not as part of getCapapbilies (for example it is updated when running a VM) and at the same time getCaps (or other updates of the vdsDynamic table) is performed, a race can happen.

- Those are the problematic two lines -

removeNameServersByDnsResolverConfigurationId(entity.getId());
saveNameServersByDnsResolverConfigurationId(entity.getId(), entity.getNameServers());

- If a context switch happens after/during the removal (thread A) and another thread  (thread B) updates the dns configuration.
When returning to thread A, the duplicate name server error will happen.

Caused by: org.postgresql.util.PSQLException: ERROR: duplicate key value violates unique constraint "name_server_pkey"
  Detail: Key (dns_resolver_configuration_id, address)=(86405e21-b3fd-4968-aa66-287daa178a81, 10.35.28.28) already exists.
  Where: SQL statement "INSERT INTO
    name_server(
      address,
      position,
      dns_resolver_configuration_id)
    VALUES (
      v_address,
      v_position,
      v_dns_resolver_configuration_id)"
PL/pgSQL function insertnameserver(uuid,character varying,smallint) line 3 at SQL statement


Version-Release number of selected component (if applicable):
4.2.2-0.1.el7

How reproducible:
100%

Steps to Reproduce:
1. Refresh caps during start VM operation

Actual results:
duplicate name server error will happen