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feeb684 sys/net/nanocoap: implement observe

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Freeze is over so if anyone wants to pick this up...

Time for squash?

Please squash!

An issue I see is that observe registrations never time out. So The server can easily fill up if clients do not unregister properly (which can easily happen when they get out of range/crash/reboot/…)

Should nanocoap_notify_observers() send CONfirmable messages and remove clients on timeout?

We sadly cannot send fully standard compliant CONs as separate response; which we use for the notification. (In fact, there is an assert that will block.)

One could probably send the notifications as CON without doing retransmissions, breaking the standard. (I think aborting retransmissions when a more recent notification is available is probably the sane thing to do anyway, as the stated goal of the observe RFC is to get the client as closely in sync with the server state as possible; blocking a more fresh update by retransmissions of stale data is not well aligned with this.)

Another approach is to just let observations time out anyway. After all, clients can in fact occasionally re-affirm their active observe registration by registering with the same token at the same resource again. This could be used to reset the timeout.

feeb684 introduced a regression that makes the release tests fail for

• make -C tests/unittests/ tests-nanocoap test and
• make -C tests/unittests/ tests-nanocoap_cache test

(part of task 1.3)

In file included from /home/mlenders/Repositories/RIOT-OS/RIOT/sys/include/net/nanocoap_sock.h:146,
                 from /home/mlenders/Repositories/RIOT-OS/RIOT/sys/net/application_layer/nanocoap/nanocoap.c:31:
/home/mlenders/Repositories/RIOT-OS/RIOT/sys/include/net/sock/udp.h:293:28: error: conflicting types for ‘sock_udp_ep_t’; have ‘struct _sock_tl_ep’
  293 | typedef struct _sock_tl_ep sock_udp_ep_t;   /**< An end point for a UDP sock object */
      |                            ^~~~~~~~~~~~~
In file included from /home/mlenders/Repositories/RIOT-OS/RIOT/sys/net/application_layer/nanocoap/nanocoap.c:30:
/home/mlenders/Repositories/RIOT-OS/RIOT/sys/include/net/nanocoap.h:104:14: note: previous declaration of ‘sock_udp_ep_t’ with type ‘sock_udp_ep_t’ {aka ‘void’}
  104 | typedef void sock_udp_ep_t;
      |              ^~~~~~~~~~~~~
/home/mlenders/Repositories/RIOT-OS/RIOT/sys/include/net/sock/udp.h: In function ‘sock_udp_ep_is_multicast’:
/home/mlenders/Repositories/RIOT-OS/RIOT/sys/include/net/sock/udp.h:809:15: error: request for member ‘family’ in something not a structure or union
  809 |     switch (ep->family) {
      |               ^~
/home/mlenders/Repositories/RIOT-OS/RIOT/sys/include/net/sock/udp.h: At top level:
/home/mlenders/Repositories/RIOT-OS/RIOT/sys/include/net/sock/udp.h:845:10: fatal error: sock_types.h: No such file or directory
  845 | #include "sock_types.h"
      |          ^~~~~~~~~~~~~~