So far there was no dedicated mechanism for replacing DNS servers with
new list. Add dns_resolve_reconfigure() API that allows to achieve that
in a thread-safe manner.
Introduce 3rd state in DNS context lifetime by converting from 'bool
is_used' to 'enum dns_resolve_context_state state'. This new
DEACTIVATING state allows to mark a DNS context as busy and safely close
context without holding lock. Closing DNS context with released lock
prevents deadlock in case net_context_close() has to synchronize with a
separate thread executing handler passed to net_context_recv() (which is
the case for example with ESP-AT WiFi driver).
Signed-off-by: Marcin Niestroj <m.niestroj@grinn-global.com>
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